|Title of Invention
" AN ON - LINE (H.V.) COUPLER FOR MONITORING OF ELECTRICAL DISCHARGES IN LARGE TURBO-GENERATORS"
|The invention relates to an On-line H.V. coupler for monitoring of electrical discharges in large turbo-generators in order to maintain high operational stability. The system is based on the following important features. (i) A HV coupler has been provided to tap the PD/RF signals from the HV terminal of a generator in service. (ii) A resistor divider circuit is provided to bring down the AC voltage level. The frequency response of the measuring cirucuit is found to be flat upto 30 MHz. (iii) A protection circuit has been provided to protect the measuring system and the operating personnel against possible switching/power frequency over-voltages. (iv) An electrical by-pass circuit is provided to avoid the damage of protection circuit by the fault current. (v) A suitable control circuit is provided for the safe operation of the system. (vi) Theoretical simulation of the generator system faults.
|The invention relates to an on-line High Voltage (H.V) coupler for monitoring of electrical discharges in large turbo-generators.
In recent years, generators of higher power rating are being introduced to meet the fast increase of power demand. The reliability of such machines is very important, as an unplanned outage of a machine leads to loss of revenue and also high repair cost. Insulation system in a generator is an important component, which needs to be maintained with high reliability. The deterioration of insulation in service is mainly caused by temperature. In large machines, forced hydrogen cooling system is employed to maintain the temperature. However, there may be hot sports in the insulation system where the temperature is more than the environment. This is due to the combined effect of prevailing thermal environment and electrical discharges. These discharges occur in insulation due to (i) presence of gas pocket (ii) absence of conducting paint on the outer surface of insulation in the slot region and (iii) conductor fatigue and breakage. The discharges increase the temperature at a spot, leading to local thermal breakdown and ultimate insulation failure. Hence, it is necessary to monitor the level of such discharges, generally known as 'partial discharges' (PD). Though the effect of partial discharges is depicted by the conventional off-line electrical measurements viz. tanδ, discharge energy, on-line PD detection and measurement
becomes absolute necessary especially for large machines in order to maintain high operational reliability.
A simple method for on-line monitoring of discharge is to provide an impedance or a high frequency current transformer at the neutral path of the generator. The measurement can be made in the form of time-based pulses or measuring the Radio Frequency Spectrum (RF Spectrum). In general, the magnitude of time based pulses is depending upon the actual discharges. However, because of amplifier bandwidth limitation there is always a cut, off frequency, beyond which the sensitivity of measurement decreases. Recently many manufacturers use amplifiers with very high bandwidth (Up to 300 MHz). In case of RF spectrum a wide range of frequency can be covered and hence the sensitivity of measurement is improved.
There are some advantages with the present system of monitoring of discharges.
One of the main disadvantages is that even though the neutral current measurement is able to detect "partial discharges', it cannot locate from which phase the "PD1 originates.
Therefore the main object of the present invention is to install an on-line H.V. coupler for monitoring of electrical discharges for turbo-generators.
Another object of the present invention of an on-line H.V. coupler for monitoring of electrical discharges in large turbo-generators is to provide a low voltage arm (resistance)
across which partial discharge signals can be tapped out for measurement.
Yet another object of the present invention of an on¬line HV coupler for monitoring of electrical discharges in large turbo generators is to provide a necessary scheme to protect the system and the operating personnel in the event of the failure of the surge capacitor.
Still another object of the present invention of an on¬line HV coupler for monitoring of electrical discharges in large turbo generators is to provide control system required for the effective functioning of the system.
According to the present invention there is provided an on-line High Voltage (H.V.) coupler for monitoring of electrical discharges in large turbo-generators comprising a Low Voltage (L.V.) arm in the form of a non-inductive resistor connected between surge capacitor and ground, a resistance divider circuit connected to a protection circuit for protection against overvoltage and a control circuit for on/off control and protection against failure output of said resistance divider circuit is connected to the measuring system.
In the aforesaid on-line high voltage (HV) coupler, said control circuit is provided with a main relay Rl with coupler "on" switch with Hold-on, (ii) "off" switch and
The nature of the invent ion,its objective and further advantages residing in the same will be apparent from the following description made with reference from exemplary embodiments of the invention represented in the accompanying drawings, wherein
Figs*. 1 to 3 shows the theoretically computed fault current by simultating the systemm on a computer.
Fig.4 shows the detailed circuit diagram of HV coupler.
SUMMARY OF THE PRESENT INVENTION;
The HV coupler mainly consists of a non-inductive resistor (1 ) (LV arm) which is connected in between the LV terminal of the surge capacitor (2), available at the bus duct and the ground. This resistor is used as a measuring impedance. Any signal that arises due to the presence of electrical discharge inside the generator (3) can be tapped across the impedance for measurement. This is a basic system of signal measurement. However, the basic circuit requires further components to bring automation and safety for measuring instrument and personnel so as to achieve a total system, which is the aim of the present invention. The total system is referred here as the HV coupler. Various components of HV coupler are described below in respect of the features and operation.
