Title of Invention | AN IMPEDANCE BASED METHOD FOR REAL TIME DETECTION OF LOCKED ROTOR CONDITION OF A MOTOR PRIOR TO ELAPSE OF MOTOR STARTING TIME |
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Abstract | The invention relates to an independence based method for real time detection of locked rotor condition of a motor prior to elapse of motor starting time in a system, the system comprising a power supply module (1) providing overload, short-circuit and input reversal protections; a microprocessor based module (2) having in-built digital input, analog input, digital output, analog output, a static memory, a static RAM, and a flash memory, the module having atleast two serial ports; a relay algorithm incorporated in the microprocessor based module (2); a voltage and current transducer module (3) connected to the relay module, the transducer module having three each voltage transformers and current transformers for stepping down input voltage and field current respectively; a six channel of second order filters (4) for correction of amplitude and phase of the analog signals, a numerical relay module (5) comprising a plurality of relays having NO (normally open) contacts, the method comprising the steps of : processing the field inputs and filtering high frequency noise, and transmitting the signals to the microprocessor based module; initializing the variables and setting an interrupt timer; checking whether the phase current is greater than I start value, if not detecting presence or absence of faults relating to overcurrent, overvoltage, undervoltage, negative phase sequence, thermal overload; comparing the current with a highest instantaneous value, and issuing a trip signal if the value is greater; checking the undervoltage and then the loss of phase, and allotting a phase accumulation if the loss of phase is present; determining the impedance in each phase and monitoring time Tstart duration if the impedance is within the boundary of mho characteristic; resetting the variables if the impedance value is beyond the mho characteristic boundary; and monitoring the impedance value when reset for each phase, if a value in respect of rate of change of the ratio of reactance value over resistance is decreasing, if not (d X /R)/dt decreasing, tripping the breaker. |
Full Text | FIELD OF INVENTION The invention relates to a method of determining a locked rotor condition of electric motors, in particular large electric motors with a view to provide locked motor protection. More particularly, the invention relates to an improved based method for real time detection of locked rotor condition of a motor prior to elapse of motor starting time incorporated in a numerical motor protection relay in a system, the system comprising: BACKGROUND OF THE INVENTION In general, numerical motor protection relays with multifunction features are well known. A few related examples can be found in the earlier patents as mentioned below. a) US Patent no. 4,757,242 dt. 12/07/88. Title: Microprocessor based motor protective relay with half wave sampling. This invention discusses about the utilization of the sensed values by half wave sampling and extrapolating the full wave to calculate the positive and negative symmetrical components from which the effective current or RMS current on a cycle-by-cycle basis can be determined. b) US Patent no.4,743,818 dt. 10.5.88. Title : Microprocessor based motor protective relay with rotor temperature detector. In this invention, the temperature of the rotating rotor is inferred from the current flowing in the lines which supplies the motor and from resistance temperature detection devices in the stator of the motor. c) US Patent no.4,743,816 dt. 10/5/88. Title : Microprocessor based motor protective relay with transition control. In accordance with this invention, a microprocessor based control system samples the three phase input lines to a motor system at a relatively high rate and determines when a motor 'start' has occurred by inferring the start from the presence of a predetermined set amount of electrical current in the lines. The apparatus monitors the line current from the time when it crosses 15% of full load current (i.e. 