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A microprocessor-based inrush restrained differential relay for transformer protection
Journal of Microcomputer Applications (1986) 9, 313-3 18 COMMUNICATION
A microprocessor-based inrush restrained differential relay for transformer protection H. K. Vera
and A. M. Basha*
Department of Electrical Engineering, University of Roorkee, Roorkee--247 667, India *Department of Electrical Engineering, Regional Engineering College, Calicut--673 601, Kerala, India
This communication reports the development of a fully selective microprocessor-based differential relay for transformer protection. Unwanted operations of the relay on magnetizing inrush, external faults and over-excitation conditions are restrained, while still allowing tripping on internal faults, by conducting suitably designed checks on the waveform of differential current. The relay behaviour has been extensively tested by applying signals of representative waveforms generated from a specially designed function generator.
I,
Introduction
The power transformer is one of the most important links in a power transmission system and its reliability of operation has assumed wider importance with the advent of EHV and UHV transmission. Several analogue schemes are in operation for the protection of transformers. A review of literature on this subject reveals that some differential protection schemes for transformers utilize the second harmonic component and others use the second along with higher harmonic components of the differential current to yield a blocking signal during magnetizing inrush (Sykes & Morrison, 1972; Kennedy & Hayward, 1937). Relays using this approach, popularly known as the harmonic restraint approach, may operate undesirably when the inrush current wave is unidirectional because of the low second harmonic content and high d.c. component content in the wave (Van C. Warrington, 1962). Another article on this subject gives statistical data showing that in about 30% of cases these relays operated incorrectly (Fedoseev, 1961). This high percentage of maloperations calls for further work to improve the differentail protection performance of transformers. The relay developed by the authors eliminates the undesired tripping of the differential relay by incorporating a waveform approach. The important features of the current waveform on inrush, over-excitation and heavy through fault conditions have been exploited to discriminate these conditions from internal short-circuits and thereby provide adequate restraint in all conditions except on internal short-circuits.
2.
Significant features of inrush current wave
The magnetizing inrush phenomenon occurs when a transformer is energized. The inrush current flows only in the winding to which the voltage is applied. The waveform of this current has several features of interest in the present context: 313 0745-7 138/86/0403 13 + 06 $03.00/O
0 1986 Academic Press (London) Limited
314 (a)
H. K. Verma and A. M. Basha
Its magnitude (peak) can reach a maximum of 8-10 times the rated current of the transformer (Blume, 1946). (b) Peak dzJ%rence. Harmonic analysis shows that the inrush current wave contains a considerable percentage of higher harmonics and the current in one or two phases may have a large d.c. component too. The difference between consecutive positive and negative peaks is appreciable even after eliminating the d.c. component. (c) Undirectionality. A typical oscillogram of the differential currents in the three phases of transformer caused by inrush surge is shown in Figure l(a) (Brown Boveri, 1968). It can be noted that the differential current in phase ‘a’ for the first few cycles after switching has peaks of one polarity only (unidirectional). Such a condition may arise when the transformer switching-in takes place at zero voltage (Austen Stigant & Franklin, 1973). (d) Peak-to-peak spacing. From the same oscillogram it can be noted that the differential current waves in phases ‘b’ and ‘c’ have peaks of one polarity much smaller than the peaks of the other polarity. Further, the positive peak to negative peak spacing for phase ‘b’ current is increased by inrush, while the same spacing for phase ‘c’ current is decreased. These features are utilized in the present relay to discriminate an inrush condition from an internal short-circuit. Amplitude.
3. Features of the differential current wave under over-excitation and heavy through fault conditions The differential current under an over-excitation condition of transformer contains strong third and fifth harmonic components and small amounts of other higher harmonics. Out of these, the third harmonic component is precluded by the transformer or CT delta connections and only the fifth harmonic component along with the fundamental would be reflected in the relay circuit. A typical relay current waveform under these conditions is depicted in Figure l(b), which has two peaks in half a cycle (Say, 1955; Staff of the Department of Electrical Engineering, MIT, 1962). Under heavy through-current fault conditions, the CTs become saturated and their ratios depart from their normal values. A typical differential current wave under these conditions is depicted in Figure l(c). It can be noted that the waveform is similar to the inrush waveform (Kennedy & Hayward, 1937).