(i) Resistance divider circuit
The purpose of providing a resistance divider circuit (4) is to reduce the voltage level, both AC (50Hz) component and discharge signal since the AC voltage across the LV arm is sufficiently high that a measuring equipment cannot take it. Normally, the value of LV arm is chosen as 50 ohms to match the surge impedance of the measuring circuit cables. When the resistance divider circuit (4) is connected across the LV arm, (1 ) the output voltage level can be matched to the requirement of measuring equipment. Though the signal
magnitude also gets reduced, it can be amplified inside the measuring equipment, while the 50 Hz component of the voltage is rejected by a filter provided at the input of the measuring equipment. Basically, each stage of the resistance divider circuit consists of two resistors in series. The input is given across the series combination of these resistors, while the output is taken across the lower resistor of the last stage. The values of resistors determine the output voltage level. In the present case, the output voltage level of the resistance divider circuit is maintained as 3V AC. Also, the resistors have a low inductance value (less than 100 nano Henry) so that they can respond to very high frequency signals.
(ii) Protection circuit
The HV coupler as a whole is connected to the surge capacitor (2). The capacitor in turn connected to the generator bus (5) which is at a system voltage of 15.75 kv. In view of this connection, there is probability that an abnormal voltage can appear at the HV coupler terminal during service. This abnormal voltage can be detrimental to both the measuring system and the safety of the operating personnel. Hence, there is a need to provide a protection system.
Firstly, a transferred impulse voltage from the generator transformer (6) can appear at the bus ( .5). A major part of
this voltage* may appear across the LV arm (1). To reduce the severity of the voltage, a gas filled lighting arrester (7) is connected across the LV arm. The arrester clips the impulse voltage at the desired level (90V across LV arm) which can be taken by the measuring circuit (?1). In the present coupler, a Transabsorber ( 150V Clipping) (8) is also connected across the arrester (7) and this acts as a second line protection. When the impulse voltage strikes the LV arm (1), the arrester (7) starts conducting and the current is limited by the impedance of the surge capacitor (2). Since, the duration of the impulse voltage is around 50 micro-sec., the arrester (7) can easily take up the current (short duration current rating for a 8/20 micro second current wave=20KA peak).
In the second case, the system voltage can appear across the LV arm (1), when the surge capacitor (2) fails. In view of this, the arrester (7) operates and the current flows through the arrester (7) is of the order of few amperes which depends upon generator neutral circuit parameters. This current has to be carried by the arrester (7.) till the generator earth fault relay operates. Since the AC (50 Hz) current rating of the arrester (7) is only 20 amperes/sec, a by-pass circuit has been designed and incorporated. During failure the fault current is sensed by a current transformer (9) and the current signal is converted into a voltage signal. This voltage signal is compared with a reference voltage in a comparator (10). Once, the fault current exceed the
pre-set value, a relay in the comparator (10) operates which in-turn activates relay R2 (11) and subsequently contactor R3 ( 12). This contactor closes and by-passes the fault current to ground.
The switching ON and OFF of the HV coupler is effected by a relay R1 (13) which also has a "hold on" circuit and ON & OFF indicators When the Relay R1 , ( 13) is switched on, it connects the measuring system to the output of the resistance divider network.
Relay R2 (11) is operated to switch off relay R1(13) and thereby switches off the HV coupler operation. This can act only during the fault conditions. The operation of R2 (11) makes the contactor R3 (12) to by-pass the fault current.
The HV coupler is normally by-passed by a mechanical switch (14). This switch is opened during measurements. The supply requirement for HV coupler is 230V AC, 50 Hz, single phase.
DESIGN OF PROTECTION SYSTEM;
The LV arm shown in Figure 4 alongwith a resistance
divider circuit (4) when connected to a surge capacitor,
(0.25 micro Farad) draws a current of 700mA for a system
voltage of 15.75 kV. Whenever the measurement is not required, the LV arm (4) is by - passed by a mechanical switch (14). By this operation, the LV terminal of the surge capacitor (2) is directly connected to ground. During the measurement,the mechanical switch (14) is opened so that the LV arm (1) comes in series with the surge capacitor (2).
Though the operation of this system appears to be simple, sufficient protection has to be provided for the safe operation against the abnormal over-voltages. A transferred surge may emerge from the generator-transformer and enter into the generator terminal. The severity of this surge can be reduced by the combined action of the surge capacitor (2) and the lighting arrester (15), available in the LA cubicle. However, during initial condition, the surge capacitor (2) offers least resistance for the surge so that almost the surge appears across the LV arm (4). The magnitude of surge is around 100kV for a 15.75kV system. It is therefore necessary to clip this voltage to a lower level in order to protect the measurement system and the operating personnel. Hence, a gas filled lightning arrester/GDT (7) with a sparkover voltage of magnitude 2 to 5 times the normal voltage is connected across the LV arm (1). The type of arrester fixed is depending upon the maximum voltage allowed across the measuring terminal, spark-over voltage and the V-I characteristics of the arrester. The current carrying capacity of the present arrester (7) is 20KA for an 8/20 micro-sec, current wave and 20 Amps., 50Hz for 1 sec. duration.
arrester (7) is further backed up by a 150 Volts Trans-absorber (8). Whenever the surge voltage appears across the 50 ohms resistor (LV arm), the arrester starts conducting and limiting the over-voltage. Also, the current through the arrester is limited by the surge capacitor (2). After spark over, the arrester (7) recovers during the first current zero of the power frequency follow current. The second type is the power frequency over-voltage that can appear across the LV arm (1) due to the following reasons.