'motor start) to 5 or 6 line cycles, and then checks if the current has fallen below 105% of full load current, which activates the voltage starter to add voltage so that the amount of voltage which is placed across the winding of the motor reaches full terminal voltage. If the current does not fall below 105% in 5 or 6 cycles, the microprocessor automatically trips the motor. A second feedback loop provides a signal from the reduced voltage motor starter which indicative that the motor starter contacts have been closed. In the event that the motor starter contacts have not been closed within a reasonable amount of time, it is presumed that there is failure in the reduced voltage starter and the relay orders a trip. Even though this method may determine if the motor is stalled during start, but then an external signal from the reduced voltage motor starter has to be monitored to check if the motor started contacts have been closed. In situations where no external signal is provided, this method would fail to detect the locked rotor condition. d) US Patent no.4,713,718 dt. 15/12/87. Title : Microprocessor based motor protective relay with maximum number of starts per time interval limiter. This invention deals with a motor protection relay, which is programmable to limit the number of starts that are allowed in a given interval of time. In accordance with the algorithm associated with the relay microprocessor for controlling the starts per time, old starts which no longer fall within the same time constraint set by the operator or user are discarded and the time constraint is updated according to the next oldest start time. A moving time window is provided which uses as its base in each case the last oldest non-timed out start time. This invention discusses about the timer feature in the relay and not about the detection of the locked rotor condition of the motor. OBJECTS OF THE INVENTION It is therefore, an object of the invention to provide a method for locked rotor detection by means of a numerical motor protection relay which eliminates the disadvantages of the prior art. Another object of the invention is to provide a method for locked rotor detection by means of a numerical motor protection relay which provides additional protections necessary for large electric motors for example, three phase over-current and earth fault, over/under voltage, thermal overload and negative phase sequence. A further object of the invention is to provide a method for locked rotor detection by means of a numerical motor protection relay which distinguishes the healthy start of a motor based on determination of current and voltage parameters thereby eliminating the need of a mechanical speed switch vis-a-vis larger frame size of the motor. A still further object of the invention is to provide a method for locked rotor detection by means of a numerical motor protection relay which is capable of sensing a faulty start of the motor when it fails to accelerate because of locked rotor condition. Yet another object of the invention is to provide a method for locked rotor detection by means of a numerical motor protection relay which constitutes an extension of the impedance method monitoring, in particular, monitoring of the rate of change of the ratio of reactance over resistance. Still another object of the invention is to provide a method for locked rotor detection by means of a numerical motor protection relay which is a cost effective and functionally reliable. SUMARY OF THE INVENTION Accordingly there is provided an independence based method for real time detection of locked rotor condition of a motor prior to elapse of motor starting time incorporated in a numerical motor protection relay, in a system, the system comprising, a power supply module providing overload, short-circuit and input reversal protections, a microprocessor based module having in-built digital input, analog input, digital output, analog output, a static memory, a static RAM, and a flash memory, the module having atleast two serial ports; a relay algorithm incorporated in the microprocessor based module ; a voltage and current transducer module connected to the relay module, the transducer module having three each voltage transformers and current transformers for stepping down input voltage and field current respectively; a six channel of second order filters for correction of amplitude and phase of the analog signals, a numerical relay module comprising a plurality of relays having NO (normally open) contacts, the method comprising the steps of processing the field inputs and filtering high frequency noise, and transmitting the signals to the microprocessor based module; initializing the variables and setting an interrupt timer; checking whether the phase current is greater than I start value, if not detecting presence or absence of faults relating to overcurrent, overvoltage, undervoltage, negative phase sequence, thermal overload; comparing the current with a highest instantaneous value, and issuing a trip signal if the value is greater; checking the undervoltage and then the loss of phase, and allotting a phase accumulation if the loss of phase is present; determining the impedance in each phase and monitoring time Tstart duration if the impedance is within the boundary of mho characteristic; resetting the variables if the impedance value is beyond the mho characteristic boundary; and monitoring the impedance value when reset for each phase, if a value in respect of rate of change of the ratio of reactance value over resistance (d