4. Features of differential current wave under internal faults The differential current wave under internal fault conditions is sinusoidal in nature with or without the decaying d.c. component. A typical differential current wave (with d.c. offset) under this condition is depicted in Figure l(d) (Sharp & Glassburn, 1958).
5.
Relay implementation
A biased differential relaying function involving comparison of the differential current magnitude with the through-current magnitude has been implemented on an 8085A
Microprocessor-based transformer protection
Figure 1. Typical differential current waveforms under through-current fault and (d) internal short-circuit conditions
(a) inrush, (b) of the transformer.
over-excitation,
(c)
3 15
heavy
microprocessor using software. Restraint on inrush, over-excitation, and external faults is provided on the basis of the waveform discrimination features identified above. The relay also incorporates unrestrained unbiased tripping at very large differential currents to allow fast tripping on internal faults. The operating value for this trip function is kept adjustable between eight and 20 times the rated current of the transformer.
6.
Relay hardware
The block schematic of the relay is shown in Figure 2. The required current signals for biased differential relaying are derived from CTs and given to the differential and through-current circuits to obtain voltages proportional to the differential and through currents respectively. The relay uses transactors to eliminate the d.c. component of the through current. The through-current signals are full-wave rectified whilst the differential-current signal is applied unrectified. These preprocessed signals (for all channels) are digitized at the rate of 48 samples over one cycle window. To ensure simultaneous sampling of all the signals, a sample and hold circuit is incorporated in each channel. The digital output of ADC is fed to microprocessor through an input port of a programmable parallel interface. The input channels are selected by giving three select signals (generated by the microprocessor) to an analogue multiplexer. The pick-up current settings for the biased and unbiased trip functions are provided by connecting the contacts of two single-pole four-way switches to the input port lines while the bias setting is obtained using a potentiometer (not shown in Figure 2). The trip signal is output by the microprocessor through an output port line to a slave relay, which in turn actuates trip circuits of circuit-breakers and an alarm circuit.
316
H. K. Verma and A. M. Basha
Accept ’ settings
Through current circuit
_
Rectifier circuits
1 Y
TCS
Ph.7
TCS
Ph.‘b’
TCS
Ph.?
.
w C :, .z
Indicators 3 ti ,x
9 o ;gg
_ !JCS Ph.%’ -h DCS Ph.‘b’ *
Differential current circuit
DCS
5
= E
E 2
Ph.? *
t TCS -Through-current Ph.
signal
-Phase
DCS -- Differential
current signal
Figure 2.
7.
Two, 1 -pole 4-way switches
Block schematic of the relay.
Relay software
The relay software checks the differential current samples continuously at intervals of O-416 ms (to establish sampling instants) and yields a blocking signal when the differential current (Id) is found to be zero for 12 successive samples. An inrush current wave having a unidirectionality feature can be detected by this prolonged zero-current check. It then finds the time interval between the consecutive positive and negative peaks of the I, wave and yields a blocking signal when the time interval in terms of number of sampling period is found to be less than 22 or greater than 27. An inrush current wave having unusual apparent half-time period can be detected by this check. The microprocessor then compares the magnitude of consecutive positive and negative peaks of the I, wave after eliminating the d.c. component and yields a blocking signal if the two peaks differ from each other by more than 25%. The above software checks for detecting inrush conditions are effective in blocking the relay operation on heavy through-current faults too since the I,, wave under this condition is similar to that under inrush condition [Figure l(c)]. The relay software checks for a double topped wave that characterizes an over-excitation condition of the transformer. It produces a restraining signal when two peaks of same polarity are found to occur within half a cycle. If an inrush condition or over-excitation or heavy through-current fault condition is not detected and the differential signal exceeds the pick-up value and through-current threshold, the microprocessor declares an internal fault and issues a trip signal. An
Trip and alarm
Microprocessor-based
transformer protection
3 17
internal fault is also indicated if the differential signal exceeds the pick-up setting for the unbiassed trip function. The relay software occupies about one kilobyte of memory locations.
8.
