A high speed three-zone distance relay using digital circuits

Electric Power Systems Research, 8 (1984/85) 187 - 195

187

A High Speed Three-Zone Distance Relay Using DigitalCircuits A. J. KELLOGG, L. P. SINGH and G. K. DUBEY

Department of Electrical Engineering, Indian Institute of Technology, Kanpur 208016 (India) (Received August 27, 1984)

SUMMARY

Polyphase distance relaying using solid state c o m p o n e n t s has been accepted in general as a secondary protection for E H V / UHF lines. It is not surprising that in developed countries solid state relaying has gained popularity as the primary means o f protection with 100% discrimination and reliability. The older schemes using rectifier bridge phase detectors are not so sensitive and are also slow in operation, especially near boundary conditions. This paper deals "with a new relaying scheme, using digital circuits with CMOS logic, which does not use a separate starting unit, nor an electromechanical unit for zone changing. The present relaying scheme with m h o and elliptic starting characteristics can give modified m h o characteristics with blinders or a quadrilateral characteristic by simply changing the measuring circuit and signal processing unit. This relay has been designed, fabricated and tested on a 'dynamic test bench' which has also been designed and fabricated in this laboratory. I t is expected that this relaying scheme may prove to be better than the existing ones.

1. INTRODUCTION

Multi-input comparators [1] are generally used for polyphase relaying and the relays are normally of three types. The first type responds only to all kinds of ground faults [2, 3], the second responds only to all kinds of phase faults [4] and the third responds to all types of polyphase faults [5]. A measuring unit is used in all the above schemes which provides a compensated line to ground voltage to the comparators. Some of the relaying schemes work on the principle of polarized mho relay [6], others on the principle of 0378-7796/85/$3.30

phase sequence detection [7, 8], and the remainder on the principle of phase coincidence [9]. The last two schemes are recent developments which simplify the relay connection and circuitry. A 'General purpose static relay using digital techniques' [10] has been described earlier which can generate all types of tripping characteristics by the use of two-input and multi-input comparators. The above relay works on the principle of phase coincidence and generates different characteristics simply by changing the measuring circuit and signal processing unit. The variable angular criterion [11] is achieved by change of clock frequency. The present paper is based on a similar technique but provides three-zone operation by the use of two-input comparators in a novel way. The basic comparator is transient free and remains operative even if the voltage falls to zero. The single-phase three-zone relay can be converted to a three-phase relay if the output of each 'phase module' is 'OR' gated. The same basic comparator can become a polyphase relay if the three-phase compensated line to ground voltages are connected to it via 'AND' and 'NOR' gates. Three or four such units can detect and clear all ten types of shunt faults; hence the basic relaying scheme can provide economy, reliability and simplicity in the field of protection. Blocking during a power swing can be achieved either by modifying the third-zone characteristic or using the d l F / d t property of the fault current, which induces insufficient voltage, to cut off the final tripping.

2. P R O P O S E D

RELAYING

SCHEME

Figure 1 shows the block diagram of the relay. It can be seen that comparator I handles zone I and zone II, whereas compar© Elsevier Sequoia]Printed in The Netherlands

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Fig. 1. Block diagram of three-zone distance relay (phase module).

ator II is used for zone III only. Both these comparators are independent, hence t h e y give trip signals every half cycle, which are OR gated to start a monostable circuit for about 100 cycles. If the trip signal from the lower (zone III) comparator is due to a power swing, then L d l F / d t is n o t sufficient to maintain logic level 1 to the clear terminal (C1) of the monostable circuit and hence it remains inoperative. When the power swing disappears and the fault persists, the L d I F / d t is sufficient to maintain logic level 1 and hence the monostable circuit is put into operation and thus there is a final trip signal to the circuit breaker. If by chance the relay voltage vanishes, then the OR gate issues an instantaneous trip signal which is desirable as the fault may be severe or a close-up one. The lower comparator gives an elliptic characteristic and hence it is used as a third-zone starting element and at the same time it blocks tripping due to a power swing. 2.1. O p e r a t i o n o f a t h r e e - z o n e d i s t a n c e relay