X /R)/dt is decreasing, if not decreasing, tripping the breaker. dt. The invention proposes an additional technique over the impedance based detection of the locked motor condition, in particular where the safe locked motor time is less than the motor starting time and in the cases where mechanical speed switches are not present. Fig.3 depicts the block diagram of the relay hardware; Fig.4 shows the various software modules incorporated in the relay; Fig.5 shows a flowchart of performing the process of the present invention; Fig.6 illustrates PRIOR ART on impedance based detection; Figs. 7A and 7B show the variation of the ratio of reactance over resistance with reference to speed; Fig. 8 shows the test setup to test the relay; Figs. 9A to 9C show the relay performance characteristics; BRIEF DESCRIPTION OF THE INVENTION Referring now to Fig.l, the numerical motor protection relay incorporating the present invention is a low cost and size effective multifunction numeric relay, configured for the protection of medium and large rated induction motors. The relay is capable of handling the protection requirements of motors, and works based on three phase voltages and currents. Current based protections are the over-current, earth-fault, negative phase sequence and thermal overload functions. Voltage based protections are over-voltage and under-voltage functions. The locked rotor protection is based on the principle of impedance measurement using the three phase voltages and currents. One other deterministic feature is the monitoring of the rate of change of the ratio of reactance to resistance to enable prior information much before the start time if the motor is failing to accelerate. The numerical motor protection relay, can sense the healthy start of the motor with special emphasis on locked rotor protection. Locked rotor protection based on impedance method and the rate of change of reactance over resistance has been incorporated. The other protection functions developed are three phase over-current, earth-fault, over and under voltage, negative sequence protection and thermal overload. The relay has a front panel with keypad and display. Software functions have been built in to gather motor data that can be used for monitoring, data logging and trouble shooting functions. The numerical motor protection relay of the invention is configured to perform: - Locked rotor protection : Impedance based protection for the motor during starting, specifically against stalled rotor conditions. This function is responsive to voltage, current, and the phase angle between them. It has an mho characteristic, a separate circle for each setting of the relay, and the relay can be used to detect a change in impedance to verify rotor rotation. The method for locked rotor protection comprises gathering information before the time Tstart has elapsed, if the motor is beginning to accelerate or whether it has stalled, the following steps are carried out:- (i) checking for a phase current greater value than hiset set value if it is so, tripping the breaker. (ii) Checking for an under voltage, if it is true, exit (iii) Checking for an impedance in each phase, if it is inside the mho characteristic boundary. If yes, monitoring the time Tstart duration. If it continues for the Tstart duration, tripping the breaker. If the impedance has come out, reset the variables and exit. (iv) If step (ii) is true, monitoring d/dt (X/R) for each phase. dt. This value should be decreasing i.e. negative, if the motor is accelerating monitor this value for 500 msec duration. If this value is not decreasing then trip the breaker. Otherwise wait for the Tstart duration and step (iv) will trip the breaker. -Thermal Overload : Provides protection against motor running overload by a thermal replica element, without the necessity to embed RTDs in the motor stator windings. Negative sequence heating is included in the thermal protection algorithm, and is added to the normal heating effects to provide an overall heating from all sources to the rotor and stator. The thermal image takes into account both the heating and cooling time constants. -Hiset Instantaneous & IDMT Phase Overcurrent : Provided to limit damage from phase faults. -Hiset Instantaneous & IDMT Earthfault : The earth fault protecdtion scheme with the zero sequence component of the three phase currents. - Phase-current Unbalance : Provided to detect an open-phase or motor winding to winding short circuit, and usually includes a negative-sequence overcurrent as well. This relay responds to the negative-sequence component of the phase currents with the inverse -time characteristic. - Overvoltage & Undervoltage : Three phase definite time voltage function. - Frequency of starts (66): Provided to alarm or trip when the number of attempted starts within a given time frame exceeds a set number. - Maximum start time (48) : Provides protection for incomplete start sequence. - Time taken to start, Istart(max), Vstart(min), Tstart(max) over the period, number of starts. The program continuously monitors to determine the status of the associated motor in real time. This provides information related to the running / stopped conditions and whether any alarms or trips are issued by the relay. All the set parameters and status can be read, such as root- mean-square values of the three phase voltage, current, phase, earth- fault current, negative sequence current, frequency, three phase resistance and reactance values. The keypad provides access for reading the parameter values set in the relay, to modify the settings, read the event/fault record etc. The sequence of event record feature marks the time and date of significant events. It allows for the recording of all trips and alarms of the motor in addition to historical data, such as, number of starts, time taken to start etc. There is also a provision for printing the stored values over the serial RS232 port. This relay senses the healthy start of the motor from the current and voltage parameters and this obviates the need for the mechanical speed switch. The method used to distinguish the healthy start from a faulty one is more reliable. Figure 2 shows the relay connected to a motor for monitoring the current and voltages in real-time. The auxiliary d.c. supply to the relay is given to terminal blocks 1 & 2 of TB3. The potential free contacts outputs of the relay is available in terminal blocks 1-12 of TB2 and 3-4 of TB3. The field inputs viz. the three phase currents are connected in star mode to terminal blocks 7-12 of TB1. The three phase voltages are connected to the terminal blocks 1-6 of TB1. The RS-232 port is available in the rear of the relay which can be connected to a PC for PC based MMI, or to a printer to print the event & fault logs. The signals from the field, viz. phase currents and voltages, are processed in real-time by the system. When a fault occurs, the relay senses it and issues a command in the form of an annunciation or a trip, depending upon the severity of the fault. Relay status is indicated by means of lamps provided on the front side of the panel. In addition, alarms and trip contacts are brought out in the form of potential-free contacts, which can, in turn, be used for either operating the circuit breaker or to drive audio/visual alarm. The contacts and lamps are of self - resetting type. Fig. 3 shows a block diagram of the hardware of the system. The relay hardware comprises of 5 modules. Module no. 1 is a power supply module which takes in either 220V/110V/48V/30V dc voltage and multi outputs are 24Vdc of 2A, + 15 Vdc of 250 MA, -15V of 250 mA. The module has overload, short circuit & input reversal protections. Module no. 2 is a single board computer (SBC) BL2120, which is a high performance module comprising of RABBIT microprocessor (H) operating at 22.1 Mhz for first data processing. It has built-in digital input (I), analog input (converter) E. digital output F, analog output G, 128 static RAM A, 250K flash memory B. Module no. 3 is a voltage and current transducer module which comprises of 3 voltage transformer (VTs) and 3 current transfoemers (CTs). Each voltage transformer (VT) steps down the field input of 110V (phase-phase voltage) into %Vpeak a.c. voltage. Each current transformer (CT) meets the 5P20 class specification, and it steps down 1A of field current into 500 mV peak voltage. The transformers (VT,CT) serve the dual purpose of stepping down the field signals to a lower in linear mode and also provide galvanic isolation. Module no. 4 comprises a 6 channels of second order chebyshev filters along with amplitude and phase correction for the analog signals. Module no. 5 is a relay module consisting of eleven members of relays with NO (normally open) contacts. These contacts (NO) have the capability to either operate the circuit breaker or to drive audio or visual alarm. Contact rating : 10 A max, resistive load at 30Vdc, 250 Vac. Coil voltage : 24 Vdc Coil resistance : 1100 ohms. FUNCTION OF THE MODULES AND THEIR INTERCONNECTTVTTY The auxilliary D.C. voltage given to the relay is first stepped down to 24Vdc in module No. 1. This voltage energises module no. 2 and module no. 5. The +15Vdc and -15 Vdc is used to energise module no. 4. The field inputs viz three phase current and the three voltages (described in Fig. 2), connected to the relay terminal blocks, is first processed by module no. 3. The stepped down signals are then filtered by module no. 4 for nay high frequency disturbances. These signals are then sent to section E of module no. 2. The module no 2 has the relay algorithm residing in section B. When the module is powered the program control first digitizes the analog data in section E and process the data checking for any