Relay testing
A prototype of the inrush restrained differential relay was assembled in laboratory. The software was developed for its operation and loaded in an EPROM chip. Various types of waveforms representing inrush, over-excitation, internal fault and heavy through-current fault conditions were simulated for laboratory testing. To begin with, waveforms representing inrush having prolonged zero current, d.c. transient and both d.c. and a.c. transients were generated by a suitably designed function generator and applied to the prototype relay one after the other. It was observed that the relay yielded a blocking signal in each case. The current waveforms under through-current fault conditions being similar to those under inrush conditions, these tests also confirm restraint of the relay on the former. The test was repeated on a double peak waveform representing an over-excitation condition and generated by the function generator. The relay again restrained. For simulating internal fault conditions, waveforms having dc. transients (exponentially decaying d.c. offset), a.c. transients and both d.c. and a.c. transients respectively, were generated by a function generator and fed to the relay one after the other. The relay gave out a trip signal in these three cases. The relay operation on heavy internal faults was verified by applying an amplified sinusoidal signal to the relay. The test observations indicate that the relay functions satisfactorily.
9.
Conclusions
An 8-bit 8085A microprocessor-based differential relay restraining on inrush, over-excitation and heavy external faults has been successfully developed for power transformer protection. The existing second-harmonic restraint differential relays applied for short-circuit protection of the power transformers may fail to block tripping on a unidirectional magnetizing inrush as already explained. The present relay is an improvement over the principle of harmonic restraint and the logic adopted in it to achieve this discrimination is based on certain definite features of the differential current waveforms. The relay generates a trip signal on internal short-circuits and restrains on all other conditions. The relay prototype has been tested to study its response to normal and various abnormal conditions of the protected transformer and it was found to be completely discriminative. The internal heavy faults and light faults are detected in the present relay in half a cycle and one cycle, respectively. Because most of the relay functions have been achieved through software, the hardware requirement is fairly simple.
Acknowledgement The facilities provided by the Department of Electrical Engineering and the Centre for Microprocessor Applications, University of Roorkee, Roorkee, India, for carrying out the work reported in this paper are gratefully acknowledged.
318
H. K. Verma and A. M. Basha
References Austen Stigant, S. & Franklin, A. C. 1973. The Jand P Transformer Book. London: Butterworths. Biume, L. F. 1946. Transformer Engineering. London: Chapman and Hall. Brown Boveri. 1968. Relays for the Protection of Electrical Installations. Brown Boveri Literature, Switzerland. Fedoseev, A. M. 1961. Osnwy Velgoni Zashchity (Gosenergoizdat). Kennedy, L. F. & Hayward, C. D. 1937. Harmonic current restrained relays for differential protection. Transactions of the AIEE, 262-270. Staff of the Department of Electrical Engineering, MIT. 1962. Magnetic Circuits and Transformers. New York: John Wiley. Say, M. G. 1955. The Performance and Design of Alternating Current Machines. London: Pitman. Sharp, K. L. & Glassburn, W. E. 1958. A transformer differential relay with second-harmonic restraint. Transactions of the AIEE. 913-918. Sykes, J. A. 8c Morrison, I. F. 1972. A proposed method of harmonic restrained differential protection of transformers by digital computer. Transactions of the IEEE, 91, 12661273. Van C. Warrington, A. R. 1962. Protective Relays: Their Theory and Practice, Vol. 1. London: Chapman and Hall. H. K. Verma is a professor of electrical engineering at University of Roorkee, Roorkee, India. He
obtained a BE degree in 1967 from the University of Jodhpur, and an ME degree in 1969 and a PhD in 1977, both from the University of Roorkee. For two years he worked as R and D Manager, Power and Industrial Systems Division, Universal Electrics Ltd, Faridabad. Prof. Verma is actively engaged in research on the application of microprocessors in power system protection, bioelectric signal analysis and measurements. He is a fellow of the Institution of Engineers (India), Institution of Electronics and Telecommunication Engineers and Institution of Instrumentation Scientists and Technologists. A. M. Basha graduated in electrical and electronic engineering
from the Government College of Engineering, Salem, in 1974 and gained an ME in Power Systems from PSG Institute of Technology, Coimbatore in 1976. He is a lecturer at Regional Engineering College, Calicut and is presently engaged in research at the University of Roorkee on application of microprocessors to power system protection.