Figure 2 gives single-phase relay The signals to the mho characteristic

the circuit diagram of the using digital components. upper comparator to give a in zone I are:

S' 1 = IZR1 -- V

S'2 = V

and the signals to give a m h o characteristic in zone II are:

S' 2 = V S '3 = I Z a 2 - - V

Since the operating criterion is - - 9 0 ° < / 3 < 90 °, a cos comparator is used. A flip-flop (B-1 and B-2) acts as a zone synchronizer with clock frequency f0 to give +90 ° operation. The digital integration is done by a binary counter (BC-1 and BC-2) over a period of 5 ms (1/4 cycle), thus the frequency f0 is taken to be 12.8 kHz. If the fault lies in zone II, the o u t p u t of B-2 is connected to a digital integrator (BC-1 and BC-2) which gives a pulse after 5 ms and operates a monostable (M-3) through a flip-flop (B-5). Thus the o u t p u t of M-3 goes to logic level 0 and resets B-6, hence B-6 does not pass the pulse as it comes after time delay t D. The BC-2 gives a pulse every 1/2 cycle which is reset after time delay t D . The M-3 resets after time delay t2 and thus B-6 passes one pulse for every two input pulses. The o u t p u t of M-1 and M-2 has a time lag of 10 ms and duration of about 15 ms. Thus the o u t p u t of A-5 is a regular pulse of 5 ms duration at intervals of 10 ms. These pulses operate M-7 via gate OR-2 for about 100 cycles, which is the final trip signal to the circuit breaker. If the fault happens to take place in zone I also, A-1 sends a pulse along with BC-2 and sets M-3 to logic level 1 once again, hence the zone II time delay is eliminated enabling M-7 to operate with a zone I time delay (about

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15 ms). The signals to the lower comparator for third-zone operation are: S '4 = I Z R 3 - - V

S's = KIZR3 + V with clock frequency fo, which give the offset mho circular characteristic normally required by starting elements. If the fault lies in zone III, the upper comparator does not give a trip pulse, but the lower comparator gives a trip pulse after a time delay of t3 s due to B-7 and M-4. The BC-4 gives a trip pulse every 1/2 cycle which is reset when its 'clear' goes to logic level 1. The rest of the operation is similar to that of the upper comparator. The shape o f the characteristic of zone III can be changed using a variable angular criterion, by change of clock frequency fl, where fl-

f0 2 --~i/90

f0 is the frequency required to give a -+90° angular criterion, ]'i is the frequency required to give a variable angular criterion, and ~i is the angular criterion (<90°). If the operating criterion is +60 ° , then the required clock frequency fl = 3f0/4, i.e. fl f0. The characteristic produced by the lower comparator with the new clock frequency is elliptical and hence the tripping due to a power swing is avoided.

2.2. Testing The relaying scheme is tested on a singlephase dynamic test bench (described later), designed and fabricated by us in the laboratory, which has an overall accuracy of not better than -+5%. This is due to the fact that an accuracy in current and power transformer ratios, line and replica impedance, etc., of better than -+2% could not be obtained. The following tests were performed on the distance relay. (a) Open~ircuit line energized and deenergized from the bus-bar; the relay was found to be completely stable. (b) The accuracy (Zm/ZR) under steady state and transient conditions with respect to 'range' (Zsl/Zm) was plotted against the range, as shown by Fig. 3(a). It can be seen that at a range of 5 the under-reach is less than 5%. (c) The dynamic characteristic of the relay was plotted for different phase angles of t h e line impedances. A transient over-reach of less than 5% near the balance point was observed. The characteristic is as shown by Fig. 3(c). (d) The 'accuracy' under transient conditions with respect to the switching angle of the fault current was plotted as shown b y Fig. 3(b). A transient over-reach of 10% was ob-

served.

