Title of Invention

DYNAMIC REACTIVE POWER COMPENSATOR

Abstract

Disclosed herein is a dynamic reactive power compensator to be known as 'STATION' which continuously compensates the varying load reactive power and allows unity power factor operation at the input supply within its capacity. For addressing the load requirements beyond the declared capacity/ ratings of the STATION, the STATION units can be paralleled. The power compensator of the present invention reduces the system losses due to continuous reactive power reduction and hence continuous power factor correction and requires less maintenance. It is also environment friendly and can easily be re-installed in any other location.

Full Text

Hold is generated by processor in @6 usee after power supply availability during panel switch ON. 8. Main contactor is turned ON by processor after @120 sees, after panel being switched ON. 9. DC bus gets charged to @ 560 V dc. 10. Protection card LED's are to be RESET using respective toggle switch in the Protection card. 11. Two LEDs (L8 and L9) on Protection card glow indicating presence of Hold signal from processor and hold being generated by protection card. 12. Red LEDs on Gate drive card also glow indicating presence of Hold on gate drive ou^uts. 13. Meantime processor checks health of RAM on the Digital card. 14. Then processor checks health of NVRAM on the Digital card. 15. Processor generates intemal start command, which operates corresponding relay on the Relay card (This signal is generated after about 3.3sec subsequent to main contactor being turned ON). 16. Processor waits for START INPUT (given through a push button in the panel) to go to next step. 17. On receipt of start input. Processor executes series of control actions without any user interaction in healthy condition of the panel. 18. Processor measures frequency through available zero crossing signal. If frequency is not foimd within 45 to 55Hz for next 4 cycles, it declares faulty condition and withdraws the contactor as per withdrawal procedxire. 19. Processor measures R phase voltage with available R phase voltage peak detector signal. If voltage exceeds the limiting value 323 V rms, it declares faulty condition after checking for 20 cycles and withdraws the contactor as per withdrawal procedure. phase input voltages, three phase load currents, dc bus voltage feedback and PI control value as discussed earlier in analog card. The Digital status / control signals at TTL logic levels received by the card are buffered internally before being fed to core control circuit. This includes start contactor feedback contact. Bypass contactor feedback contact, stop command from USER through front panel pushbutton, zero crossing detector signal for dynamic frequency calculation, master fault input from the protection card, synchronizing signal corresponding to 'R' phase positive zero cross over, and negative zero cross over signals for all the three phases for facilitating their respective current measurements so as to derive corresponding phase reactive current component. This is as disciissed in Analog and Protection cards. The final outputs (modulating signals) are in the form of dynamically computed 24 step waveforms for all the three phases with +/- 5V as peak amplitude in real time with appropriate phase relationship and in close synchronization with 'R' phase positive zero cross over. These signals are received by the Analog card for fiirther processing through Sinusoidal PWM generation hardware to deliver the gate pulses to the IGBT's. Operating sequence of three phase STATCON panel 1. Connect the three phase 415 V supply input with neutral and earth connections to the panel. Source capacity should be at least 200 A. 2. Keep IGBT gate control switch in ^propriate ON/OFF condition (as desired during testing). 3. Now panel incoming power source can switched "ON". 4. Panel incoming MCB to be switched ON. 5. Power supply LED's for 5V,+/-12V, +/-15V, as well as for Gate drive power supply will glow. 6. Processor (micro-controller) initializes. INTRODUCTION This invention relates to a Dynamic Reactive Power Compensator. This power compensator will be known as 'STATCON' and will be referred by the said name 'STATCON' throughout the specification. BACKGROUND Over a period of time the industrial requirements for reactive power compensation have undergone substantial change and have become extremely demanding / challenging. The conventional solutions for reactive power compensation like LT / HT side fixed / switched capacitors, automatic power factor correction through contactor switching or thyristorised switching (Thyristor Switched C^acitor - TSC or Thyristor Controlled Reactor -TCR) are falling short, one way or the other, in meeting the reactive power compensation requirements of industrial and other loads. There are various draw backs with the known power compensators, such as higher response time (with respect to load changes), nonsinusoidal current being drawn fi-om the supply and dependence on the short circxiit capacity available at the coupling point on the network. On the other hand, considering the present load demands, the reactive power compensator, proposed by the present invention, satisfies the following requirements: • Very fast dynamic response (with respect to load changes) • Sinusoidal current to be drawn all the time fi-om the supply • Almost independent of the short circuit capacity available at the coupling point on the network The powCT compensator proposed by the present invention meets the above specified load reactive power compensation reqiiirements. It is used on low-tension side. BASIC PRINCIPLE The basic operating principle of STATCON is given in fig. lof the accompanying drawings, wherein: Vs indicates instantaneoxis supply phase voltage; Vji indicates fundamental component of the instantaneous switching phase voltage Vi produced by STATCON converter; i Li indicates instantaneous fundamental component of load current; i SI indicates instantaneous fundamoital component of supply current; ici indicates instantaneous fundamental component of STATCON current; wLs indicates supply short circuit impedance at supply fi-equency; and wLb indicates STATCON Boost Reactor impedance at supply fi-equency. It is very clear from fig. 1 that by controlling the voltage Vii (its amplitude and displacement from supply voltage), STATCON can be made to draw either capacitive or inductive current. This method of current control is called as "Indirect Current Control (ICC)". The load CT gives information of load cvirrent from which the reactive current compensation is calculated and Vji then is given by: -Vs ± (ici • wLb) + Vii = 0 (1) where + sign denotes inductive mode operation of STATCON - sign denotes capacitive mode operation of STATCON Since STATCON uses active power converter (based on power devices) it will have to support a small but varying power loss in the devices and other power components [elaborated imder equations (3) to (8) and (11) to (14)], The varying loss needs to be accounted by a variable resistor Ry in the equivalent circuit. The equivalent circuit shown in fig. 1 (b) gets modified as given in fig. 2(a). Equation (1) now changes to: -Vs±(ici*Rv)±(ici«wLb) + Vii=0 (2) The modified vector diagrams are given in figs. 2(b) and 2(c). It should be noted that the displacement angle '6' is extremely small and could vary within less than 3 degrees as will be seen later. It should, however, be noted that since the resistance Ry is variable, it needs to be simulated in a closed loop condition. Further, the dynamic placement of vector Vji or its calculated value depends on it. STATCON controls will have to calcvilate this Ry continuously and adjust the vector Vji accordingly so that the entire closed loop fimctions properly to meet the dynamic reactive power requirement of the load. This will be explained in detail herein after. STATCON can be used in a single phase or three phase electric supply and has a wide variety of ^plications. These are: • All core sector industries (paper, cement, steel). • Refineries. • Arc furnace loads / furnace converters. • Wind Mills. • Distribution transformers. • Agricultural loads. • Railways. • Residential cimi commercial complexes. • Public utility systems like escalators, conveyor belts, ropeways etc. • Automobile industry loads (spot welding, tag welding, painting, robotic processes etc.). In general the loads to be catered to include dynamically varying, unpredictable, balanced or unbalanced loads. The three phase STATCON caters to balanced loads while the single phase STATCON caters to single phase loads and unbalanced three phase loads where individual phase incorporates a single phase STATCON. Accordingly, it is an object of the present invention to provide, dynamic reactive power compensator, which continuously compensates the varying load reactive power and allows unity power factor operation at the input supply within its capacity. For addressing the load requirements beyond the declared edacity/ ratings of the STATCON, the STATCON units can be paralleled. A further object of the present invention is to provide, a smooth operating dynamic reactive power compensator and not a stepwise operating compensator. Yet another object of the present invention is to provide a power compensator, which provides dynamic reactive power in capacitive as well as inductive modes and which has a very fast response, i.e. close to one power cycle (20 msec for 50 Hz frequency). The power compensator of the present invention reduces the system losses due to continuous reactive power reduction and hence continuous power factor correction and requires less maintenance. It is also environment friendly and can easily be re-installed in any other location. This invention will now be described with reference to the accompanying drawings, wherein: Fig. 1 Basic operating principle of STATCON; (a) Single line diagram for a typical load using STATCON; (b) Equivalent circuit for the compensating reactive current drawn by STATCON; (c) Vector diagram for capacitive compensation; (d) Vector diagram for inductive compensation; Fig. 2 Modified equivalent circuit and vector diagram for STATCON considering variable loss resistor; (a) Modified equivalent circuit including the variable loss component of STATCON; (b) Modified vector diagram for cq)acitive compensation; (c) Modified vector diagram for inductive compensation The above figures are common to both single and three phase STATCON's. Three phase STATCON: Fig. 3 Power scheme for the three phase STATCON; Fig. 4 PWM generation based on SPWM method; Fig. 5 Circuit diagram for the voltage distortion analysis; Fig. 6 (a) Control Logic; (b) Control Electronics; Fig. 7 Flow chart; Fig. 8 Signals received and delivered by various control cards; Fig. 9 Block diagram of digital card; Fig. 10 General arrangement of three phase STATCON; Fig. 11 (a) Inductive mode of operation; (b) Capacitive mode of operation; Fig. 12 Mode changeover response; Fig. 13 Dynamic response Smgle phase STATCON: Fig. 14 Power scheme for the single phase STATCON; Fig. 15 PWM generation based on SPWM method; Fig, 16 Circuit diagram for the voltage distortion analysis; Fig. 17 (a) Control logic; (b) Control electronics; Fig. 18 Flow chart; Fig. 19 Signals received and deUvered by various control cards; Fig. 20 Block diagram of digital card; Fig. 21 General arrangement of single phase STATCON; Fig. 22 Dynamic response Note : PWM denotes " Pulse Width Modulation ". SPWM denotes "Sinusoidal Pulse Width Modulation". Description of all the figures shown in the accompanying drawings Before the figures are described, it is necessary to understand certain basic as well as overall aspects with respect to figs. 1 and 2 and the STATCON operation in relation to the same. The STATCON is connected in shunt with the load, as in fig. 1(a). The load current (ILI ) is measured through a current transformer (CT) and its reactive component is established, as in figs. 1(b) and (c) and as indicated by ILI * sin (|). Note: 1. Instantaneous values are mentioned by low case letters, such as ILI, VU. 2. Root Mean Square (RMS) values are mentioned by ILI, Vii. STATCON is supposed to nullify or compensate this load component In * sin In STATCON, the power devices used are called as Insulated Gate Bi-polar Transistors (IGBT's). The power converter using these IGBT's (fig. 3) is controlled by Pulse Width Modulation (PWM) process in such a way that the three fiindamental fi-equency voltages appearing at the terminals Rl, Yl, and Bl of the converter (fig. 3) with respect to virtual zero (midpoint of the dc capacitors or voltage Vdc in fig. 3) are as shown by voltage VH in fig. 4. This fimdamental component Vii of the switching voltage Vj in fig.4 is controlled by varying the amplitude of the modulating signal in fig. 4. The varying voltage vn opposes the supply voltage Vs shown in fig. 1 (b), which is the single phase equivalent circuit. The difference between the supply voltage Vi and the appUed voltage Vii , say AV, could be positive or negative. Based on whether the AV is positive or negative, STATCON will draw inductive or capacitive current fi-om the supply (RMS value as AV/ wLb), as shown in figs. 1(d) and (c) respectively, to compensate the load reactive component 1LI * sin (|) . Next requirement is to understand how STATCON generates the voltage Vi and Vn and how many components and subassembUes are required to do so. The IGBT based converter in fig. 3 has to have controlled switching of the IGBT devices, based on the required compensation process as explained in figs. 1 and 2. It should be noted that the fig. 2 accoxmts for small but active power losses in STATCON (power loss in devices, and all other components in the power scheme of fig. 3) and modifies figs. 1(b), (c), and (d) to figs. 2(a), (b), and (c) respectively. Accounting for the small power loss as above, the STATCON has to dynamically solve and find out value of Vii continuously. The voltage VH is produced by superimposing the carrier wave of necessary fi-equency (in this case 2.8 kHz) with the varying modulating signal corresponding to required Vii as shown in fig. 4.The various components and subassemblies required to establish the control over this voltage Vii and hence on the compensating current Id , are given and explained in the figures below. The converter in fig. 3 finally produces the voltage Vn at its terminals Rl, Yl, and Bl with respect to the virtual zero ( midpoint of the dc capacitors or the voltage Vdc). The Digital controllCT, which is part of the Control Electronics, sequences the converter and the power scheme. However, the Control Electronics and the Control Logic control the sequencing and interlocking operations of the complete STATCON as a whole. Thus, the Power scheme and its components, the Control Electronics and the Control Logic form major parts of STATCON. The details are now given in figures below. Fig. 1(a) This figure gives a typical single line diagram of coimecting the STATCON (three phase or single phase) in parallel with the load while receiving a load CT feedback and compensating the load reactive power. From the load current, reqiiired compensation for the inductive or capacitive load current component is calculated by STATCON and same compensating current is then drawn the supply. Please refer figs. 1(c) and (d) and figs. 2 (b) and (c). Fig. 1(b) This figure is for understanding the fimdamental fi-equency voltage loop including the supply voltage, the fimdamental component Vii of