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A high-speed two-stage phase comparator
Electric Power Systems Research, 4 (1981) 147 - 152 © Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands
147
A High-Speed Two-Stage Phase Comparator A. A. EL-ALAILY and OMAR A. S. EL-DIN*
Faculty of Engineering, Cairo University, Cairo (Egypt) (Received November 10, 1980)
SUMMARY The paper reports further work on phasecomparison techniques by the same authors. The technique reported here describes the operation o f a double-input phase comparator based on a two-stage phase-comparison process between two relaying signals used to measure the phase difference between them, and hence it can be applied to either the phase-comparison carrier or distance protection systems. The comparator reduces the maximum operating time to a value corresponding to half the trip angle o f the system. An example is given o f its application to a mho characteristic distance relay when applied to protect a typical 40 km, 132 k V single-circuit transposed line.
INTRODUCTION The phase-comparison carrier system in its simplest form may be regarded as a singlephase comparison of two signals which are functions of the line currents at each end of the protected section [1], whereas, in distance-type protection schemes, the conventional double-input phase comparator measures the phase difference between the two relaying signals which are functions of the line current and phase voltage at one end of the protected line section [2]. Although numerous techniques have been reported [3 - 5] for developing the phasecomparison process, most of them are susceptible to system transients and other spurious signals by virtue of their nearly instantaneous operation or a higher measuring
*Present address: Electrical Consultants Group, 8 EI-Heussain Ebn-Ali St., West Heliopolis, Cairo, Egypt.
accuracy may be obtained b y making sacrifices in the operating speed (block average comparator), in which case the comparator may either fail to trip or will give rise to unacceptably long operating times at the threshold conditions. The phase comparator described here reduces the maximum operating time to a value corresponding to half the trip angle of the system, irrespective of the value of the prefault or postfault phase difference between the relaying signals. The hardware necessary to realise the comparator consists of readily available and well-established small-scale and medium-scale integrated circuits. Results are given for off-line tests on a model transmission line for the comparator used in a mho characteristic application.
BASIC PRINCIPLE OF OPERATION It has been shown in a previous paper [2] that the operating criterion for double-input symmetrical phase comparator is expressed by
where ¢ is the phase difference between the two relaying signals and a is the limit of comparison (trip angle or the phase-angle setting of the system). The double-input symmetrical phase comparator has been widely used in phase-comparison carrier systems and distance-type protective schemes to produce certain characteristics,as shown in Fig. 1. In a previous paper [6], a new technique of phase comparison was presented, in which comparison periods equal to the trip angle of the system and related to the positive-tonegative and negative-to-positive zero crossings are assigned to each relaying signal in such a way that
148 j.X ZL
__. SA
-|.-'X~ IA
ZL
27-.I A.~ [~IS.-'" I i
(a)
I(oad
SA - ~ [ - - " ~
SB~q
t
IB
.- _..su e/
-,
/
t rj p
~ I
SA
I
', )
OUTPUT OF COMPARATOR -:=
( a
t
)
TRIP ZONE "
//-
SB ~
(b)
Fig. 1. D o u b l e - i n p u t s y m m e t r i c a l , p h a s e c o m p a r a t o r . (a) Simplified p o w e r s y s t e m . P.C. -- p h a s e c o m p a r a t o r . ( b ) P h a s e - c o m p a r i s o n carrier s y s t e m . S A = I A = locale n d c u r r e n t signal; S B = I B = r e m o t e - e n d c u r r e n t signal d e t e c t e d a t t h e local end. (c) M h o characteristic d i s t a n c e relay. SA -- --VA = local-end p h a s e voltage; S B = - - V A + I A Z r w h e r e Z r is t h e replica i m p e d a n c e ; a 2 -- 90 ° = trip angle o f t h e s y s t e m .
n = 180/~ where a is the width of the comparison period, equal to the trip-angle of the system, and n is the number of comparison periods, assigned per half cycle of the relaying signal (integral number). This is shown in Fig. 2 for a trip angle o f 90 °, i.e. t w o comparison periods per half cycle of the relaying signal. The comparison process is performed in such a way that each two corresponding comparison periods are compared in a separate coincidence gate and the o u t p u t s of all coincidence gates are combined in an OR gate delivering the o u t p u t of the phase comparator [6, 7]. This method will be defined b y the single-stage phase~comparison m e t h o d in which the maximum operating time is 5 ms (corresponding to the width of the 90 ° trip angle). It should be mentioned that there are certain specified trip angles which satisfy the operating criterion; these are indicated in Table 1. It can be concluded that for large trip angles the maximum operating time may reach a large value, as in mho characteristic distance relays where the trip angle is 90 °, and in phase-comparison carrier systems incorporating EHV transmission lines where the trip angle may reach 60 °. In such cases it is desirable to reduce the operating time to a minimum.