(e) The relay operating time was plotted against the switching angle of the fault

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current and was found to vary between 15 and 18 ms. (f) The relay was tested for reverse faults by passing steady state and transient currents through the transformer in the opposite direction to the normal current flow. Observation. The relay was found to be completely stable.

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The block diagram shown in Fig. 4 converts a single-phase relay to a three-phase relay. The outputs of phase modules A, B and C are 'OR' gated to give a final trip signal. The phase modules A, B and C are, in fact, single-phase comparators, as described in §2.1. Suitable

PHASE-A MODULE

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input signals are sent to the comparators, via the measuring circuit, in such a way that the relay responds to all types of faults.

3.1. Measuring circuit The circuit diagram as shown in Fig. 5 is the measuring circuit which provides the necessary signals to each phase module. Compensated fault point voltages V~, Vy and Vz are provided for each phase module A, B and C along with line to ground voltages V~, Vrb and Vrc. It is a known fact that such a measuring scheme will not give the same reach for phase to phase faults and phase to ground faults owing to the different short-circuit line currents I,, Ib and Ic. But, if zero sequence current is taken into account, the measuring circuit will give the correct measurement both for phase faults as well as for line to ground faults. The compensated fault point voltages can now be derived as explained below. A

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Vfa -- V r a - - I a l Z L I

-- Ia2ZL2 -- Ia0ZLo

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where n = ZLo/ZL1 and assuming ZL1 = ZL2 for transmission lines. Hence, the three compensated voltages are: Vx

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(Ia + Klao)ZR = Vxl + Vx2 + Vxo ( 3 )

Vy = Vrb - - (Ib + Klao)ZR = ol2Vxl + olVx2 + Vxo

(4) Vz = Vrc - - (Ic + KIao)ZR = olVxl + o~2Vx2+ Vx0

(5) where

Z L 1 ----Z R ~-

replica impedance, and

V x l = Vral - - I . l Z r

Yx: = V r ~ - - I ~ Z R Vxo = V,.o - - I~onZR

The signals sent to the phase A module are Vra and --Vx, to phase module B are V,b and --Vy and to phase module C are V~¢ and --Vz. The auxiliary power transformer is used to provide three-zone operation of each phase with a certain percentage of first-zone impedance ZRI. Thus the characteristic of each phase module will be mho type and the final characteristic will be a combination of these characteristics, depending upon the type of fault. The final characteristics can be predicted mathematically.

3.2. Mathematical analysis for operation The signals to phase module A for 1st and 2nd none-operation are: S 1 -~ Y r a

and S2 = --Vx = (Ia + Klao)ZR -- ~'a

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Now converting Ia into its sequence components and changing ZL likewise, we get:

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192 t h e n t h e c r i t e r i o n for tripping is - - 9 0 ° ~< ff 90 °. Thus t h e characteristic will be m h o passing t h r o u g h the origin, which will satisfy t h e a b o v e c o n d i t i o n . T h e criterion for a line t o g r o u n d fault o f d i f f e r e n t phase will be t h e same as above.

t h r e e - z o n e selection is r a t h e r unusual and h e n c e a special m e t h o d o f testing is n e e d e d which m a d e it necessary to design and fabricate a special t y p e o f d y n a m i c test bench. It simulates all the fault c o n d i t i o n s in o r d e r t o test t h e p e r f o r m a n c e o f the static relay during t r a n s i e n t c o n d i t i o n s and also t o verify t h e a c c u r a c y o f t h e relay with r e s p e c t to its r e a c h and range. T h e d y n a m i c test b e n c h , as s h o w n by Figs. 6 and 7, consists o f t h e following units: 1. Point-on-wave selector and fault simulator

4. DYNAMIC TEST BENCH T h e t h r e e - z o n e distance relay d e v e l o p e d uses a novel m e t h o d o f a u t o m a t i c z o n e changing by using solid state c o m p o n e n t s o n l y . T h e

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193 2. Voltage and current simulator 3. Phase angle measurement 4. Digital time measurement and display 5. Source and line impedance 6. Replica impedance with three zones 7. Power supply In order to understand the design and operation of the dynamic test bench, a brief explanation of these units is necessary.