the switching voltage Vji of the STATCON and the boost reactor. This leads to proper understanding of figs. 1 (c) and (d). Fig. 1(c) and fig. 1(d) These figures explain the vector relationship amongst the supply voltage, the fimdamental component of the switching voltage of the STATCON and the current drawn by the STATCON when it works in either capacitive mode or in inductive mode. Fig. 2 (a) This is modified fig. 1(b) incorporating the variable power loss component of STATCON. The power loss component is denoted by Ry. This loss resistance depends upon the actual active loss in the components of the power scheme given in fig. 3. Fig. 2(b) and fig. 2(c) These are modified vector diagrams given in figs. 1(c) and (d) incorporating the variable power loss resistor Ry of STATCON. Fig. 3 This is the power scheme diagram for the three phase STATCON which includes all the power components starting from acceptance of input power supply to the IGBT power stack. Basically, it uses a three phase half bridge construction for the Voltage Source Converter. It also shows ac voltage sensing transformer, control transformer for control logic and electronic cards, power supplies, incoming diode rectifier for surge absorption, incoming voltage spike or surge suppressor, R-C-D snubber arrangement, and the power contactor arrangement. When the main Moulded Case Breaker (MCB) [1] is closed, it gives the three phase power to STATCON and impressing the three phase voltage across its input terminals. The HRC fuses [2] are meant for short circuit protection. The incoming smoothening inductors [3] and the diode rectifier based surge energy absorber [5,6,7,and8] are effective when there is a sudden power failure. The rectifier output capacitors absorb trapped energy in the network and save the STATCON fi-om any damage due to overvoltage or voltage transients. The R-C based surge suppressor [9] absorbs transients on the incoming supply network. The main contactor [11] closes initially and charges the DC c^acitor stack [20] to peak of incoming phase to phase voltage ( approximately 587 V for a three phase supply of 415 V) through the charging resistor [12] with a limit on the supply current drawn. These DC capacitors are also provided with discharge resistors [21] across their terminals to discharge them when the power fails or STATCON is switched off for any reason. The bypass contactor [13] closes when the micro-controller releases the command for it to operate. It then bypasses the main contactor and carries the supply line currents. Once the bypass contactor is closed, the IGBT converter or STATCON as a whole is ready to perform, but will be controlled by the micro-controller in the Digital card. It then charges the DC bus voltage from the nominal voltage of 587 V to 850 V dc ( called as boost charging of the DC capacitor stack) and subsequently the converter or STATCON performs as the reactive power compensator. This will be explained later in subsequent sections. The control transformer 240 / 3 V [4] is meant for providing incoming voltage information to control electronics. The 415 / 240 V transfonner [10] is used for supplying isolated power to control logic and control electronics. The filter capacitors [14] after the bypass are meant for filtering the input current ripple generated by the switching of the IGBT devices. The protection CT's [15] measure the input currents and provide information to be used for sensing overcurrent in supply lines. The boost reactors [16] allow boost charging of the stack to 850 V dc and isolate the incoming supply from the IGBT converter. The IGBT switches, as shown by [19 and 22], are bi-directional allowing the current to flow in both directions. In forward direction, the switch is controlled based on pulse delivery to the IGBT gate with respect to its emitter, while the reverse direction conduction depends on the IGBT not having a pulse to its gate and the diode being forward biased. The snubber devices ( R-C-D) [18] are meant for providing dv / dt, di / dt and hole storage / reverse recovery protection for the IGBT's. Additional protection for the devices is also given using snubber capacitors [17] across the positive and negative DC bus of the converter and located near each IGBT module. The capacitor stack is maintained at 850 V dc during running condition of the STATCON. Since the output dc ripple current is small, only two dc parallel capacitor sections are used to filter the same. Fig. 4 This figure shows how switching Pulse Width Modulated (PWM) voltage is produced at the terminals (Rl, Yl, Bl terminals marked in fig. 3) of the converter using Sinusoidal Pulse Width Modulation process. It uses a 2.8 kHz carrier and a sinusoidal modulating waveform and reflects the comparison in terms of the switching voltage as shown in the figure. The switching voltage, as explained earlier, is expressed as +/- Vdc / 2 with respect to the virtual zero (midpoint of the dc side capacitors [21] in fig. 3. The fundamental component Vii of the switching voltage Vi is as discussed in figs. 1 and 2. Fig. 5 This figure is used for calculation of the supply voltage distortion based on 'n'th harmonic current ripple fed to the supply and being filtered through an appropriately sized capacitor connected in shunt with the supply terminals. The capacitor presents relatively a small impedance (1/ nwC ) to the 'n'th harmonic frequency as compared to the short circuit impedance of the supply network (nwLb ) and hence restricts the harmonic current flowing into the supply network. Fig.6(a) This figure gives the Control Logic (sequencing and interlocking operations) for the three phase STATCON fed through a 240 V isolation transformer. The logic is simple and utilizes basically control contactors and relays (from Relay card). On receipt of the start command through a start push button [1], the start contactor [2] closes. Then the micro-controller gives command for the main contactor [3] to operate to charge the capacitor stack of the converter. Further, the micro-controller will give command for the bypass contactor [5] to close to allow the STATCON function as a reactive power compensator. On receipt of stop command, given from a stop push button [7], or sensing of any fault, the micro-controller will first stop the IGBT firing, bring down the supply currents to zero value, and give command for the bypass contactor [5] to open. It will then give command for the main contactor [3] to open. The auxiliary contactor [4] is used for operating the bypass contactor coil. The stop contactor [6] operates when stop command is delivered by a stop push button. Fig. 6(b) This figure explains how the Control Electronics is interfaced with the Control Logic, regulated / isolated IGBT Gate power supplies, DC voltage sensor, filtered incoixiing ac voltage and the Gating for the IGBT's. It also shows the connections within various control cards. These cards and their functions are explained later. The required controlled / regulated power supplies for the control cards are as given below. +5 V dc for the Digital card +/- 12 V dc for the Digital card, Gate Drive cards, and Analog card +/- 15 V dc for the Analog card, Protection card. Clamp card, and DC Voltage sensor card + 20 V dc, 6 numbers isolated power supplies for the IGBT triggering / firing. All the power supplies are routed through the Relay card, except six numbers of the + 20 V dc power supplies. The major interconnections are done using flat cables between the cards. Fig. 7 This is the flow chart for the sequence and interlocking operations of the three phase STATCON as executed by the micro-controller through its controlling software. It broadly explains the actions taking place in the STATCON after the power is switched on through the incoming MCB, and various checks the micro-controller carries out at different stages of the operation, till the required operation of reactive power control is effected. It also gives the information on how the STATCON folds back if there is any fault sensed. Fig. 8 This figure is for providing the understanding of basic inputs and outputs of each card of the Control Electronics. It also establishes and explains how the incoming signals are received processed at various cards and finally how the output signals are delivered to the Gate drive cards, which control the IGBT firing. The analog signals vary between +/-10 V , +/- 5 V or 0 to +5 V while the digital signals have maximum value + 5 V dc. This figure also gives block level schematic and understanding of each control card. The input / output and functional details of the cards are given later. Fig. 9 This figure explains the architecture of the Digital card in a block form and also explains input / output relationship of the signals received / delivered by the card. Input / output and functional details of this card are given later. Fig. 10 This figure gives the general arrangement of various components and subassemblies within the three phase STATCON panel or cubicle. These are numbered and explained. It gives an impact of how a panel / cubicle housing the complete STATCON assembly looks like. The panel / cubicle is basically of IP 31 protection class in construction but needs deration for higher protection classes (typically up to 35% when the class reaches IP54). Fig. 11(a) and fig. 11(b) These figures depict the basic operation of the STATCON while working in Inductive mode and Capacitive mode and confirm that the current drawn is at 90 degrees lagging or leading with respect to the supply voltage. Fig.l2 This figure gives the dynamic response of the three phase STATCON when it changes its mode of operation. The response is programmable and can be tuned with the load demanded response. Fig. 13 This figure gives the dynamic response of the three phase STATCON current with respect to the load current. This response can be tuned and made to track the load current demand as close as possible based on demand and / or based on the system needs. Fig. 14 This is the power scheme diagram for the single phase STATCON, which includes all the power components starting from acceptance of input power supply to the IGBT power stack. Basically it uses a single phase full bridge construction for the Voltage Source Converter with two bridges connected in parallel to improve the kVAR rating. It also shows ac voltage sensing transformer, control transformer for control logic and electronic cards, power supplies, incoming diode rectifier for surge absorption, incoming voltage spike or surge suppressor, R-C-D snubber arrangement, and the power contactor arrangement. When the main Moulded Case Breaker (MCB) [1] is closed, it gives the single phase power to STATCON and impressing the single phase voltage across its input terminals. The HRC fuses [2] is meant for short circuit protection. The incoming smoothening inductor [3] and the diode rectifier based surge energy absorber [5,6,7,and8] are effective when there is a sudden power failure. The rectifier output capacitors absorb trapped energy in the network and save the STATCON from any damage due to overvoltage or voltage transients. The R-C based surge suppressor [9] absorbs transients on the incoming supply network. The main contactor [11] closes initially and charges the DC capacitor stack [20] to peak of incoming phase voltage ( approximately 339 V for a single phase supply of 240 V) through the charging resistor [12] with a limit on the supply current drawn. These DC capacitors are also provided with discharge resistors [21] across their terminals to discharge them when the power fails or STATCON is switched off for any reason. The bypass contactor [13] closes when the micro-controller releases the command for it to operate. It then bypasses the main contactor and carries the supply line currents. Once the bypass contactor is closed, the IGBT converter or STATCON as a whole is ready to perform, but will be controlled by the micro-controller in the Digital card. It then charges the DC bus voltage from the nominal voltage of 587 V to 850 V dc ( called as boost charging of the DC capacitor stack) and subsequently the converter or STATCON performs as the reactive power compensator. This will be explained later in subsequent sections. The control transformer 240 / 3 V [4] is meant for providing incoming voltage information to control electronics. The 240 / 240 V transformer [10] is used for supplying isolated power to control logic and control electronics. The filter capacitor [14] after the bypass is meant for filtering the input current ripple generated by the switching of the IGBT devices. The protection CT [15] measures the input currents and provides information to be used for sensing overcurrent in supply lines. The boost reactors [16] allow boost charging of the stack to 600 V dc and isolate the incoming supply from the two IGBT converters operating in parallel. This single phase STATCON uses two converters in parallel to provide the necessary current rating. The IGBT switches, as shown by [19 and 22], are bi-directional allowing the current to flow in both directions. In forward direction, the switch is controlled based on pulse delivery to the IGBT gate with respect to its emitter, while the reverse direction conduction depends on the IGBT not having a pulse to its gate and the diode being forward biased. The snubber devices ( R-C-D) [18] are meant for providing dv / dt, di / dt and hole storage / reverse recovery protection for the IGBT's. Additional protection for the devices is also given using snubber capacitors [17] across the positive and negative DC bus of the converter and located near each IGBT module. The capacitor stack is maintained at 600 V dc during running condition of the STATCON. Since the output dc ripple current is high, five dc parallel capacitor sections are used to filter the same. Fig. 15 This figure shows how switching Pulse Width Modulated (PWM) voltage is produced at the terminals ( PI, P2 and Nl , N2 terminals marked in fig. 14) of the converters using Sinusoidal Pulse Width Modulation process . It uses a 1.5 kHz carrier and a sinusoidal modulating waveform and reflects the comparison in terms of the switching voltage as shown in the figure. The switching voltage, as explained earlier, is expressed as + Vdc, 0, - Vac with respect to the virtual zero (midpoint of the dc side capacitors [21] in fig. 14. The fundamental component Vii of the switching voltage Vi is as discussed in figs. 1 and 2. Fig. 16 This figure is used for calculation of the supply voltage distortion based on 'n'th harmonic current ripple fed to the supply and being filtered through an appropriately sized capacitor connected in shunt with the supply terminals. Fig. 17(a) This figxire gives the Control Logic (sequencing and interlocking operations) for the single phase STATCON fed through a 240 V isolation transformer. The logic is simple and utilizes basically control contactors and relays (fi-om Relay card). On receipt of the start command through a start push button [1], the start contactor [2] closes. Then the micro-controller gives command for the main contactor [3] to operate to charge the capacitor stack of the converters. Further, the micro-controller will give command for the bypass contactor [5] to close to allow the STATCON function as a reactive power compensator. On receipt of stop command, given fi-om a stop push button [7], or sensing of any fault, the micro-controller will first stop the IGBT firing, bring down the supply currents to zero value, and give command for the bypass contactor [5] to open. It will then give command for the main contactor [3] to open. The auxiliary contactor [4] is used for operating the bypass contactor coil. The stop contactor [6] operates when stop command is delivered by a stop push button. Fig. 17(b) This figure explains how the Control Electronics is interfaced with the Control Logic, regulated / isolated IGBT Gate power supplies, DC voltage sensor, filtered incoming ac voltage and the Gating for the IGBT's. It also shows the connections within various control cards. These cards and their functions are explained later. The required controlled / regulated power supplies for the control cards are as given below. +5 V dc for the Digital card +/-12 V dc for the Digital card, Gate Drive cards, and Analog card +/- 15 V dc for the Analog card. Protection card, Clamp card, and DC Voltage sensor card + 20 V dc, 6 numbers isolated power supplies for the IGBT triggering / firing. All the power supplies are routed through the Relay card, except six numbers of the + 20 V dc power supplies. The major interconnections are done using flat cables between the cards. Fig 18 This is the flow chart for the sequence and interlocking operations of the single phase STATCON as executed by the micro-controller through its controlling software. It broadly explains the actions taking place in the STATCON after the power is switched on through the incoming MCB, and various checks the micro-controller carries out at different stages of the operation, till the reactive power control is effected. It also gives the information on how the STATCON folds back if there is any favdt sensed. Fig.19 This figure is for providing the understanding of basic inputs and outputs of each card of the Control Electronics. It also establishes and explains how the incoming signals are received processed at various cards and finally how the output signals are delivered to the gate drive cards, which control the IGBT firing. The analog signals vary between +/- 10 V, +/- 5 V or 0 to +5 V while the digital signals have maximum value + 5 V dc. This figure also gives block level schematic and understanding of each control card. The input / output and functional details of the cards are given later. Fig. 20 This figure explains the architecture of the Digital card in a block form and also explains input / output relationship of the signals received / delivered by the card. Input / output and functional details of this card are given later. Fig. 21 This figure gives the general arrangement of various components and subassemblies within the single phase STATCON panel or cubicle. These are numbered and explained. It gives an impact of how a panel / cubicle housing the complete STATCON assembly looks like. The panel / cubicle is basically of IP 31 protection class in construction but needs deration for higher protection classes (typically up to 35% when the class reaches IP54). Fig. 22 This figure gives the dynamic response of the single phase STATCON current with respect to the load current. The responses are taken for a spot welding application in an automobile industry. This response can be tuned and made to track the load current demand as close as possible based on demand and / or based on the system needs. THREE PHASE STATCON Power scheme and component selection The three phase STATCON has following basic specifications. • 415V(±10%),±50Hz(±5%),±85A • Incoming supply as 4 wire (3 phases and one neutral) • All the internal power supplies to be derived from the incoming supply only • Dynamic response time close to one cycle The power scheme is given in fig. 3. It uses a three phase half bridge construction for the power converter. The power converter produces terminal voltage Vi, (vjr, Viy , vn, with respect to the neutral or the midpoint of the output c£5)acitor bank voltage Vdc), as can be seen from fig. 4. Fundamental component of this voltage is v ii, (v iu, v iiy, v iib), as can be seen from the same figure. Each power switch (in this case IGBT, i.e. Insulated Gate Bipolar Transistor) is bi-directional. The converter is called as Current-controlled Voltage Source Converter operating in boost mode. In this case input current is (isr, isy, isb) is controlled indirectly by controlling the voltage Vii. As such, this current control method is called as 'Indirect Current Control (ICC)". The dc voltage Vdc needs to be more than the peak of supply phase to phase voltage so that the supply current can be forced in both the directions. This requirement gives the converter it's above defined name. Various formulae to be used in the component selection of the converter and their relevance 1. Maximum vn required for capacitive operation = (415*1.15)/V3 + (85*wLb) 2. Minimum Vji required for inductive operation = (415 * 0.8) / V3 -(85*wLb) Design factor is 15% overvoltage and 20% imdervoltage. 3. Peak ripple in the supply current at 50% duty ratio of the terminal switching voltage v i switching between the levels +/- Vdc /2 is given by (2*Vdc)/(7i*mf*wLb) where mf=ratio of switching frequency of the IGBT devices to supply frequency. Note tiiat the switching frequency is also the same carrier frequency as explained above. 4. The converter equation (2) given earlier can be rewritten as C^acitive mode Error E decides the angle '5' in fig. 2 or the angular displacement of voltage Vii from the supply voltage. The switching voltage Vi and hence the Vji voltage is produced by using comparison of a triangular wave with a fundamental frequency signal' Vc' as shown in fig. 4. The converter thus uses Sinusoidal Pulse Width Modulation (SPWM) method for producing the switching voltage Vi. The ratio of amplitude of Vc to the amplitude of the triangular waveform is called as modulation index (Mj). It should normally be below 1.0. However, over modulation is possible iising a third harmonic which can be injected on the modulating signal Vc. The third harmonic is to be injected in phase with the fundamental signal Vc and its amplitude should be l/6th as that of the Vc. This gives aroxmd 15.5% increased vn for the same fundamental modulation index Mi. Since the loss component Icm * Rv or E in equations (5) and (6) is quite small, the displacement angle '5' of Vii is also qmte small. It is hence not necessary to consider both sine and cosine components of the third harmonic and one can consider only sine component only. Thus l/6th of the fimdamental ampUtude sine term, i.e. (Vm ± Icm *wLb) sin 3wt needs to be added to equations (5) and (6). These equations hence get modified as under. Capacitive mode (7) Inductive mode These are the equations the Digital card micro-controller has to solve on a continuous basis. The STATCON basic functioning is based on these equations. With only Vc considered superimposed on the triangular waveform, the Viivoltge is given by Vi,= (Mi*Vdc)/(2*V2) (9) With superimposition of 1/6 the third harmonic, as discussed earlier, this voltage Vii is given as (10) The linear relation between M, and Vji is valid for nif (switching frequency to fundamental frequency ratio) greater than nine. Converter component selection The converter component selection is an interactive process based on • Formulae given above • Various IGBT devices available and their characteristics • Proving the power stack for requisite rating integrating the devices, the snubber components, dc capacitors and the forced cooling etc. • Switching frequency choice related to above • Integrated protection approach for the IGBT turn off within less than 12 microseconds • Digital card developed around 80196 micro controller and its clock frequency, and • Few other parameters The component and parameters selected are as xmder (please see power scheme in fig. 3 and fig. 10 also). Components 1. IGBT 200 A, 1400 V (Fuji make 2MBI200PB -140,2 in one with isolated base). 2. DC cq>acitors 4700 ^F, 450 V dc used for the IGBT stack. 3. Three phase, boost reactor 1.65 mH, 120 A, 4. Snubber - Diode 3 A, 2000 V, type UF 5408,2 groups in series with each having 4 in parallel. Resistor 11 ohms, 100 W C^acitor 0.1 ^iF, 2000V dc The snubber is connected across each device. Further, there is also 0.33 {JF, 2000 V dc capacitor connected across the dc terminals of each IGBT module. 5. Blower - Single phase, 240 V, 500 cubic feet / min. 6. Heatsink - AFCOSET 80 AD (645 H * 126 W * 136 D ) anodized. 7. DC c^. discharge resistor - 15 K, 25 W. 8. Main MCB - 415 V, 150 A, 3 pole. 9. Main contactor - 415 V, 40 A, 3 pole with 240 V ac coil. 10. Bypass contactor -415 V, 150 A,3 pole with 240 V ac coil. 11. Pre-charging resistor -10 Ohms, 200 W. 12. Ripple filter capacitor - 12.5 ^F, 2000 V dc per phase. 13. hicoming R-C filter - 5 Ohms, 100 W and 4 ^iF, 660 V. 14. Incoming smoothening inductor 20 ^H, 120 A (three phase). 15. Incoming surge energy absorption Diode 70A, 2000 V (3 modules with 2 in one with isolated base or 6 nos.). Current limiting resistor 27 ohms, lOOW DC c^acitor 1000 ^F, 450 V dc, 2 nos. Discharging resistor 27 K, 100 W. All of these are mounted on a small anodized heatsink (300 H*150W*60D ) 16. Heatsink temperature sensor type N/C operating at 90 degrees. 17. HRC fiise in supply lines rating 125 A, 500 V. 18. Protection CT in each line rating 200 A /1 A suitable for 415 V supply voltage. 19. Control / Isolation transformer for feeding the Control Logic and power supplies for Control Electronics. This is 415 V / 240 V, with 3 kVA rating. 20. Three numbers of control transformers for the purpose of sensing incoming phase voltages. These are 240 / 3 V, with 6 VA rating each. Parameters 1. Digital processor N80 C 196 KC 20 (16 bit) operated at 12 MHz 2. Switching fi-equency of IGBT (carrier fi-equency) 2.8 kHz 3. IGBT stack cooling. Forced cooled with specified blower 3 Modulating waveform optimized to 24 pvilse (15 degrees per step) in each 15 degrees, the controller computes the modulating signal level based on fresh information of all the parameters. 4 In each cycle (before its completion) all information related to voltages, current, frequency and associated operating liniiits (dynamic) are updated. Correct synchronization is maintained. Thus, every cycle resynchronization with updated parameters takes place. However, the response of the converter is j\ist a little over one cycle (less than 1 cycle and 15 degrees). 5 'R' phase is considered as the master phase. Computations related to' Y' and 'B' phases are based on 120 and 240 degrees relationship with respect to the 'R' phase. Results based on iterations 1. The converter dc voltage has been finally selected as 850 V dc. The switching frequency selected is 2.8 kHz. 2. Input current ripple (peak) = (2 * 850) / (n * (2800/50) * 2 * TI * 50 * 1.65 x 10"^ ) = 18.64 A at 2.8 kHz 3. Vji variation reqxiired (per phase) (240 * 1.1 + 85 * 1.65 * 10-^ * 2 * 71 * 50) = 308 V (240 * 0.8 - 85 * 1.65 * lO'^ * 2 * u * 50) = 148 V Thus the variation is from 308 V to 148 V AC. At 850V dc, it means modulation index Mj varying from from (308 *2*V2)/(1.155* 850) = 0.89 to (148 *2*V2)/(1.155* 850)) = 0.426 The restricted modulation index Mj to a maximum of 0.89 allows the modulating signal Vc to remain lower than the triangular waveform amplitude and leaves some margin to account for dynamics of the load. 4. Expected displacement angle Consider fig. 2 (a). ThemaximumIc*wLbdrop(85*2* 71*50*1.65* 10"^) =44volts Assume a worst case Ic * Rv drop of 10% of this nature Thusic *Rv = 4.4 volts Thus maximimi '6' in capacitive mode is given by tan "' (4.4 / (240 x 1.1 + 44)) = 0.82 degree. Similarly maximimi '5' in inductive mode is given by tan -' (4.4 / (240 x 0.8 - 44)) = 1.7 degree. It shows that the displacement '5' angle of the vector Vji with respect to the phase voltage Vs (Vsr, Vsy or Vsb) is quite small but is essential for the converter operation. This is in line with what was stated earlier while introducing equations (3) to (8). 5. Incoming voltage distortion (calculated for a typical short circuit capacity of the network) Assume that at the coupling point of STATCON the short capacity of the network is say 30000 KVA. Refer fig. 5 for the voltage distortion analysis. Short circuit impedance (w * Lb) = (3 * 240 * 240) / (30000 * 1000) = 0.00576 Ohms Therefore, impedance at switching fi-equency = (2800 * 0.00576)/(50) = 0.265 Ohms Further, Filter capacitor impedance at 2.8 kHz = (1000 * 1000) / ((2800/50) * 2 * TI * 50 * 12.5) = 4.547 Ohms Therefore, RMS value of the incoming voltage distortion due for switching frequency will be: = 100*(18.64 * 4.547 * 0.265 ) / (240 * sqrt (4.547^ + 0.265^) * 1.4142 ) = 1.263 %. It shows that higher the short circuit edacity of the network, lesser will be the voltage distortion. 6. The converter dc side second harmonic ripple current is expected to be very small. This is because the total ripple current is summation of contribution by each phase of the three phases, which are displaced by 120 degrees, [cos 29+ cos (471 /3 +0) + cos (8 TC /3 +6) = 0] Control scheme and list of functions in control electronics cards Control scheme The control scheme is given in fig. 6. It basically consists of two parts. • Control Logic • Control Electronics These are given in figs. 6 (a) and (b) respectively. The heart of the system is the micro-controller based Digital card assembly, which performs the following important functions. • Sequencing and interlocking of the entire ST ATCON • Solving the mathematical model given by the equations (7) and (8) and outputting the necessary three modulating signals (vcr, Vcy, Vcb) for the three phases dynamically. • Updating of all parameters (voltage, current , dc voltage, frequency, error E in equations (7) and (8) etc.) and sequencing the operation once again at the start of every cycle. • Checking for receipt of any protection signal and giving an output command for the withdrawal of IGBT gate pulses. • Checking for its own hardware health. Fig. 7 gives a flow chart, which expresses the complete sequence of operations in the STATCON as controlled by the brain 'micro-controller'. The operational sequence for the STATCON is described here. In fig. 6(b), the Relay card is an important card interfacing the Controls Electronics with tiie Control Logic and the regulated power suppUes for the Control Electronics. It receives feedbacks (three phase voltages, three phase currents, DC voltage sensor output, heat sink temperature monitor, door interlock, and control contactors' on/off conditions) and gives out commands for the operation of control and the power contactor as explained later. The Control Logic is fed fi-om a separate isolation transformer 415 / 240 V. It takes 415 input firom phases "Y" and "B". The 240V power supply input for all the regulated power supplies (+/- 15V, +/- 12V, +5V and +20 V) is also supplied from the same transformer. The Control Electronics, except for the micro-controller based digital card, uses mainly a single operational amplifier IC (TL084, Quad Opamp ) for implementing all the functions like comparator, monoshot, gain ampUfier, buffer amplifier, differentiator, zero cross detector etc. The triangular waveform is the only waveform generated by using the IC 566. Thus all fimctions, protections, signal processing, PWM generation, IGBT pulse release / suppression, level shifting, IGBT deadband generation (minimum delay to avoid simultaneous firing of two IGBT's in the same leg), sequencing and interlocking signal, etc are based on use of this IC TL084 which is the discrete and significant feature of this control electronics. The number of electronic cards (refer fig. 6(b)) used are as given below. 