fault. The final relay outputs are sent through section F to the module no. 5. This module with its eleven number of No contacts have a higher driving capability and these potential free contacts are brought out to the relay terminal blocks (described in Fig. 2). The module no. 2 also has two serial ports RS232-(section D) through which the metered data is communicated to the operator interface on the front of the relay and also to the PC-based MMI (Man Machine Interface) program. It can also read the set parameters from the operator interface. The flowchart of the process as shown in Fig.5 describes the real-time data acquisition and processing. Each analog signal is sampled at the rate of 12 samples per cycle, and the root-mean-square values of the three- phase voltages, currents, earth-fault current, negative sequence current, frequency, three-phase resistance and reactance values are computed. When the current is found to be more than "Irated", it indicates that the motor has started. When the current exceeds "Ihiset", or when the broken conductor is sensed by absence of current in any one or two phases, the relay issues a trip command. "Zinit" and "Zphi" determine the locus of impedance. The relay is set to pick up each time the motor is started and resets as the motor accelerates, as shown in Fig. 6. Additionally the present invention teaches monitoring of the ratio of reactance over resistance and check continually if it is decreasing. When the relay senses that the rate of change in the gradient of X/R (i.e. reactance over resistance) is not adequate and that the impedance is within the zone for the duration more than "Tstart", i.e., the motor has failed to accelerate, the relay issues a trip command. The entire programme is executed in realtime, in the sence that the entire process is completed within the sampling interval of 1.66 ms, by the time next set of samples are available. Step 1 : When powered ON, the program first initializes all the variables, checks the memory sec (A) and sec (B) module 2 in Figure 3 analog - digital converter (E) analog and digital outputs F & G of module 2 in figure 3, and sets the interrupt timers (C). The timer (C) is programmed such that for the field input (i.e. current & voltage signals) of 50Hz, the time period of 20 millseconds; 12 evenly spaced samples are acquired. This implies that the sampling interval is 20ms = 1.66 milliseconds 12 All the six field inputs are digitized, and processed using digital fourier filter algorithm. The root mean square values of the three voltages, three currents, reactance & resistance of the 3 phases, active power, reactive power, power factor and frequency is computed. Step 2 : Each of the phase current is checked if it is greater than Istart value, and if it is so, the locked rotor check is started. Otherwise the program control goes to the other modules check stage 6. Step 3: here the current is compared with the highest instantaneous value, and if it is found greater, the trip signal is issued. Next, the under voltage is checked, and if true the module is exited. Next, the loss of phase (one or two phases) is checked, if true, loss of phase a phase accumulation is given. For each of the phase, resistance and reactance values are checked if they lie within the mho characteristic boundary, till Tstart time elapses. After Tstart/2 time, d (x 1R) is monitored dt Step 4: a) if for the duration Tstart, the impedance was within the boundary of the mho characteristic, trip command is issued in stage 5. b) For the duration Tsart + 100ms the d (x1R) was either positive or 2 dt zero, trip command is issued in stage 5. Step 6 : The other protection modules viz overcurrent, earthfault, overvolatge, under voltage, negative phase sequence and thermal overload protection functions are enabled and any other fault, if present is detected. Step 7 : The trip command to be issued by the other function is checked and issued if necessary in stage 8. The program control once again goes back to the data acquisition for the next set of fresh samples. Figures 7A and Figure 7B show the test results of TORQUE-SPEED characteristics of the 2050 kW, 18 pole and 50 HP, 10 pole Induction motors respectively. These graphs depict that the method of monitoring the gradient of the xbyr parameter is reliable. The test setup to simulate the locked rotor condition is shown in Figure 8. The relay responses as captured in the oscilloscope is shown in Figures 9A, 9B, and 9C. The monitoring program continuously monitors and determines the status of the associated motor in real time. This provides information related to the running / stopped conditions and any alarms or trips issued by the relay. All the motor set parameters such as, root-mean-square values of the three-phase voltages, currents, phase currents, earth-fault current, negative sequence current, frequency, three-phase