4.1. Point-on-wave selector and fault simulator The heart of the dynamic test bench is the point-on-wave selector. A triac is fired to simulate the fault condition, in which the time delay (phase angle control) is achieved by a push-button operating a monostable circuit. Firing during the positive cycle (0 °180 ° control) and firing during the negative cycle (180 ° - 3 6 0 ° control) is achieved by a logic 1/0 control, selected by a toggle switch. The power circuit can be turned off by a push-button switch which stops the high frequency pulses (10 kHz) to the base of the transistor through the NOR gate 74C02. Thus, the triac turns o f f at the natural current zero. The presence of a short-circuit (fault simulation) is sensed by logic level 1 using a precision detector and a Schmidt trigger. 4.2. Relay voltage and current simulator and measuremen t A power transformer of nominal voltage ratio 230/100 and a current transformer of ratio 10/5 are used in which the secondary voltage can be varied from zero to maximum and the secondary current can be varied in ratios of 1.8 to 2.5 in steps of 0.1, i.e. 1.9, 2.0, 2.1, 2.2, 2.3 and 2.4. Thus the change of ratio between two consecutive steps is approximately 4%- 5%, by which the overand under-reach of the relay can be determined under dynamic conditions. The variation of the power transformer ratio together with the current transformer ratio can give fine control over the reach of the relay under test. The precision detector used with a pointon-wave selector is also used to measure the line currents. The voltage measurement is done by a rectifier bridge and a DC voltmeter for higher accuracy and linearity. A DPDT switch with central off position, is used to inject the line current to any transformer (replica impedance) so as to make the dynamic test bench more versatile. The mea-

surement of this line current is done by a current transformer of ratio 10/1, the secondary of which also provides an IR reference vector for phase angle measurement, etc.

4.3. Phase angle measurement The phase angle between any two sine waves can be measured by 'plugging in' and the display is shown by an analog DC phasemeter calibrated directly in degrees. An XOR gate is used to measure the phase difference of the standardized sine wave. Two operational amplifiers are used to convert sine waves to square waves which can be made to pass to a 'invertor', through switch $1 or S:, depending upon the polarity of the input sine wave. 4.4. Digital time measurement and display The digital time measuring device can measure up to 999 ms, which is the maximum range of zone III operation. The device can also measure the transient operating time. The basic principle of operation of this unit is as follows. A certain number of pulses n of known frequency fc is counted during the relay operating time ton, which gives the measure of time, thus: n ton = -- an

fc

where fc = constant. A four-stage decade counter is used with three 7-segment display units. Input pulses (10 kHz) are gated for the time ton to be measured. The fault initiation (logic level 1) starts the counter and the relay operation (1) stops the counter. It can be seen that the transient time is also included in relay operating time. Four sockets are provided on the control panel so that ton, f¢ and I1 can be displayed on a cathode ray oscilloscope and the relay output (trip) can be injected to stop the counter.

4.5. Source and line impedance Two iron-cored tapped inductors, with air gaps, are used to simulate source and line reactances with a continuous rating of 5 A. Provision is made to insert line and source resistances, externally, so as to vary the source and line phase angle. The taps of the inductors are brought out to a rotary switch

194

by which a desired source and line reactance can be inserted in the main circuit. The phase angle of the source and line reactor is greater than 85 ° .

sequential tripping for end-zone faults. It could, perhaps, prove to be a simple, economical and reliable relaying scheme for EHV protection.

4.6. Replica impedance with three zones An iron-cored transformer with air gap is used to simulate replica reactance; the transformer secondary has taps with which to vary the reactance through a rotary switch. The replica resistance is simulated by passing the transducer primary current through a resistance and then 'stepping up' b y a p o w e r transformer. Taps are provided on the secondary of the p o w e r transformer so as to vary the replica resistance through a rotary switch. This simulated impedance (RR + jXR) is isolated from the voltage circuit by an auxiliary p o w e r transformer, the secondary of which has five tappings to give a voltage proportional to KZR1, ZR1, ZR2 and ZR3. Hence a three-zone replica impedance is simulated in a novel way w i t h o u t passing the line current directly through the inductor, as is normally done. The zone I impedance ZR1 can be further reduced by a factor K which varies from 0.2 to 1 b y change of a rotary switch. The voltages IRR, IXR, KIZR1, IZR1, IZR2 and IZRa are brought o u t on the control panel through the sockets.