1. Relay card 1 no. 2. AC voltage filter card : 1 no. 3. Clamp card : 1 no. 4. Analog card : 1 no. 5. Protection card : 1 no. 6. Gate drive card : 3 nos. 7.: DC voltage sensor card : 1 no. 8. Digital card 1 no. All the analog inputs accepted by the Digital card vary fi-om 0 to +5V. Similarly, the digital inputs have 0 V as the zero level and +5 V as the one level for acceptance. The PWM modulating signals (Vcr, Vcy, Vcb) generated by the Digital card, however, vary between ± 5V. Figure 8 gives mformation on the defined signals received / delivered by the control electronics cards and flow of internal signals. List of functions in control electronics cards Relay card All the terminations for the control electronics are done on this card. It receives the power supplies (+/- 15V, +/- 12V, +5V) necessary for the Analog card. Protection card. Digital card. Gate drive cards and also gives out power supply +/-15V required for Clamp card and DC voltage sensor card. It receives feedback fi-om potential free contacts of heatsink temperature sensor (type N/C), door interlock microswitch (type N/O) and main HRC fiise blown condition also through a microswitch (type N/O). It gives outputs as OV or +15V respectively when the sensor or the switches operate. The outputs are delivered to protection card. It receives +15V commands to operate +12V dc relays whose potential free N/O contacts drive the main contactor, start contactor and the bypass (power) contactor (through an additional auxiliary contactor to drive its coil). It also receives feedback when start, stop control contactors and bypass contactor are operated. These are potential free N/O, N/0 and N/C contacts respectively. Corresponding ou^uts delivered to protection card are +15V, +15V and OV respectively. The card also receives the secondary side phase voltages (Vsm, Vsyn, Vgbn) produced by the three potential transformers (240/3 V) and filtered through the filter card. The output of the dc voltage sensor (0.4V dc corresponding to lOOV dc input), the load side Current Transformer (CT) output processed through the Clamp card and the over-current signal (obtained through three protection CT's of STATCON, one in each phase) as processed through the Clamp card are also received as inputs by the Relay card. Filter card It receives input voltages (corresponding to the three phases) from the secondary side of the potential transformers (240V/3V). These are filtered using 30 μF capacitors and then given to the Relay card as inputs. Further, these inputs go to Analog card and Protection card. Clamp card It receives the three current inputs from the star connected secondary side of the three protection CT's. These are rectified and the output signal is taken as over-current signal to protection card through the relay card. It also receives the load CT feedback for any one phase. Since the three phase STATCON draws balanced reactive current (meant for balanced reactor power compensation), single load CT feedback is sufficient. The feedback is processed through a gain stage such that SV feedback voltage corresponds to reactive power compensation of 140A peak current (to be delivered by STATCON). The load CT is considered to be of 1A secondary and hence the CT burden is chosen in such a way that at peak current of 140A in CT primary, the CT burden output is 5V. The card has a provision to reduce the compensation also. The protection CT current feedback gives actual STATCON phase current. If it crosses say 95A rms (134A peak), it actuates a +12 V dc relay and reduces the gain in the load current feedback path (adjustable up to 35%). This means the STATCON will work from 100 to 65% of the actual current based on the reduction in the gain. This feature is essential since the STATCON works on 'Indirect Current Control' principle as explained in the beginning. When the demanded current reduces, the normal (rated) current can be dravm by the STATCON. This feature is extremely useful in applications such as Windmills, where the load dynamics does not change in less than a second. Analog card This card has following functions. It processes the load current feedback (filters it for harmonics through a bandpass filter and then adjusts it from 0 to 5V with zero current level as +2.5V). The input is then delivered to digital card. This filtered sine wave is scanned at 180 degrees of the corresponding phase voltage. If it is beyond +2.5 V, the difference corresponds to peak capacitive current reference for the STATCON and vise versa. It filters the ac 3V phase voltage signals received from the Filter card and delivers a dc voltage varying between 0 to 5V corresponding to 375 V ac rms for the Digital card. It also generates a triangular waveform (carrier). The dc offset in the waveform is removed using a series c^acitor and grounded resistor. It is then amplified for peak to peak voltage of 20V. The card processes the dc voltage available from the dc sensor card. The dc voltage obtained from the dc sensor (Hall effect) gives 0.4 V corresponding to lOOV dc input. This voltage is filtered, ampUfied and trimmed with a preset in such a way that the amplifier ou^ut is -8.5 V dc when IGBT dc capacitor stack voltage reaches 850 V dc. This voltage before amplifications is also used for dc over voltage protection (input to Protection card). It is also given as an input to digital card for checking dc imder-voltage and over-voltage conditions. Reference voltage generation is also done in this card. The IGBT dc capacitors are operated at 850 V dc. The reference voltage is hence selected as +8.5 V dc. This is further split as +6.0 V and +2.5 V dc. The +6.0 V dc is generated using a buffer ampUfier and a preset. The +2.5 V dc is also generated similar way. However, it comes in operation only after controller gives a pulse release command. The card has Proportional and Integral (PI) error ampUfier. This accepts the reference dc voltage +6.0 and +2.5 V dc) and the feedback dc voltage available from the dc voltage sensor and processed as discussed above. The PI amplifier function is the most important function of the STATCON. The error ±E indicated in equations (7) and (8) is the ou^ut of this PI amplifier. The PI amplifier uses two presets and an error voltage gain stage to output the error E between 0 to 5V dc to be delivered to the digital card. This is called as PI output. It has an offset of +2.5 V dc. This differential value alone +2.5 V is recognized as capacitive mode operation and below +2.5 V is recognized as inductive mode operation by the Digital card with a scale factor of 100. There is a +12 V dc relay whose potential free contact is used for operating the PI amplifier. When the Digital card gives hold command, the relay shorts the first stage of the PI amplifier (operational error amplifier with a simple RC network connected across its input / output). The final ou^ut of the PI ampUfier is now +2.5 V dc. When the Digital card gives a release command, the relay contact opens out, the proportional integration action on the error between dc voltage reference (+6.0 V plus 2.5 V dc) and the feedback dc voltage ( -8.5 V dc) is initiated. The PI final ou^ut now starts working between OV dc to +5V dc depending upon actual STATCON current demand and its nature (capacitive or inductive). The error E holds the dc voltage of the IGBT stack dc c^acitors to +850 V dc during the required current delivery by the STATCON and answers how the dynamic value Ry (variable loss representing resistor) discussed in equations (3) to (8) is simulated. PWM waveform and IGBT pulse generation is also done in this card. The modulating signals (vcr, Vcy, Vcb) are received from the Digital card. These signals have ± 5V as the maximum amplitudes. These are amplified to ±10V before being compared with the triangular waveform. The PWM generator uses basically Sinusoidal Pulse Width Modulation. It also incorporates a dead time around 10-12 microseconds for avoiding two IGBT's in the same leg fi-om getting fired simultaneously which otherwise can lead to short-circuiting the 850 V dc charged capacitor bank and damage the IGBT's. This AND gating is done for all the three phases (legs). Fault signal gating and pulse inhibiting is an important feature of this card. The three AND gating signals as above and six IGBT short circuit signals (this is described in Protection card next) are OR gated to operate a latch. When the OR gated signal is ' 1' the latch operates and gives out a' 1' level output, which goes to the protection card. As will be seen in Protection card later, this signal is processed as +5V level signal, which gives to the Digital card as faxilt information. It also is processed as a +10 V signal, which is returned back to all the six gate drives to inhibit all IGBT pulses. The latch operates for 14 - 16 msec. If it does not see any fault input beyond this period, it delaches the ou^ut to zero level. Protection card This card generates fault ou^ut commands based on ac overcurrent (based on input received fhjm clamp card), ac overvoltage and undervoltage (based on input received from filter card through analog card) and dc overvoltage (based on input received from DC sensor card and processed through Analog card). The ac voltages are first rectified to process the information. All the three fault-processing circuits use simple comparators. Overcxurent adjustment has a present facility available to set the limit. Usually it is set for 200 A instantaneous. The other fault inputs accepted by this card (based on inputs available from Relay card) are Heatsink temperature. Door interlock and HRC fiise blown conditions. These inputs are also processed through comparators to generate the fault condition outputs. All the six faxilt condition ou^uts along with the latch output as discussed in analog card are OR gated with digital controller hold / release output and power 'ON' hold / release input. The power ON input holds the OR gate output to '1' for about 800 msec, when STATCON is switch 'ON' or when the grid fails and resumes back. This avoids any spurious IGBT firing. The ou^ut of the OR gate ('O' means release and' 1' means hold) goes to the Digital card for the controller information, to the gate drive card as discussed in Analog card and also to the PI amplifier relay in Analog card. The card also outputs certain signals at +5 V level (maximimi) to the Digital card as discussed here. First is the positive ZCD (Zero Cross Detector) which gives square wave output for positive part of the 'R' phase PT voltage Vsm) to be used for frequency measurement. Second is the negative ZCD but is presently not used. It then has positive monoshot for 'R' phase (monoshot at zero cross over of Vsm) used for cycle to cycle synchronization and three monoshots, at 180 degrees cross over for all the three phases, used for current measurement synchronization. The card processes infonnation related to operation of start command/contactor, stop command/contactor and bypass contactor. The information related to the operation of all the three (command operated / not operated) is given to Digital card at +5V level. Similarly, the +5V commands available from Digital card for operating the main, start and bypass contactors are processed in this card and outputs are delivered at +15V level to +12 V dc relays on the Relay Card. The card has following RED LED indications (LED glowing when fault occurs) • DC overvoltage (latched) • AC undervoltage (latched) • AC overvoltage (latched) • AC overcurrent (latched) • Heatsink temperature excess (latched) • Door interlock operated (latched) • HRC fiise blown (latched) (switch available for delatching the fault indications) • Pulse release by the digital card • Fault OR gate operated (pulse hold / release by the OR gate) Additional switch controlled hardware based "hold" for IGBT is also provided to check the panel sequence in 'cold' run condition. In this case the digital card releases the modulating signals and pulses. However, IGBT pulses remain in suppressed condition since the switch in ON condition gives hold command to the Gate drive cards. Gate drive card There are three Gate drive cards used in the three phase STATCON panel. Each has two channels required for top and bottom IGBT firing of each leg. The card receives following inputs and power supplies. 1. ± 12 V power supply. 2. Input PWM (0 / +10V level) for top IGBT and simUar for bottom IGBT. 3. Hold / release command generated through Protection card and buffered through Analog card for both the chaimels. 4. Isolated +20 V supply for each channel. The PWM signal is processed through a driver IC EXB 841 (Fuji make). It gets isolated in fhe IC (opto-isolation with delay less than 0.5 microseconds) and is available as +15V/-5V level waveform as the final Gate Emitter driving pulse voltage. The -5V level ensures IGBT shut off condition. When there is a short circuit current flowing through IGBT, the Vce (collector to emitter) voltage rises. It is sensed approximately at TV level and the feedback is obtained, (after opto-isolation through an IC TLP 521). This feedback is processed at +10 V level (0 / lOV) and goes to Analog card as stated earlier. It requires approximately 12 microseconds to stop the conduction of an IGBT when short circuit happens. The 12 microseconds include sensing at 7 V, opto- isolation, delivering the input to latch in Analog card, processing the same in Protection card, reprocessing of the hardware inhibit in analog card, hold to be given to all IGBT's and finally stopping the conduction of all the IGBT's properly. Additional internal hardware command is also generated after the opto-isolation to stop the concerned IGBT locally under short circuit. This is to ensure that there is no delay in withdrawing the pulses for the concerned IGBT, which has seen a short circuit condition. DC voltage sensor card The dc voltage sensor card uses an Hall Effect sensor LV-25P (open loop). It takes the 850 V dc voltage feedback fi-om the IGBT cj^acitor stack output and uses a series resistor of 82 K, 100 W for adjusting the input current and the output voltage. The power supply required is +/- 15V. Output voltage is available as 0.4 V dc / 100 V dc as the input. The sensor card output is given to Relay card, and is used in Analog card as a feedback to the PI amplifier as stated earlier. It also is given to Digital card for understanding undervoltage condition as well as it is processed in Protection card for processing the dc overvoltage condition as said earlier. Digital card This is the card where the STATCON intelligence is embedded since the core STATCON control philosophy is implemented through this card. It becomes the heart of the entire STATCON. Failure in this card brings the STATCON operation to a grinding halt. Its block diagram is given in fig. 9. To achieve the desired function, this card incorporates 16 bit Intel Micro-controller N80C196KC20 operating at 12 MHz. To achieve the desired core control function, Micro-controller is assisted by its peripherals, which occupy the balance space on the board. Some of the key peripherals used include EPROM bank of 64K space, RAM of 16K space, NVRAM space of 8k, Programmable peripheral interface, Progranmiable Timer, High speed D to A converters. Buffers for input signals and Control regulators to reference the analog section of the circuit. The card receives the power supphes (+5V, +/-12V). It also receives analog and digital inputs and gives out analog and digital outputs. It receives the TTL logic buffered signals which include Hold for the entire Gate circuit firing pulse control (active Low), Main contactor ON control (active high), START input enable (active high), Bypass contactor ON control (active high). The Unipolar, conditioned analog signals (0-5V) are received from the Analog card, which form the key control parameters for dynamic compensation. These include three 20. After reading R Phase voltage, it generates the limiting values for any subsequent voltage read for R phase as well as Y & B phase (limits: + 20%, - 30%). 