A microprocessor-based inrush restrained differential relay for transformer protection H. K. Vera
and A. M. Basha*
Department of Electrical Engineering, University of Roorkee, Roorkee--247 667, India *Department of Electrical Engineering, Regional Engineering College, Calicut--673 601, Kerala, India
This communication reports the development of a fully selective microprocessor-based differential relay for transformer protection. Unwanted operations of the relay on magnetizing inrush, external faults and over-excitation conditions are restrained, while still allowing tripping on internal faults, by conducting suitably designed checks on the waveform of differential current. The relay behaviour has been extensively tested by applying signals of representative waveforms generated from a specially designed function generator.
I,
Introduction
The power transformer is one of the most important links in a power transmission system and its reliability of operation has assumed wider importance with the advent of EHV and UHV transmission. Several analogue schemes are in operation for the protection of transformers. A review of literature on this subject reveals that some differential protection schemes for transformers utilize the second harmonic component and others use the second along with higher harmonic components of the differential current to yield a blocking signal during magnetizing inrush (Sykes & Morrison, 1972; Kennedy & Hayward, 1937). Relays using this approach, popularly known as the harmonic restraint approach, may operate undesirably when the inrush current wave is unidirectional because of the low second harmonic content and high d.c. component content in the wave (Van C. Warrington, 1962). Another article on this subject gives statistical data showing that in about 30% of cases these relays operated incorrectly (Fedoseev, 1961). This high percentage of maloperations calls for further work to improve the differentail protection performance of transformers. The relay developed by the authors eliminates the undesired tripping of the differential relay by incorporating a waveform approach. The important features of the current waveform on inrush, over-excitation and heavy through fault conditions have been exploited to discriminate these conditions from internal short-circuits and thereby provide adequate restraint in all conditions except on internal short-circuits.
2.
Significant features of inrush current wave
The magnetizing inrush phenomenon occurs when a transformer is energized. The inrush current flows only in the winding to which the voltage is applied. The waveform of this current has several features of interest in the present context: 313 0745-7 138/86/0403 13 + 06 $03.00/O
0 1986 Academic Press (London) Limited
314 (a)
H. K. Verma and A. M. Basha
Its magnitude (peak) can reach a maximum of 8-10 times the rated current of the transformer (Blume, 1946). (b) Peak dzJ%rence. Harmonic analysis shows that the inrush current wave contains a considerable percentage of higher harmonics and the current in one or two phases may have a large d.c. component too. The difference between consecutive positive and negative peaks is appreciable even after eliminating the d.c. component. (c) Undirectionality. A typical oscillogram of the differential currents in the three phases of transformer caused by inrush surge is shown in Figure l(a) (Brown Boveri, 1968). It can be noted that the differential current in phase ‘a’ for the first few cycles after switching has peaks of one polarity only (unidirectional). Such a condition may arise when the transformer switching-in takes place at zero voltage (Austen Stigant & Franklin, 1973). (d) Peak-to-peak spacing. From the same oscillogram it can be noted that the differential current waves in phases ‘b’ and ‘c’ have peaks of one polarity much smaller than the peaks of the other polarity. Further, the positive peak to negative peak spacing for phase ‘b’ current is increased by inrush, while the same spacing for phase ‘c’ current is decreased. These features are utilized in the present relay to discriminate an inrush condition from an internal short-circuit. Amplitude.
3. Features of the differential current wave under over-excitation and heavy through fault conditions The differential current under an over-excitation condition of transformer contains strong third and fifth harmonic components and small amounts of other higher harmonics. Out of these, the third harmonic component is precluded by the transformer or CT delta connections and only the fifth harmonic component along with the fundamental would be reflected in the relay circuit. A typical relay current waveform under these conditions is depicted in Figure l(b), which has two peaks in half a cycle (Say, 1955; Staff of the Department of Electrical Engineering, MIT, 1962). Under heavy through-current fault conditions, the CTs become saturated and their ratios depart from their normal values. A typical differential current wave under these conditions is depicted in Figure l(c). It can be noted that the waveform is similar to the inrush waveform (Kennedy & Hayward, 1937).