4.7. Power supply unit A well-regulated built-in p o w e r supply provides +10 V DC and +5 V for the electronic control, measurement and instrumentation purposes.

5. C O N C L U S I O N

The single-phase three-zone relay which was designed, fabricated and tested is compact and uses few digital components. The same relay, by triplication, can be converted to a polyphase relay. The polyphase relay is under development and its performance will be reported later. The dynamic test bench designed and fabricated in the laboratory uses a different approach for fault simulation and signal processing. It is c o m p a c t and reliable as many functions are included in a single unit. The relay thus made can be used satisfactorily for autoreclosing of circuit breakers and

NOMENCLATURE

a, b, c

K

three phases phase to ground voltages at source point phase current referred to current transformer secondary n--1

n

ZLo/ZLI

Ea, Eb, Ec [a, lb, le

RR, XR

replica resistance and reactance different o u t p u t signals from S l , 8 2.... , S . measuring unit SP 1, S ?2, . . . , S nf different input signals to relay after standardization through an operational amplifier, etc. compensated line to ground Vx, Vy, voltages at relaying point Vta, Vfb, Vfc phase to ground voltages at fault point Yra, Yrb, Yrc phase to ground voltage of power transformer secondary at relaying point line, source and replica imZL, Z s , Z R pedances all referred to relaying side ZR1, ZR2, ZR3 replica impedance of zones I, II and III phase angle between two input quantities $1 and $2 angular limits of phase com~1, ~2 parison positive, negative and zero se1, 2, 0 quence c o m p o n e n t s

REFERENCES 1 A. J. Kellogg, L. P. Singh and G. K. Dubey, Protection of E H V transmission lines by using static relays, Conf. on Power System Protection, Madras, April 1980, Vol. 1, Inst. Eng., India, p. B5. 2 G. D. Rockfeller, Zone packaged ground distance relay- I, Principle of operation, Trans. IEEE, PAS 85 (1966) 1021 - 1044. 3 S. Choudhary, S. K. Basu and S. P. Patra, Polyphase ground distance relaying by phase coincidence principle, IEEE Summer Power Meeting, 1972, Paper No. T-72, 428-I.

195 4 W. K. Sonnemam, Compensator distance relaying, Proc. Inst. Elect. Eng., Part 3, 77 (1958) 372 382. 5 D. Bhattacharya, S. K. Basu, K. P. Bose and S. P. Patra, A static polyphase distance relay, Trans. IEEE, PAS 91 (May/June) (1972). 6 L. M. Wedepohl, Polarised mho distance relay -- a new approach to analysis of practical characteristics, Proc. Inst. Elect. Eng., 112 (1965) 525 535. 7 Y. G. Paithankar, Versatile phase comparator based on detection of phase sequence for protective relay, Proc. Inst. Elect. Eng., 117 (1970) 1703.

8

S. C. Gupta, Static polyphase distance relay scheme for the protection of transmission lines, Ph.D. Thesis, Roorkee University, India, 1969. 9 M. Ramamoorthy, Static polyphase distance relay using comparison principle. Paper presented at Inst. Eng., India, Meeting, Calcutta, Dec. 1975. 10 A. J. Kellogg, L. P. Singh and G. K. Dubey, General purpose static relay using digital techniques, J. Inst. Eng. (India), Part EL1, 63 (1983) 5 -11. 11 M. Ramamoorthy and S. N. Lall, A versatile phase comparator using digital circuits, J. Inst. Eng. (India), Part 6, 59 (1979) 309 - 314.