(i) In case, the connection to ground is disconnected, the system voltage appear at the terminal of LV arm. (ii) If there is a failure of surge capacitor (short circuit), the system voltage almost appears across the resistor.
When the above events occur, the arrester (7) acts immediately, thus protecting the entire measuring system. The current through the arrester (7) is limited by the surge capacitor (2) in the first case, whereas by the resistive load ( 16) of the neutral grounding transformer (17) in the second case. The secondary winding of the neutral grounding transformer ( 17) is short-circuited by a high voltage resistor ( 16), so that during the faiure of surge capacitor (2), the energy produced is dissipated by this resistor. This type of protection does not allow the generator to supply the fault beyond certain level i.e. 4.12A peak, in the present
case. In the meantime, generator earth fault relay operates and trips the system.
This fault current is theoretically computed by simulating the entire generator system by EMTP (Electro Magnetic Transient (Programme) on a computer. The results of the analysis is shown in Figs. 1 to 3.However, the arrester () can not take up the fault current for a longer time because of its short-time rating. A special feature has been made in the present design to by-pass the short circuit current in the arrester (7) through a contact switch ( 18). In this case, an air-cored current transformer (CT) (9) senses the fault current. The secondary current of this transformer is converted into a voltage signal. This signal is compared to a pre-determined voltage, computed based on the fault current level, by a comparator ( 10). When the CT secondary voltage exceeds the preset voltage level during a fault, the comparator output activates its relay, which in turn operates a main contactor R3 (12) and thereby connecting the LV arm (1) to ground. The fault current is then allowed to flow directly to ground till the fault is cleared and thereby protecting the arrestor (7) from failure due to over-current. The system developed is designated as HV Coupler and its circuit diagram is shown in Fig. 4.
DESIGN OF HV COUPLER CONTROLS
The main relay (R1 ) is provided with (i) coupler "ON"switch with 'Hold On1, (ii) 'OFF1 switch and (iii) ON/OFF indicators. When this relay is off, the measurement system is disconnected from the resistance divider (4) output and
connected to ground. The output switch of the comparator is connected in such a way that during a fault, the comparator output activities the relay R2, which in turn switches off relay R1 . The relay R2 also activates relay R3 that bypasses the current in arrester directly to ground.
TESTS CONDUCTED ON THE HV COUPLER
(i) The entire coupler system consisting of the surge capacitor, the LV arm, protection and controls have been successfully tested by applying an AC high voltage of 15.75KV/ 3 for hours duration. The operation of the mechanical switch was monitored for any possible spark-over.
(ii) A high voltage of 15.75 KV/ √3 was suddenly applied across the protection unit and limiting the current to the required level of 2.9A (4.12/ √2) by suitable resistance network simulating the effect of the neutral grounding transformer. The comparator output was set for this current so that when fault current exceeds 2.9A, relay R3 (12) operates.
(iii) Frequency response to the resistance divider upto 300 MHz was done.
The invention described hereinabove is in relation to non-limiting embodiments and as defined by the accompanying
1. An on-line High Voltage (H.V.) coupler for monitoring of electrical discharges in large turbo-generators comprising a Low Voltage (L.V.) arm in the form of a non-inductive resistor connected between surge capacitor and ground (1), a resistance divider circuit (4) connected to a protection circuit (19) for protection against overvoltage and a control circuit (20) for can/off control and protection against failure output of said resistance divider circuit is connected to the measuring system (21) .
2. An on-line High Voltage (H.V.) coupler as claimed in Claim I wherein said control circuit (20) is provided with a main
relay Rl (13) with (1) coupler 'on' switch with 'Hold On', (ii) 'off' switch and (iii) On/Off indicators.
3. An on-line High (H.V.) coupler as claimed in Claim 2 wherein when said relay is 'off' position the measurement system gets disconnected from three stage resistance divider (4) and connected to ground.
4. An on-line High Voltage (H.V.) coupler as claimed in Claim: 2 or 3
wherein during fault, the output relay of the comparator (10) activates the
relay R2(ll) to switch off said relay Rl(13).
5. An on-line High Voltage (H.V.) coupler as claimed in Claim 4
wherein said relay R2(l 1) activate the relay R3 (12) to bypass the current
in the arrester (7) directly to ground.
6. An on-line High Voltage (H.V.) coupler for monitoring of electrical
discharge in large turbo-generators as herein described and illustrated in
the accompanying drawings.
|Indian Patent Application Number
|PG Journal Number
|Date of Filing
|Name of Patentee
|BHARAT HEAVY ELECTRICALS LTD
|BHEL HOUSE, SIRI FORT, NEW DELHI, 110 049, INDIA
|PCT International Classification Number
|PCT International Application Number
|PCT International Filing date