21. Processor measures Y phase voltage with available Y phase voltage peak detector signal. If voltage exceeds the limiting value 323 V rms, it declares faulty condition after checking for 20 cycles and withdraws the contactor as per withdrawal procedure. 22. Processor measures B phase voltage with available B phase voltage peak detector signal. If voltage exceeds the limiting value 323 V rms, it declares faulty condition after checking for 20 cycles and withdraws the contactor as per withdrawal procedure. 23. Processor now waits for about 10 sec. 24. Processor measures DC bus voltage. If voltage is below the limiting value 400 V, it declares faulty condition after checking for 20 cycles and then withdraws contactor as per withdrawal procedure. 25. Now processor switches on the bypass contactor (Power contactor to carry STATCON current). This switching ON is about 50 sec from subsequent to START input being made available as mentioned in step 16. 26. On operation of bypass contactor, resistance in the series path of Inductor &. IGBT is bypassed in the power circuit and the DC bus voltage changes by @10volts. 27. Processor now waits for further 30 sec so as to allow stabilization of conducted noise due to sudden switch on operation of bypass contactor. 28. Processor now checks receipt of feedback on closing of the bypass contactor and ensures that bypass contactor has operated. If feedback signal is not available it declares faulty condition and withdraws the contactor as per withdrawal procedure. 29. Processor does basic house keeping i.e. update of freq., phase voltage and DC bus voltage. If any of the values are outside the limits as set earlier, it ignores and continues with the last valid value. 30. Processor now is waiting for SYNC interrupt to come. This is generated by R phase positive half cycle start monoshot, which is fed through Protection card to Digital Control card. 31. Processor remains in infinite loop if monoshot is not available. 32. On availability of SYNC interrupt. Processor starts throwing sinusoidal dummy cycles on Modulating Signal output without release of hold signal for about 100 cycles. During this dunamy cycles, it checks for presence of tripping signals viz. STOP/Bypass withdrawn/Start withdrawn signal. If it finds so, it declares faulty condition and withdraws the contactor as per withdrawal procedure. 33. Subsequent to dummy cycles. Processor releases Hold so as to facilitate generation of Gate drive pulses and switching ON of power devices, i.e. IGBT. 34. Release of hold marks beginning of active cycles with reactive current magnitude considered as ZERO for about 40 cycles. During this time when active cycles are released, it checks for presence of tripping signals viz. STOP/Bypass withdrawn/Start withdrawn/ Fault signal. If it finds so, it declares faulty condition and withdraws the contactor as per withdrawal procedure. 35. Subsequent to completion of active cycles, Processor is ready for generating demanded Reactive Compensation. Before entering into this final Core operating mode, it checks for DC bus voltage has been boosted during Active cycles i.e. beyond reference value 750V. If it finds the value below the set value, it declares faulty condition and withdraws the contactor as per withdrawal procedure. 36. Now processor enters in the final control mode of "Dynamic Statcon Current Generation Cycle", i.e. infinite loop of control by generating appropriate Modulating Signals based on available current feedback in accordance with respective phase voltage and appropriate internally generated Error Signal (PI). During this control of Core-Reactive Cycles, it checks for presence of tripping signals viz. STOP/Bypass withdrawn/Start withdrawn/ Fault signal. If it finds so, it declares faulty condition and withdraws the contactor as per withdrawal procedure Withdrawal procedure 1. Generation of Hold signal immediately on detection of fault'tripping condition 2. Waiting period of 3 sec 3. Withdrawal of start contactor 4. Waiting period of 5 sec 5. Withdrawal of Bypass contactor (If in energised condition) 6. Waiting period of 5 sec 7. Withdrawal of main contactor 8. Waiting period of 38 sec 9. Loop back to step 16 where processor waits for start Input Software realization Entire STATCON Real Time control operation is achieved through software written completely in Assembly Language (residing in the EPROMs of the Digital card), to help execute the following: (please refer fig. 7 and the operating procedure of the STATCON also).: 1. 166 usecs, is the processor operating time base with majority of the instructions getting executed @ 4 such states. 2. Frequency computation is done to the second order of decimal. It is updated at every cycle. System latches on the frequency value at the start up as reference and accepts 0.2Hz (programmmable) deviation per cycle. This ensures healthy operation and dynamic frequency updates even in noisy environment once the power stack is charged and IGBT's are fired. Respective phase voltages are computed with appropriate correction factor from the available analog signal corresponding to the peak of incoming voltage and are also computed to second order of decimal for accurate processing. These voltage updates are achieved once in a cycle for each phase. Based on start up voltage condition, dynamic limits for the phase voltage magnitudes are computed so as to prevent the system from accepting unwanted voltage magnitudes outside the limits in presence of noisy environment once the power stack is charged and the IGBT's are fired. The reactive component of current corresponding to each phase is also calculated based on the magnitude of the conditioned analog current signals available from the Analog card for each phase. For balanced loads single current feedback is adequate. However, the provision is there to accept all the three phase current feedbacks. The compensation by STATCON then can be based upon minimum OR maximum OR average of the reactive current components of the three load currents. These current updates are also done once in each cycle. However, rate of change of current effective to be offered by STATCON for compensation is progranraiable. This is an essential feature of the STATCON which is decided by an application demanded response and also to avoid excess current jerks on IGBT's as well as EMI (Electro Magnetic Interference) jerks in the given environment. Based on rate of change current value permissible, the limits for current are computed once the STATCON current value is given out starting from "zero as initial value". If the phase relationship, i.e. In case of inductive to capacitive or vice versa, gives cross over, the system forces current change through zero at the permissible rate of change. The dynamic PI value is also accepted by the controller and is mapped with appropriate magnitude while solving the control equations based on current, voltage, frequency values for each phase. The PI value is also accepted within set limits and is updated three times during each cycle. 6. The dc bus voltage also is updated once in the cycle and is used as a factor [ 2 - (dc voltage actual / dc voltage reference of 850 V) ] during capacitor voltage buildup and then as important protection parameter during the dynanwc operation of STATCON. 7. To ensure proper synchronization, 'R' phase positive zero cross over is used as the sync signal trigger and this trigger is accepted only in the last step of the 24 step calculation (i.e. between 345 to 360 degrees) and when the 24th step waveform has already been given out. Also to overcome EMI effect, if any, on receipt of the trigger signal, the physical condition of zero cross over is also verified by the microcontroller before accepting it as the valid signals. 8. System divides the entire control cycle in the form of 24 steps and at each step it updates the modulating signal values for each phase with the last updated parameters (voltage, current, PI, fi-equency). 9. Depending upon the modulating signal step number, one update or calculation related job is also executed by the micro-controller. This job can be any one of the following. • Voltage updates for R phase • Voltage updates for Y phase • Voltage updates for B phase • Current updates for R phase • Current updates for Y phase • Current updates for B phase • DC Bus voltage update • PI value update • Frequency value update • STATCON compensating current value update based on the maximum / minimum / average etc. as required by the load application logic • Current dynamic limits calculation 10. The left over time during each step is utilized for protection checks, such as, abnormal dc bus voltage, start contactor withdrawn, stop signal activated, and presence of fault signal from Protection card. Product photographs and field responses The General Arrangement of the single phase STATCON is given in fig. 21. These cover the product component and subassembly details. The field responses taken for an 1800 kVAR spot welding application are given in fig. 22. These cover the various dynamic responses of single phase STATCON. In the Dynamic Reactive Power Compensator proposed herein, various improvements and modifications are possible. For example, the converter stacks can be changed for using different IGBT's and appropriate cooling arrangement with selection of heatsink and the blower to account for same or increased KVAR from the STATCON. Different IGBT's will also mean different gate drive requirement. This is being currently attempted for by using ABB make IGBT (5SNS 0225U170100), which is a 6 IGBT based isolated base single module. A +/-15 V gate drive is separately under development for this IGBT modiile and also a different stack. Similarly, the higher kVAR ratings are also being evaluated for introduction using other make IGBT modules( Seraikron make) which also have different gate drive requirements. Further, development scope is also provided for using higher level micro-controller or a DSP. Currently a DSP (TMS320F240PQ) based control electronics is under development allowing improved specifications (response time lower than one power cycle) and enhanced features (data logging for voltage and current, event logging for fault interruptions, standard communication interfaces, mode operation of STATCON etc.). Better and powerful DSP's are also being separately evaluated for introduction. These can be introduced keeping the upward compatibility of the Digital control board with the rest of the STATCON and including necessary user / Man Machine Interfaces and fault and operational data logging. All above improvements and modifications to follow will be applicable for three phase as well as single phase STATCON. SINGLE PHASE STATCON Power scheme and component selection The single phase STATCON has following basic specifications. • 240 V (± 10%), ± 50 Hz (± 5%), ± 210A. • Incoming supply as 2 wire (phase and neutral). • All the internal power suppUes to be derived from the incoming supply only. • Dynamic response time close to one cycle. The power scheme is given in fig. 14. It uses a single phase full bridge construction for the power converter with two converters operating in parallel. Each power converter produces terminal voltage Vi, as can be seen from fig. 16. Fimdamental component of this voltage is v ii, as can be seen fi-om the same figure. Each power switch (in this case IGBT, i.e. Insulated Gate Bi-polar Transistor) is bi-directional. The converter is called as Current-controlled Voltage Source Converter operating in boost mode. In this case input current is, is controlled indirectly by controlling the voltage Vii. As such, this current control method is called as 'Indirect C\irrent Control (ICC)". The dc voltage Vdc needs to be more than the peak of supply phase to neutral voltage so that the supply current can be forced in both the directions. This requirement gives the converter it's above defined name. Various formulae to be used in the component selection of the converter and their relevance 1. Maximum Vii required for capacitive operation = (240 * 1.15) + (21012) * (wLb) Note that Lb is used in a split form as is seen from the power scheme in fig. 15 . Further, the current is equally shared by the two converters as 105 A which is equal to 210 / 2 A. 2. Minimum Vii required for inductive operation = (240 * 0.8) - (210/2) * (wLb) Design factor is 15% over-voltage and 20% under-voltage. 3. Peak ripple in the supply current at 50% duty ratio of the terminal voltage v i switching between the levels + Vdc and 0 & - Vac and 0 is given by 2*(2 * Vdc) / (K * (2*mf) * wLb) where mf = ratio of switching frequency of the IGBT devices to supply frequency. Further, the factor 2 in the beginning is because of two converters operating in parallel and the factor 2 in bracket along with mf is because of frequency doubling effect in a full wave converter. Thus the carrier frequency and the IGBT switching frequency are same but the input current of each converter will have ripple frequency which is double of the IGBT switching frequency or the carrier frequency. 4. The converter equation (2) given previously can be rewritten as Capacitive mode (11) Inductive mode (12) or C^acitive mode (13) Inductive mode Vii= (Vn, - Icm *wLb) sin wt + (E) cos wt (14) Herelcm = 105 * V2 A = 149 A This is to be considered for single converter and not for two converters in parallel. Error E decides the angle '5' in fig. 2 or the angular displacement of voltage Vii from the supply voltage. The switching voltage Vi and hence the Vii voltage is produced by using comparison of a triangular wave with a fundamental frequency signal' Vc' as shown in fig. 16. The converter thus uses Sinusoidal Pulse Width Modulation (SPWM) method for producing the switching voltage vi. The ratio of amplitude of Vc to the amplitude of the triangular waveform is called as modulation index (Mi). It should normally be below 1.0. Since the loss component Icm * Rv or E in equations (13) and (14) is quite small, the displacement angle '5' of Vii is also quite small. These are the equations the digital card micro-controller has to solve on a continuous basis. The STATCON basic fimctioning is based on these equations. With only Vc considered superimposed on the triangular waveform, the Viivoltge is given by (15) The linear relation between Mi and Vii is valid for mf (switching frequency to fundamental frequency ratio) greater than nine. Converter component selection The converter component selection is an interactive process based on • Formulae given above. • Various IGBT devices available and their characteristics. • Proving the power stack for requisite rating integrating the devices, the snubber components, dc capacitors and the forced cooling etc. • Switching frequency choice related to above. • Integrated protection approach for the IGBT turn off within less than 12 microseconds. • Digital card developed around 80196 micro controller and its clock frequency, and • Few other parameters. The component and parameters selected are as under (please see power scheme in fig. 14 and fig. 21 also) Components 1. IGBT 200 A, 1400 V (Fuji make 2MBI200PB -140,2 in one with isolated base). 