4. Features of differential current wave under internal faults The differential current wave under internal fault conditions is sinusoidal in nature with or without the decaying d.c. component. A typical differential current wave (with d.c. offset) under this condition is depicted in Figure l(d) (Sharp & Glassburn, 1958).
5.
Relay implementation
A biased differential relaying function involving comparison of the differential current magnitude with the through-current magnitude has been implemented on an 8085A
Microprocessor-based transformer protection
Figure 1. Typical differential current waveforms under through-current fault and (d) internal short-circuit conditions
(a) inrush, (b) of the transformer.
over-excitation,
(c)
3 15
heavy
microprocessor using software. Restraint on inrush, over-excitation, and external faults is provided on the basis of the waveform discrimination features identified above. The relay also incorporates unrestrained unbiased tripping at very large differential currents to allow fast tripping on internal faults. The operating value for this trip function is kept adjustable between eight and 20 times the rated current of the transformer.
6.
Relay hardware
The block schematic of the relay is shown in Figure 2. The required current signals for biased differential relaying are derived from CTs and given to the differential and through-current circuits to obtain voltages proportional to the differential and through currents respectively. The relay uses transactors to eliminate the d.c. component of the through current. The through-current signals are full-wave rectified whilst the differential-current signal is applied unrectified. These preprocessed signals (for all channels) are digitized at the rate of 48 samples over one cycle window. To ensure simultaneous sampling of all the signals, a sample and hold circuit is incorporated in each channel. The digital output of ADC is fed to microprocessor through an input port of a programmable parallel interface. The input channels are selected by giving three select signals (generated by the microprocessor) to an analogue multiplexer. The pick-up current settings for the biased and unbiased trip functions are provided by connecting the contacts of two single-pole four-way switches to the input port lines while the bias setting is obtained using a potentiometer (not shown in Figure 2). The trip signal is output by the microprocessor through an output port line to a slave relay, which in turn actuates trip circuits of circuit-breakers and an alarm circuit.
316
H. K. Verma and A. M. Basha
Accept ’ settings
Through current circuit
_
Rectifier circuits
1 Y
TCS
Ph.7
TCS
Ph.‘b’
TCS
Ph.?
.
w C :, .z
Indicators 3 ti ,x
9 o ;gg
_ !JCS Ph.%’ -h DCS Ph.‘b’ *
Differential current circuit
DCS
5
= E
E 2
Ph.? *
t TCS -Through-current Ph.
signal
-Phase
DCS -- Differential
current signal
Figure 2.
7.
Two, 1 -pole 4-way switches
Block schematic of the relay.
Relay software
The relay software checks the differential current samples continuously at intervals of O-416 ms (to establish sampling instants) and yields a blocking signal when the differential current (Id) is found to be zero for 12 successive samples. An inrush current wave having a unidirectionality feature can be detected by this prolonged zero-current check. It then finds the time interval between the consecutive positive and negative peaks of the I, wave and yields a blocking signal when the time interval in terms of number of sampling period is found to be less than 22 or greater than 27. An inrush current wave having unusual apparent half-time period can be detected by this check. The microprocessor then compares the magnitude of consecutive positive and negative peaks of the I, wave after eliminating the d.c. component and yields a blocking signal if the two peaks differ from each other by more than 25%. The above software checks for detecting inrush conditions are effective in blocking the relay operation on heavy through-current faults too since the I,, wave under this condition is similar to that under inrush condition [Figure l(c)]. The relay software checks for a double topped wave that characterizes an over-excitation condition of the transformer. It produces a restraining signal when two peaks of same polarity are found to occur within half a cycle. If an inrush condition or over-excitation or heavy through-current fault condition is not detected and the differential signal exceeds the pick-up value and through-current threshold, the microprocessor declares an internal fault and issues a trip signal. An
Trip and alarm
Microprocessor-based
transformer protection
3 17
internal fault is also indicated if the differential signal exceeds the pick-up setting for the unbiassed trip function. The relay software occupies about one kilobyte of memory locations.
8.