resistance and reactance values and status of the relay can be viewed on the display. The keypad provides access for reading the parameter values set in the relay, for modifying the settings, and for reading the event / fault record etc. The sequence of event record feature marks the time and date of significant events. It allows for the recording of all trips and alarms of the motor. It can also record historical data for the motor, such as, number of starts, time taken to start etc. There is also a provision for printing the stored values over the serial RS232 port. The operator interface is a menu-driven sequence. The operator can read the various metered data through the display and can read / modify the parameters settings for the various functions of the relay. Event / fault records can also be read from the front-panel. All the records are stored in a cyclic buffer, which can hold a maximum of 40 events and 40 faults. WE CLAIM 1. An independence based method for real time detection of locked rotor condition of a motor prior to elapse of motor starting time in a system, the system comprising: a power supply module (1) providing overload, short-circuit and input reversal protections, a microprocessor based module (2) having in-built digital input, analog input, digital output, analog output, a static memory, a static RAM, and a flash memory, the module having atleast two serial ports; a relay algorithm incorporated in the microprocessor based module (2); a voltage and current transducer module (3) connected to the relay module, the transducer module having three each voltage transformers and current transformers for stepping down input voltage and field current respectively; a six channel of second order filters (4) for correction of amplitude and phase of the analog signals, a numerical relay module (5) comprising a plurality of relays having NO (normally open) contacts, the method comprising the steps of: processing the field inputs and filtering high frequency noise, and transmitting the signals to the microprocessor based module; initializing the variables and setting an interrupt timer; checking whether the phase current is greater than I start value, if not detecting presence or absence of faults relating to overcurrent, overvoltage, undervoltage, negative phase sequence, thermal overload; comparing the current with a highest instantaneous value, and issuing a trip signal if the value is greater; checking the undervoltage and then the loss of phase, and allotting a phase accumulation if the loss of phase is present; determining the impedance in each phase and monitoring time Tstart duration if the impedance is within the boundary of mho characteristic; resetting the variables if the impedance value is beyond the mho characteristic boundary; and monitoring the impedance value when reset for each phase, if a value in respect of rate of change of the ratio of reactance value over resistances (d X /R)/dt decreasing, if not decreasing, tripping the breaker. 2.An independence based method for real time detection of locked rotor condition of a motor prior to elapse of motor starting time incorporating a numerical motor protection relay as substantially described and illustrated herein with reference to the accompanying drawings. |
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248-KOL-2005-CANCELLED PAGES.pdf
248-KOL-2005-CORRESPONDENCE 1.2.pdf
248-KOL-2005-CORRESPONDENCE-1.1.pdf
248-kol-2005-correspondence.pdf
248-kol-2005-correspondence1.3.pdf
248-KOL-2005-DESCRIPTION (COMPLETE) 1.1.pdf
248-kol-2005-description (complete).pdf
248-kol-2005-description (provisional).pdf
248-kol-2005-examination report.pdf
248-kol-2005-granted-abstract.pdf
248-kol-2005-granted-claims.pdf
248-kol-2005-granted-description (complete).pdf
248-kol-2005-granted-drawings.pdf
248-kol-2005-granted-form 1.pdf
248-kol-2005-granted-form 2.pdf
248-kol-2005-granted-specification.pdf
248-KOL-2005-REPLY TO EXAMINATION REPORT.pdf
248-kol-2005-reply to examination report1.1.pdf
248-kol-2005-specification.pdf
Patent Number | 247927 | ||||||||||||
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Indian Patent Application Number | 248/KOL/2005 | ||||||||||||
PG Journal Number | 23/2011 | ||||||||||||
Publication Date | 10-Jun-2011 | ||||||||||||
Grant Date | 06-Jun-2011 | ||||||||||||
Date of Filing | 30-Mar-2005 | ||||||||||||
Name of Patentee | BHARAT HEAVY ELECTRICALS LIMITED | ||||||||||||
Applicant Address | REGIONAL OPERATIONS DIVISION (ROD), PLOT NO: 9/1, DJBLOCK 3RD FLOOR, KARUNAMOYEE, SALT LAKE CITY, KOLKATA-700091, HAVING ITS REGISTERED OFFICE AT BHEL HOUSE, SIRI FORT, NEW DELHI-110049, INDIA | ||||||||||||
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PCT International Classification Number | G01R 29/12 | ||||||||||||
PCT International Application Number | N/A | ||||||||||||
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