2. DC capacitors 4700 [xF, 450 V dc used for the IGBT stack. 3. Single phase, boost reactor L b as 2.5 mH, 150 A ( or 1.25 mH 2nos. for split arrangement pa* converter). 4. Snubber - Diode 3 A, 2000 V, type UF 5408,2 groups in series with each having 4 in parallel Resistor 11 ohms, 100 W Capacitor 0.1 ^iF, 2000V dc. The snubber is connected across each device. Further, there is also 0.33 \iF, 2000 V dc capacitor connected across the dc terminals of each IGBT module. 5. Blower - Single phase, 240 V, 500 cubic feet / min. 6. Heatsink - AFCOSET 80 AD (845 H * 126 W * 136 D) anodized. 7. DC cap. discharge resistor - 15 K, 25 W. 8. Main incoming breaker MCB or Switch Fuse Unit SFU - 400 A, 240V AC. 9. Main contactor - 415 V, 40 A, 3 pole with 240 V ac coil (all three poles paralleled). 10. Bypass contactor - 415 V, 150 A, 3 pole with 240 V ac coil (all three poles paralleled). 11. Pre-charging resistor -10 Ohms, 200 W. 12. Ripple filter capacitor - 10.0|iF, 2000 V dc (common for two converters). 13. Incoming R-C filter - 5 Ohms, 100 W and 4 ^iF, 660 V. 14. Incoming smoothening inductor 20 (xH, 250 A (single phase). 15. Incoming surge energy absorption. Diode 70A, 2000 V (2 modules with 2 in one with isolated base or 4 nos. separate) Current limiting resistor 27 ohms, lOOW DC capacitor 1000 ^F, 450 V dc, 2 nos. Discharging resistor 27 K, 100 W. All of these are mounted on a small anodized heatsink (300 H * 150 W * 60 D ) 16. Heatsink temperature sensor type N/C operating at 90 degrees. 17. HRC fiise in supply lines rating 400 A, 500 V 18. Protection CT combined for both the converters rating 300 A /1 A. 19. Control / Isolation transformer for feeding the Control Logic and power supplies for Control Electronics. This is 240 V / 240 V, with 3 kVA rating. 20. One niunber of control transformer for the piirpose of sensing incoming phase voltages. These are 240 / 3 V, with 6 VA rating each. Parameters 1. Digital processor N80 C 196 KC 20 (16 bit) operated at 12 MHz. 2. Switching frequency of IGBT (carrier frequency) 1.5 kHz. 3. IGBT stack cooling. Forced cooled with specified blower. 4. Modulating waveform optimized to 24 pulse (15 degrees per step) In each 15 degrees, the controller computes the modulating signal level based on fresh information of all the parameters. 5. In each cycle (before its completion) all information related to voltages, current, frequency and associated operating limits (dynamic) are updated. Correct synchronization is maintained. Thus, every cycle resynchronization with updated parameters takes place. However, the response of the converter is just a little over one cycle (less than 1 cycle and 15 degrees). Results based on iterations 1. The converter dc voltage has been finally selected as 600 V dc. The switching frequency selected is 1.5 kHz for the IGBT's. 2. Input current ripple (peak), considering both converters (parallel converters) = 2 * (2 * 600) / (7t * (2*1500/50) * 2 * TI * 50 * 2.5 x 10"^ ) = 16.2 A 3. Vii variation required (per phase) (240 * 1.1 + 105 * 2.5 * 10-^ * 2 * 7t * 50) = 346 V (240 * 0.8 - 105 * 2.5 * lO"^ * 2 * 7i * 50) = 110 V Thus the variation is from 346 V to 110 V AC. At 600V dc, it means modulation index M, varying from from (346*^2)/(600) = 0.815 to (110*V2)/(600)) = 0.26 The restricted modulation index Mi to a maximxun of 0.8IS allows the modulating signal Vc to remain lower than the triangular waveform amplitude and leaves some margin to accoimt for dynamics of the load. 4. Expected displacement angle Consider fig. 2 (a). The maximum Ic * wUdrop for single converter (105 * 2 * n * 50 * 2.5 * 10'^) 82.4 volts Assume a worst case Ic * Rv drop of 10% of this nature Thusic * Rv = 8.24 volts Thus maximum '6' in capacitive mode is given by tan -^ (8,24 / (240 x 1.1 + 82.4) = 1.364 degrees Similarly maximum '5' in inductive mode is given by tan ~' (8.24 / (240 x 0.8 - 82.4) = 4.28 degrees. It shows that the displacement angle of the vector Vii with respect to the phase voltage Vs is quite small but is essential for the converter operation. This is in line with what was stated earlier while introducing equations (11) to (14). 5. Incoming voltage distortion (calculated for a typical short circuit capacity of the network) Assume that at the coupling point of STATCON the network short edacity is say 30000 KVA. Refer fig. 16 for the voltage distortion analysis. Short circuit impedance (w * Lb) = (3 * 240 * 240) / (30000 * 1000) = 0.00576 Ohms Therefore, impedance at switching firequency = (3000 * 0.00576)7(50) = 0.3456 Ohms Further, Filter capacitor impedance at 3.0 KHz = (1000 * 1000) / ( (3000/50) * 2 * 7t * 50 * 12.5) = 4.265 Ohms Therefore, RMS value of the incoming voltage distortion due for switching frequency will be = 100*(16.2 * 4.265 * 0.3456 ) / (240 * sqrt (4.265^ + 0.3456^) * 1.4142 ) = 0.38%. It shows that higher the short circuit capacity of the network, lesser will be the voltage distortion. 6. The peak second harmonic ripple current on the dc side is as given here. = [1.1 * Phase voltage * Phase current / DC voltage) = (1.1*240*210/600) =92.4 A. Control scheme and list of functions in control electronics cards. Control scheme The control scheme is given in fig. 17. It basically consists of two parts. • Control Logic • Control Electronics These are given in figs. 17 (a) and (b) respectively. The heart of the system is the micro-controller based Digital card assembly, which performs the following important functions. • Sequencing and interlocking of the entire STATCON • Solving the mathematical model given by the equations (13) and (14) and outputting the necessary modulating signals Vc, for the three phases dynamically. • Updating of all parameters (voltage, current, dc voltage, frequency, error E in equations (13) and (14) etc.) and sequencing the operation once again at the start of every cycle. • Checking for receipt of any protection signal and giving an output commnd for the withdrawal of IGBT gate pulses. • Checking for its own hardware health. Fig. 18 gives a flow chart, which expresses the complete sequence of operations in the STATCON as controlled by the brain 'micro-controller'. The operational sequence for the single phase STATCON is explained here. In fig. 17(b), the Relay card is an important card interfacing the controls electronics with the control logic and the regulated power supplies for the control electronics. It receives feedbacks (the phase voltage, the phase current, DC voltage sensor output, heat sink temperature monitor, door interlock, and control contactors' on/off conditions) and gives out commands for the operation of control and the power contactor as explained later. The control logic is fed from a separate isolation transformer 240 / 240 V. The 240V power supply input for all the regulated power supplies (+/- 15 V, +/- 12V, +5V and +20 V) is also suppUed from the same transformer. The Control Electronics except, for the micro-controller based Digital card, uses mainly a single operational amplifier IC (TL084, Quad Opamp ) for implementing all the functions like comparator, monoshot, gain amplifier, buffer amplifier, differentiator, zero cross detector etc. The triangular waveform is the only waveform generated by using the IC 566. Thus all functions, protections, signal processing, PWM generation, IGBT pulse release / suppression, level shifting, IGBT deadband generation (minimum delay to avoid simultaneous firing of two IGBT's in the same leg), sequencing and interlocking signal. etc are based on use of this IC TL084 which is the discrete and significant feature of this control electronics. The number of electronic cards (refer fig. 17(b)) used are as given below. 1. Relay card : 1 no. 2. AC voltage filter card : 1 no. 3. Clamp card : 1 no. 4. Analog card 1 no. 5. Protection card : 1 no. 6. Gate drive card : 4nos. 7.: DC voltage sensor card : 1 no. 8. Digital card : 1 no. All the analog inputs accepted by the digital card vary from 0 to +5V. Similarly, the digital inputs have 0 V as the zero level and +5 V as the one level for acceptance. The PWM modulating signal Vc generated by the Digital card, however, varies between ± 5 V. Figure 19 gives information on the defined signals received / delivered by the control electronics cards and fiow of internal signals. List of functions in control electronics cards Relay card All the terminations for the control electronics are done on this card. It receives the power supplies (+/- 15V, +/- 12V, +5V) necessary for the analog card, protection card, digital card, gate drive cards and also gives out power supply +/-15V required for clamp card and DC voltage sensor card. It receives feedback from potential free contacts of heatsink temperature sensor (type N/C), door interlock micro-switch (type N/0) and main HRC fuse blown condition also through a micro-switch (type N/0). It gives outputs as OV or +15V respectively when the sensor or the switches operate. The outputs are delivered to protection card. It receives +15V commands to operate +12V dc relays whose potential free N/O contacts drive the main contactor, start contactor and the bypass (power) contactor (through an additional auxiliary contactor to drive its coil). It also receives feedback when start, stop control contactors and bypass contactor are operated. These are potential free N/O, N/O and N/C contacts respectively. Corresponding outputs delivered to protection card are +15V, +15V and OV respectively. The card also receives the secondary side phase voltage Vsn produced by the potential transformer (240/3 V) and filtered through the filter card. The output of the dc voltage sensor (0.4V dc corresponding to 100V dc input), the load side Current Transformer (CT) output processed through the Clamp card and the over-current signal (obtained through the protection CT's of STATCON, one in each phase of the converter) as processed through the Clamp card are also received as inputs by the Relay card. Filter card It receives input voltage from the secondary side of the potential transformer (240V/3V). This is filtered using 30 ^iF capacitors and then given to the Relay card as input. Further, this input goes to Analog card and Protection card. Clamp card It receives the current input from the combined protection CT. This is rectified and the output signal is taken as over-current signal to protection card through the relay card. It also receives the load CT feedback. The feedback is processed through a gain stage such that 5V feedback voltage corresponds to reactive power- compensation of 330 A peak current (to be delivered by STATCON). The load CT is considered to be of lA secondary and hence the CT burden is chosen in such a way that at peak current of 330 A in CT primary, the CT burden output is 5V. The card has a provision to reduce the compensation also. The protection CT current feedback gives actual STATCON phase current. If it crosses say 240 A rms (340 A peak), it actuates a +12 V dc relay and reduces the gain in the load current feedback path (adjustable up to 35%). This means the STATCON will work from 100 to 65% of the actual current based on the reduction in gain. This feature is essential since the STATCON works on 'Indirect Current Control' principle as explained in the beginning. When the demanded current reduces, the normal (rated) current can be drawn by the STATCON. This feature is extremely useful in many applications where this dynamics does not change in less than a second. Analog card This card has following functions. It processes the load current feedback (filters it for harmonics through a bandpass filter and then adjusts it from 0 to 5V with zero current level as +2.5 V). The input is then delivered to digital card. This filtered sine wave is scanned at 180 degrees of the incoming phase voltage. If it is beyond +2.5 V, the difference corresponds to peak capacitive current reference for the STATCON and vise versa It should be noted that the actual converter current reference is half of the measured current since there are two converters operating in parallel. It filters the ac 3V phase voltage signal received from the Filter card and delivers a dc voltage varying between 0 to 5V corresponding to 375 V ac rms for the Digital card. It also generates a triangular waveform (carrier). The dc offset in the waveform is removed using a series capacitor and grounded resistor. It is then amplified for peak-to-peak voltage of 20V. The card processes the dc voltage available from the dc sensor card. The dc voltage obtained from the dc sensor (Hall effect) gives 0.4 V corresponding to lOOV dc input. This voltage is filtered, amplified and trimmed with a preset in such a way that the amplifier output is -6.0 V dc when IGBT dc capacitor stack voltage reaches 600 V dc. This voltage before amplifications is also used for dc over voltage protection (input to Protection card). It is also given as an input to digital card for checking dc under voltage and over voltage conditions. Reference voltage generation is also done in this card. The IGBT dc capacitors are operated at 600 V dc. The reference voltage is hence selected as +6.0 V dc. This is further split as +4.0 V and +2.0 V dc. The +4.0 V dc is generated using a buffer amplifier and a preset. The +2.0 V dc is also generated similar way. However, it comes in operation only after controller gives a pulse release command. The card has Proportional and Integral (PI) error amplifier. This accepts the reference dc voltage +4.0 and +2.0 V dc) and the feedback dc voltage available from the dc voltage sensor and processed as discussed above. The PI amplifier function is the most important function of the STATCON. The error +E indicated in equations (13) and (14) is the output of this PI amplifier. The PI amplifier uses two presets and an error voltage gain stage to output the error E between 0 to 5V dc to be delivered to the digital card. This is called as PI ou^ut. It has an offset of +2.5 V dc. This differential value alone +2.5 V is recognized as c^acitive mode operation and below +2.5 V is recognized as inductive mode operation by the Digital card with a scale factor of 100. There is a +12 V dc relay whose potential free contact is used for operating the PI amphfier. When the Digital card gives hold command, the relay shorts the first stage of the PI amplifier (operational error amplifier with a simple RC network connected across its input- output). The final output of the PI ampUfier is now +2.5 V dc. When the Digital card gives a release command, the relay contact opens out, the proportional integration action on the error between dc voltage reference (+4.0 V plus 2.0 V dc) and the feedback dc voltage (-6.0 V dc) is initiated. The PI final output now starts working between OV dc to +5V do depending upon actual STATCON current demand and its nature (capacitive or inductive). The error E holds the dc voltage of the IGBT stack dc capacitors to +600 V dc during the required current delivery by the STATCON and answers how the dynamic value Rv (variable loss representing resistor) discussed in equations (11) to (14) is simulated. PWM waveform and IGBT pulse generation is also done in this card. The modulating signal Vc is received from the Digital card. This signal has ± 5V as the maximum amplitude. This is amplified to ±10V before being compared with the triangular waveform. The PWM generator uses basically Sinusoidal Pulse Width Modvdation. It also incorporates a dead time around 10-12 microseconds for