Relay testing
A prototype of the inrush restrained differential relay was assembled in laboratory. The software was developed for its operation and loaded in an EPROM chip. Various types of waveforms representing inrush, over-excitation, internal fault and heavy through-current fault conditions were simulated for laboratory testing. To begin with, waveforms representing inrush having prolonged zero current, d.c. transient and both d.c. and a.c. transients were generated by a suitably designed function generator and applied to the prototype relay one after the other. It was observed that the relay yielded a blocking signal in each case. The current waveforms under through-current fault conditions being similar to those under inrush conditions, these tests also confirm restraint of the relay on the former. The test was repeated on a double peak waveform representing an over-excitation condition and generated by the function generator. The relay again restrained. For simulating internal fault conditions, waveforms having dc. transients (exponentially decaying d.c. offset), a.c. transients and both d.c. and a.c. transients respectively, were generated by a function generator and fed to the relay one after the other. The relay gave out a trip signal in these three cases. The relay operation on heavy internal faults was verified by applying an amplified sinusoidal signal to the relay. The test observations indicate that the relay functions satisfactorily.
9.
Conclusions
An 8-bit 8085A microprocessor-based differential relay restraining on inrush, over-excitation and heavy external faults has been successfully developed for power transformer protection. The existing second-harmonic restraint differential relays applied for short-circuit protection of the power transformers may fail to block tripping on a unidirectional magnetizing inrush as already explained. The present relay is an improvement over the principle of harmonic restraint and the logic adopted in it to achieve this discrimination is based on certain definite features of the differential current waveforms. The relay generates a trip signal on internal short-circuits and restrains on all other conditions. The relay prototype has been tested to study its response to normal and various abnormal conditions of the protected transformer and it was found to be completely discriminative. The internal heavy faults and light faults are detected in the present relay in half a cycle and one cycle, respectively. Because most of the relay functions have been achieved through software, the hardware requirement is fairly simple.
Acknowledgement The facilities provided by the Department of Electrical Engineering and the Centre for Microprocessor Applications, University of Roorkee, Roorkee, India, for carrying out the work reported in this paper are gratefully acknowledged.
318
H. K. Verma and A. M. Basha
References Austen Stigant, S. & Franklin, A. C. 1973. The Jand P Transformer Book. London: Butterworths. Biume, L. F. 1946. Transformer Engineering. London: Chapman and Hall. Brown Boveri. 1968. Relays for the Protection of Electrical Installations. Brown Boveri Literature, Switzerland. Fedoseev, A. M. 1961. Osnwy Velgoni Zashchity (Gosenergoizdat). Kennedy, L. F. & Hayward, C. D. 1937. Harmonic current restrained relays for differential protection. Transactions of the AIEE, 262-270. Staff of the Department of Electrical Engineering, MIT. 1962. Magnetic Circuits and Transformers. New York: John Wiley. Say, M. G. 1955. The Performance and Design of Alternating Current Machines. London: Pitman. Sharp, K. L. & Glassburn, W. E. 1958. A transformer differential relay with second-harmonic restraint. Transactions of the AIEE. 913-918. Sykes, J. A. 8c Morrison, I. F. 1972. A proposed method of harmonic restrained differential protection of transformers by digital computer. Transactions of the IEEE, 91, 12661273. Van C. Warrington, A. R. 1962. Protective Relays: Their Theory and Practice, Vol. 1. London: Chapman and Hall. H. K. Verma is a professor of electrical engineering at University of Roorkee, Roorkee, India. He
obtained a BE degree in 1967 from the University of Jodhpur, and an ME degree in 1969 and a PhD in 1977, both from the University of Roorkee. For two years he worked as R and D Manager, Power and Industrial Systems Division, Universal Electrics Ltd, Faridabad. Prof. Verma is actively engaged in research on the application of microprocessors in power system protection, bioelectric signal analysis and measurements. He is a fellow of the Institution of Engineers (India), Institution of Electronics and Telecommunication Engineers and Institution of Instrumentation Scientists and Technologists. A. M. Basha graduated in electrical and electronic engineering
from the Government College of Engineering, Salem, in 1974 and gained an ME in Power Systems from PSG Institute of Technology, Coimbatore in 1976. He is a lecturer at Regional Engineering College, Calicut and is presently engaged in research at the University of Roorkee on application of microprocessors to power system protection.