avoiding two IGBT's in the same leg from getting fired simultaneously which otherwise can lead to short-circuiting the 600 V dc charged capacitor bank and damage the IGBT's. This AND gating is done for both the legs of the converter. Fault signal gating and pulse inhibiting is an important feature of this card. The three AND gating signals as above and six IGBT short circuit signals (this is described in Protection card next) are OR gated to operate a latch. When the OR gated signal is ' 1' the latch operates and gives out a' 1' level output, which goes to the Protection card. As will be seen in Protection card later, this signal is processed as +5V level signal, which gives to the Digital card as fault information. It also is processed as a +10 V signal, which is returned back to all the six gate drives to inhibit all IGBT pulses. The latch operates for 14 - 16 msec. If it does not see any fault input beyond this period, it detaches the output to zero level. Protection card This card generates fault output commands based on ac over current (based on input received fi'om clamp card), ac overvoltage and undervoltage (based on input received from filter card through analog card) and dc overvoltage (based on input received from DC sensor card and processed through Analog card). The ac voltages are first rectified to process the information. All the three fault- processing circuits use simple comparators. Overcurrent adjustment has a present facility available to set the limit. Usually it is set for 3 80 A instantaneous. The other fault inputs accepted by this card (based on inputs available from Relay card) are Heatsink temperature, Door interlock and HRC fuse blown conditions. These inputs are also processed through comparators to generate the fault condition outputs. All the six fault condition outputs along with the latch output as discussed in analog card are OR gated with digital controller hold / release output and power 'ON' hold / release input. The power ON input holds the OR gate output to ' 1' for about 800 msec, when STATCON is switch 'ON' or when the grid fails and resumes back. This avoids any spurious IGBT firing. The output of the OR gate ('0' means release and '1' means hold) goes to the Digital card for the controller information, to the Gate Drive card as discussed in Analog card and also to the PI amplifier relay in Analog card. The card also ou^uts certain signals at +5 V level (maximum) to the Digital card as discussed here. First is the positive ZCD (Zero Cross Detector) which gives square wave output for positive part of the phase and the PT voltage Vsn to be used for frequency measurement. Second is the negative ZCD but is presently not used. It then has positive monoshot for the phase voltage (monoshot at zero cross over of Vsn) used for cycle to cycle synchronization and three monoshots, at 180 degrees cross over for all the three phases, used for current measurement synchronization. The card processes information related to operation of start command/contactor, stop command / contactor and bypass contactor. The information related to the operation of all the three (command operated / not operated) is given to Digital card at +5V level. Similarly, the +5V commands available from Digital card for operating the main, start and bypass contactors are processed in this card and outputs are delivered at +15V level to +12 V dc relays on the Relay Card. The card has following RED LED indications (LED glowing when fault occurs) • DC overvoltage (latched) • AC undervoltage (latched) • AC overvoltage (latched) • AC overcurrent (latched) • Heatsink temperature excess (latched) • Door interlock operated (latched) • HRC fuse blown (latched) (switch available for delatching the fault indications) • Pulse release by the digital card • Fault OR gate operated (pulse hold / release by the OR gate) Additional switch controlled hardware based "hold" for IGBT is also provided to check the panel sequence in 'cold' run condition. In this case the digital card releases the modulatmg signals and pulses. However, IGBT pulses remain in suppressed condition since the switch in ON condition gives hold command to the Gate Drive cards. Gate drive card There are four Gate drive cards used in the single phase STATCON panel. Each has two channels required for top and bottom IGBT firing of each leg. The card receives following inputs and power supplies. 1. ± 12 V power supply 2. hiput PWM (0 / +10V level) for top IGBT and similar for bottom IGBT 3. Hold / release command generated through Protection card and buffered through Analog card for both the channels 4. Isolated +20 V supply for each channel The PWM signal is processed through a driver IC EXB 841 (Fuji make). It gets isolated in the IC (opto isolation with delay less than 0.5 microseconds) and is available as +15V/-5V level waveform as the final Gate Emitter driving pulse voltage. The -5V level ensures IGBT shut off condition. When there is a short circuit current flowing through IGBT, the Vce (collector to emitter) voltage rises. It is sensed approximately at TV level and the feedback is obtained, (after opto-isolation through an IC TLP 521). This feedback is processed at +10 V level (0 / lOV) and goes to Analog card as stated earlier. It requires approximately 12 microseconds to stop the conduction of an IGBT when short circuit happens. The 12 microseconds include sensing at 7 V, opto- isolation, delivering the input to latch in Analog card, processing the same in Protection card, reprocessing of the hardware inhibit in analog card, hold to be given to all IGBT's and finally stopping the conduction of all the IGBT's properly. Additional internal hardware command is also generated after the opto-isolation to stop the concerned IGBT locally under short circuit. This is to ensure that there is no delay in withdrawing the pulses for the concerned IGBT which has seen a short circuit condition. DC voltage sensor card The dc voltage sensor card uses an Hall Effect sensor LV-25P (open loop). It takes the 850 V dc voltage feedback from the IGBT capacitor stack output and uses a series resistor of 82 K, 100 W for adjusting the input current and the output voltage. The power supply required is +/- 15V. Output voltage is available as 0.4 V dc / 100 V dc as the input. The sensor card output is given to Relay card, and is used in Analog card as a feedback to the PI amplifier as stated earlier. It also is given to Digital card for understanding under voltage condition as well as it is processed in Protection card for processing the dc over voltage condition as said earlier. Digital card This is the card where the STATCON intelligence is embedded since the core STATCON control philosophy is implemented through this card. It becomes the heart of the entire STATCON. Failure in this card brings the STATCON operation to a grinding halt. Its block diagram is given in fig. 20. To achieve the desired function, this card incorporates 16 bit Intel Micro-controller N80C196KC20 operating at 12 MHz. To achieve the desired core control function, Micro-controller is assisted by its peripherals, which occupy the balance space on the board. Some of the key peripherals used include EPROM bank of 64K space, RAM of 16K space, NVRAM space of 8k, Programmable peripheral interface, Programmable Timer, High speed D to A converters. Buffers for input signals and Control regulators to reference the analog section of the circuit. The card receives the power supplies (+5V, +/-12V). It also receives analog and digital inputs and gives out analog and digital outputs. It receives the TTL logic buffered signals which include Hold for the entire Gate circuit firing pulse control (active Low), Main contactor ON control (active high), START input enable (active high). Bypass contactor ON control (active high). The Unipolar, conditioned analog signals (0-5V) are received fi-om the Analog card, which form the key control parameters for dynamic compensation. These include the input phase voltage, the phase load current, dc bus voltage feedback and PI control value as discussed earlier in analog card. The Digital status / control signals at TTL logic levels received by the card are buffered internally before being fed to core control circuit. This includes start contactor feedback contact. Bypass contactor feedback contact, stop command from USER through front panel pushbutton, zero crossing detector signal for dynamic frequency calculation, master fault input from the protection card, synchronizing signal corresponding to the phase voltage positive zero cross over, and negative zero cross over signal for the phase voltage for facilitating the current measurement so as to derive corresponding phase reactive current component This is as discussed in Analog and Protection cards. The final output (modulating signal) is in the form of dynamically computed 24 step waveform for the phase with +/- 5V as peak amplitude in real time with appropriate phase relationship and in close synchronization with its positive zero cross over. This signal is received by the Analog card for further processing through Sinusoidal PWM generation hardware to deliver the gate pulses to the IGBT's. The inverse modulating signal also required for PWM generation is separately obtained through hardware in Analog card itself Operating sequence of single phase STATCON panel 1. Connect the single phase 240 V supply input (phase and neutral) with earth connection to the panel. Source capacity should be at least 400 A. 2. Keep IGBT gate control switch in appropriate ON/OFF condition (as desired during testing). 3. Now panel incoming power source can switched "ON". 4. Panel incoming MCB to be switched ON. 5. Power supply LED's for 5V,+/-12V, +/-15V, as well as for Gate drive power supply will glow. 6. Processor (micro-controller) initializes. 7. Hold is generated by processor in @6 usec after power supply availability during panel switch ON. 8. Main contactor is turned ON by processor after @120 secs, after panel being switched ON. 9. DC bus gets charged to @ 340 V dc. 10. Protection card LED's are now to be RESET using respective toggle switch in the Protection card. 11. Two LEDs ( L8 and L9) on Protection card glow indicating presence of Hold signal from processor and hold being generated by Protection card. 12. Red LEDs on Gate drive card also glow indicating presence of Hold on gate drive outputs. 13. Meantime processor checks health of RAM on the Digital card. 14. Then processor checks health of NVRAM on the Digital card. 15. Processor generates internal start command, which operates corresponding relay on the Relay card (This signal is generated after about 3.3sec subsequent to main contactor being turned ON). 16. Processor waits for START INPUT (given through a push button in the panel) to go to next step. 17. On receipt of start input. Processor executes series of control actions without any user interaction in healthy condition of the panel. 18. Processor measures frequency through available zero crossing signal. If frequency is not found within 45 to 55Hz for next 4 cycles, it declares faulty condition and withdraws the contactor as per withdrawal procedure. 19. Processor measures the phase voltage with available phase voltage peak detector signal. If voltage exceeds the limiting value 323 V rms, it declares faulty condition after checking for 20 cycles and withdraws the contactor as per withdrawal procedure. 20. After reading the phase voltage, it generates the limiting values for any subsequent voltage read for the phase voltage (Limits: + 20%, - 30%). 21. Processor now waits for about 10 sec. 22. Processor measures DC bus voltage. If voltage is below the limiting value 240 V, it declares faulty condition after checking for 20 cycles and then withdraws contactor as per withdrawal procedure. 23. Now processor switches on the bypass contactor (Power contactor to carry STATCON current). This switching ON is about 50 sec from subsequent to START input being made available as mentioned in step 16. 24. On operation of bypass contactor, resistance in the series path of Inductor & IGBT is bypassed in the power circuit and the DC bus voltage changes by @10volts. 25. Processor now waits for further 30 sec so as to allow stabilization of conducted noise due to sudden switch on operation of bypass contactor. 26. Processor now checks receipt of feedback on closing of the bypass contactor and ensures that bypass contactor has operated. If feedback signal is not available it declares faulty condition and withdraws the contactor as per withdrawal procedure. 27. Processor does basic house keeping i.e. update of freq., phase voltage and DC bus voltage. If any of the values are outside the limits as set earlier, it ignores and continues with the last valid value. 28. Processor now is waiting for SYNC interrupt to come. This is generated by the phase positive half cycle start monoshot, which is fed through Protection card to Digital card. 29. Processor remains in infinite loop if mono-shot is not available. 30. On availability of SYNC interrupt. Processor starts throwing sinusoidal dummy cycles on Modulating Signal output without release of hold signal for about 100 cycles. During this dummy cycles, it checks for presence of tripping signals viz. STOP/Bypass withdrawn/Start withdrawn signal. If it finds so, it declares faulty condition and withdraws the contactor as per withdrawal procedure. 31. Subsequent to dummy cycles, Processor releases Hold so as to facilitate generation of Gate drive pulses and switching ON of power devices, i.e. IGBT. 32. Release of hold marks beginning of active cycles with reactive current magnitude considered as ZERO for about 40 cycles. During this time when active cycles are released, it checks for presence of tripping signals viz. STOP/Bypass withdrawn/Start withdrawn/ Fault signal. If it finds so, it declares faulty condition and withdraws the contactor as per withdrawal procedure. 33. Subsequent to completion of active cycles, Processor is ready for generating demanded Reactive Compensation. Before entering into this final Core operating mode, it checks for DC bus voltage has been boosted during Active cycles i.e. beyond reference value 440V. If it finds the value below the set value, it declares faulty condition and withdraws the contactor as per withdrawal procedure. 34. Now processor enters in the final control mode of "Dynamic Current Generation Cycle", i.e. infinite loop of control by generating appropriate Modulating Signals based on available current feedback in accordance with respective phase voltage and appropriate internally generated Error Signal (PI). During this control of Core-Reactive Cycles, it checks for presence of tripping signals viz. STOP/Bypass withdrawn/Start withdrawn/ Fault signal. If it finds so, it declares faulty condition and withdraws the contactor as per withdrawal procedure. Withdrawal procedure 1. Generation of Hold signal immediately on detection of fault/tripping condition 2. Waiting period of 3 sec 3. Withdrawal of start contactor 4. Waiting period of 5 sec 5. Withdrawal of Bypass contactor (If in energised condition) 6. Waiting period of 5 sec 7. Withdrawal of main contactor 8. Waiting period of 38 sec 9. Loop back to step 16 where processor waits for start Input. Software realization Entire STATCON Real Time control operation is achieved through software written completely in Assembly Language (residing in the EPROMs of the Digital card) to help execute the following ( please refer fig. 18 and operating procedure of the STATCON also). 1. 166 nsecs. is the processor operating time base with majority of the instructions getting executed @ 4 such states. 2. Frequency computation is done to the second order of decimal. It is updated at every cycle. System latches on the frequency value at the start up as reference and accepts 0.2 Hz (programmable) deviation per cycle. This ensures healthy operation and dynamic frequency updates even in noisy enviromment once the power stack is charged and IGBT's are fired. 3 Phase voltage is computed with appropriate correction factor from the available analog signal corresponding to the peak of incoming voltage and is also computed to second order of decimal for accurate processing. The voltage update is achieved once in a cycle for each phase. Based on start up voltage condition, dynamic limits for the phase voltage magnitude is computed so as to prevent the system from accepting unwanted voltage magnitudes outside the limits in presence of noisy enviromnent once the power stack is charged and the IGBT's are fired. 4. The reactive component of current of the phase is also calculated based on the magnitude of the conditioned analog current signal available from the Analog card. This current update is also done once in each cycle. However, rate of change of current effectively to be offered by STATCON for compensation is programmable. This is an essential feature of the STATCON, which is decided by an application demanded response and also to avoid excess current jerks on IGBT's as well as EMI (Electro Magnetic Interference) jerks in the given environment. Based on rate of change current value permissible, the limits for current are computed once the STATCON current value is given out starting from "zero as initial value". If the phase relationship, i.e. In case of inductive to capacitive or vice versa, gives cross over, the system forces current change through zero at the permissible rate of change. 5. The dynamic PI value is also accepted by the controller and is mapped with appropriate magnitude while solving the control equations based on current, voltage, and frequency values for each phase. The PI value is also accepted within set limits and is updated three times during each cycle. 6. The dc bus voltage also is updated once in the cycle and is used as a factor [2 - (dc voltage actual / dc voltage reference of 600 V)] during capacitor voltage buildup and then as important protection parameter during the dynamic operation of STATCON. 7. To ensure proper synchronization, the phase positive zero cross over is used as the sync signal trigger and this trigger is accepted only in the last step of the 24 step calculation (i.e. between 345 to 360 degrees) and when the 24th step waveform has Depending upon the modulating signal step number, one update or already been given out. Also to overcome EMI effect, if any, on receipt of the trigger signal, the physical condition of zero cross OVER is also verified by the micro-controller before accepting it as the valid signals. 8. System divides the entire control cycle in the form of 24 steps and at each step it updates the modulating signal values for each phase with the last updated parameters (voltage, current, PI, frequency). 9. Depending upon the modulating signal step number, one update or calculation related job is also executed by the micro-controller. This job can be any one of the following. • Voltage update for the phase • Curent update for the phase • DC Bus voltage update • PI value update • Frequency value update • STATCON compensating current value update based on the maximum / minimum / average etc. as required by the load plication logic. • Current dynamic limits calculation. 10. The left over time during each step is utilized for protection checks, such as, abnormal dc bus voltage, start contactor withdrawn, stop signal activated, and presence of fault signal from Protection card. Product photographs and field responses The General Arrangement of the single phase STATCON is given in fig. 21. These cover the product component and subassembly details. The field responses taken for an 1800 kVAR spot welding application are given in fig. 22. These cover the various dynamic responses of single phase STATCON. In the Dynamic Reactive Power Compensator proposed herein, various improvements and modifications are possible. For example, the converter stacks can be changed for using different IGBT's and appropriate cooling arrangement with selection of heatsink and the blower to account for same or increased KVAR from the STATCON. Different IGBT's will also mean different gate drive requirement. This is being currently attempted for by using ABB make IGBT (5SNS 0225U170100), which is a 6 IGBT based isolated base single module. A +/-15 V gate drive is separately under development for this IGBT module and also a different stack. Similarly, the higher kVAR ratings are also being evaluated for introduction using other make IGBT modules (Semikron make), which also have different gate drive requirements. Further, development scope is also provided for using higher level micro-controller or a DSP. Currently a DSP (TMS320F240PQ) based control electronics is under development allowing improved specifications (response time lower than one power cycle) and enhanced features (data logging for voltage and current, event logging for fault interruptions, standard communication interfaces, mode operation of STATCON etc.). Better and powerful DSP's are also being separately evaluated for introduction. These can be introduced keeping the upward compatibility of the Digital control board with the rest of the STATCON and including necessary user / Man Machine Interfaces and fault and operational data logging. All above improvements and modifications to follow will be applicable for three phase as well as single phase STATCON. WE CLAIM; 1. A control system for a real time dynamic reactive power compensator comprising: a. control logic having relays and contactors for sequencing and interlocking of all the commands, required for the proper operation of the compensator and as received from control electronics; b. said control electronics consists of a 16 bit micro-controller based Digital card (with necessary set of software instructions written in assembly language and residing in EPROMs based on the flowcharts described) wherein the controller works at 12 MHz, said Digital card comprising means and microcontroller intelligence used for interlocking and sequencing the compensator operation through control logic, for solving the mathematical model such as herein described, ou^utting the necessary modulating signals, updating of all parameters i.e. voltage, current, dc voltage, frequency, error E in equations described herein above and sequencing the operation once again at the start of every cycle, checking for receipt of fault signal (available as an completely ORgated fault signal for all the faults in the compensator), giving an output command for maintaining and withdrawal of IGBT gate pulses and also checking and maintaining the health of its own hardware; the said control electronics also having other control cards such as: c. a relay card for receiving and relaying of various controls and used for interfacing the control electronics with control logic and to deliver the regulated power supplies to the control electronics. d. a filter card to receive the input voltages from the potential transformer, filter the same using capacitors and relay it to the relay card as inputs, e. a clamp card to receive the three phase or single phase current inputs (for the three phase or single phase operation) from a current transformer, rectify the same and then pass the output signal as an over-cvirrent signal to a protection card through the said relay card, f. an analog card having necessary circuits for processing the load current feed back, input voltage obtained through step down Potential transformer, the dc voltage feedback from the converter dc voltage sensor, dc voltage referencing, the dc voltage loop PI amplifier, to be delivered as the processed inputs to the Digital card; and also having triangular / carrier waveform generator and processing for the modulating signals received from the Digital card to deliver appropriate PWM signals for the IGBT's, g. a protection card for generating the fault output commands in response to AC overcurrent (based on input received from clamp card), ac overvoltage and undervoltage (based on input received from filter card through analog card) and dc overvoltage (based on input received from DC sensor card and processed through Analog card), door interlock, fuse failure and heatsink temperature excess (based on inputs received through the relay card), and also generatmg the final ORgated fault output for the Digital card / microcontroller, receiving back pulse stop output from the same and delivering it to the gate drive cards through the analog card, to stop the IGBT pulses and enter the stop sequence mode. h. gate drive cards for accepting the IGBT device firing / triggering signals from the analog card, isolating these signals and delivering them to the IGBT Gates, generating the signal for overcurrent / short circuit current flowing in the IGBT converter stack, isolating this signal and the delivering it to analog card for further processing as a fault input from Gate drive, i. a DC voltage sensor card for sensing the dc voltage of the IGBT converter stack through a sensor for required closed loop operation of the stack and the Voltage Source Converter at its respective dc voltage. 2. The control system as claimed in claim 1, wherein the micro-controller based Digital card includes EPROM bank of 64K space, RAM of 16K space, NVRAM space of 8k, Programmable peripheral interface. Programmable Timer, High speed D to A converters, Buffers for input signals and Control regulators to reference the analog section of the circuit. 3. The control system as claimed in claim 1, except for the micro-controller based Digital card, uses mainly one type of a quad operational amplifier (TL084) for implementing all functions like comparator, monoshot, gain amplifier, buffer amphfier, differentiator, zero cross detector and others. 4. The control system as claimed in claim 1, wherein the relay card is designed to receive all the power supplies necessary for the analog card, protection card, digital card, gate drive cards and also gives out power supply required for clamp card and DC voltage sensor card. 5. The control system as claimed in claim 1, wherein the real time control logic, is fed from a separate isolation transformer, consisting of a start push button which delivers the start command through a start contactor to the Digital card and the microcontroller in response delivers commands to operate a start contactor, a main contactor and a bypass contactor, a stop push button which delivers the stop command through a stop contactor in a similar way as the start command and to stop the IGBT pulses and withdraw bypass contactor (while using an auxiliary contactor for operating the bypass contactor coil). 6. Real time Dynamic Reactive Power Compensator based on "Indirect Current Control" principle • working in close loop with the DC voltage control of the converter (no separate dc voltage excitation for the converter) and properly accounting for the variable power loss / loss resistance of the converter / compensator • working with 24 pulse modulating signal produced by a 16 bit microcontroller (operated at 12 MHz frequency) to control the required Pulse Width Modulation (PWM) of the reflected DC voltage on the ac terminals of the converter • working with programmable tracking response (up to a limit of maximxim current correction per cycle) for dynamic reactive power compensation of the load and within compensator capacities, achieved through the use of 16 bit microcontroller • working with every power cycle synchronization • working with current controlled boost type IGBT based converter developing synchronized and proper fundamental opposition voltage to the supply voltage through the PWM process and drawing the necessary reactive current from the supply • working with two converters fully paralleled with no isolation on DC side as well as on AC side (in case of single phase version of the compensator) • working with specified 1.5 kHz / 3.0 kHz switching frequency of the IGBT power device fuing in case of single / three phase of the compensator respectively , to achieve close to sinusoidal input current drawn from the corresponding ac supply • working with specified 600 / 850 V dc voltage for the converter in case of single / three phase compensator respectively • working in either capacitive or inductive mode at any given time offering stepless / smooth dynamic reactive power compensation as commanded by the load CT feedback and within the capacity of the compensator • working with sets of equations (7), (8), (10) and (13), (14), (15) for the proper single phase and three phase operation on per cycle basis and where the error "E" is obtained through the output of a single Proportional and Integral (PI) controller and havmg two types as under. • Three phase, 415V, +/-85 A (capacitive or inductive) • Single phase, 240V, +/- 2 lOA (capacitive or inductive) 7. A Dynamic Reactive Power Compensator, as claimed in 1, for single phase and three phase operation utilizing a single phase or three phase power circuits / power supply and comprising: i) An MCB , HRC Fuse (s) and a smoothening reactor in the incommg power supply line ii) an incoming surge energy absorbing rectifier, iii) an incoming surge suppressor network, iv) means for incoming ac voltage and the dc capacitor (stack) voltage sensing. v) a transformer for the power supplies to relay and contactor based Control Logic and various electronic cards forming the control electronics and a micro-controller based Digital card assembly, vi) a main contactor, vii) a by-pass contactor, viii) a pre-charging resistor, ix) a switching current ripple filter capacitor for each phase, x) protection CT's for each phase, xi) a single phase or three phase boost inductor/reactor, xii) Insulated Gate Bi-polar Transistor (IGBT) power device based converters also having R-C-D snubber, additional snubber capacitor for each IGBT module (two IGBT's in series in one leg of the converter) connected directly across the dc Bus, suitable heatsink to accommodate the IGBT modules and snubber resistors and a blower for the force cooling of the heatsink to deliver the required power xiii) a DC capacitor bank and discharge resistors for the said power switch/insulated gate bipolar transistor stack, xiv) Control Electronics, as claimed in claims 2 to 5, to execute necessary mmiber of analog and digital functions xv) Control Logic, as claimed in claims 2 and 6, which provides sequencing and interlocking of the entire compensator based on commands received from a micro-controller based Digital card (with embedded software residing in EPROMs dictating micro-controller ftmctioning)

Documents:

0207-che-2003 drawings.pdf

0207-che-2003 others.pdf

0207-che-2003 abstract.pdf

0207-che-2003 claims duplicate.pdf

0207-che-2003 claims.pdf

0207-che-2003 correspondence others.pdf

0207-che-2003 correspondence po.pdf

0207-che-2003 description (complete) duplicate.pdf

0207-che-2003 description (complete).pdf

0207-che-2003 description (provisional).pdf

0207-che-2003 form-1.pdf

0207-che-2003 form-13.pdf

0207-che-2003 form-19.pdf

0207-che-2003 form-26.pdf

0207-che-2003 form-3.pdf

0207-che-2003 form-5.pdf