]
-.COMPARISON ut R[OD
B4 I1."B, ~ B2"~" B~ I /"
~
/
. 90%.-
J1
t
B4/ / 17" J
COINCIDENCE PERIOD
HzR (b)
R
r,
Fig. 2. M o d e o f c o m p a r i s o n in the single-phase comparison s y s t e m for a t r i p angle o f 90 °. (a) C r i t e r i o n
f o r tripping. ( b ) C r i t e r i o n for blocking.
The two-stage phase-comparison technique which is described here is considered to be two single-stage phase-comparison processes in which another set of comparison periods is generated in precisely the same way as that of the single-stage technique but displaced b y half the trip angle, as shown in Fig. 3 for the relaying signal SA at a trip angle of 90 °. For such a trip angle, the comparison periods are as follows. (1) Stage-1 comparison periods: A 1 , A 2 , A 3 and A4 related to the positive-tonegative and negative-to-positive zero crossings. (2) Stage-2 comparison periods: A 1 ' , A 2 ', A 3 ' and A4' displaced by half the trip angle (45 °) from the stage-1 comparison periods. In the two-stage comparison process, the comparison periods of the restraining signal SB are generated in the same way as those of TABLE 1 No. o f c o m p a r i s o n p e r i o d s assigned/ h a l f cycle
T r i p angle (deg)
Max. o p e r a t i n g t i m e (ms)
1 2 3 4 5 6 8
180 90 60 45 36 30 22.5
10 5 3.33 2.5 2 1.66 1.25
149 __.SA
STAGE-I C O M P A R I S O N 5A .......-., PERIODS
--I,,,'k Ii / i
.,
" -I
': .__.%:
~'/~5'% 90"--~ "",.
SA L--" ....
,~.4"1 ~:1 I ' " # ~ 1
,-"
"..... A'31
,~ ' '
"'STAGE-2
COMPARISON PERIODS
"" .... ""
Fig. 3. Comparison periods of relaying signal S A in the two-stage phase-comparison system for a trip angle o f 90 °.
the polarising signal SA, as shown in Fig. 4. These are as follows. (1)Stage-1 comparison periods: B1, B2, Bs and B4 related to the positive-tonegative and negative-to-positive zero crossings. (2) Stage-2 comparison periods: BI', B2', B~' and B4' displaced by half the trip angle in the same direction as the trip angle for the comparison periods of SA. The stage-1 phase comparator performs the comparison process between the stage-1 comparison periods of the two relaying signals SA and SB in such a way that each two corresponding comparison periods are compared in a separate coincidence gate as follows: (A1, B1), ( A 2 , B 2 ) , ( A s , B s ) and ( A 4 , B 4 ) , the outputs of the four coincidence AND gates, are combined in an OR gate, delivering a train of pulses in the blocking region of period 90 ° or a zero DC level in the trip region. The stage-2 phase comparator performs the comparison process between stage-2 comparison periods of the two relaying signals SA and SB in the same way as that performed by the stage-1 comparator as follows: (AI', BI'), (A2', B2'), ( A 3 ', B3 '} and ( A 4 ' , B 4 ' ) . The output of the stage-1 phase comparator is combined with that of the stage-2 phase comparator by an OR gate, delivering a train of pulses in the blocking region having a period equal to half the trip angle of the sys. tern {45°), and a width depending on the prefault phase angle between the two relaying signals or a zero DC level in the trip region. The o u t p u t of the OR gate is applied to an a/2 pulse stretcher which acts either on the negative going edge of the pulses or on the positive going edge. The m a x i m u m operating time in this m e t h o d is reduced to half the value o f that in the single-stage process. Figure 5 illustrates the operating criterion for a trip angle of 60 ° of a typical phase-com-
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', I k.-90"~4 COINCI;?ENCE
i-
STAGE-2
i--i
PULSES
~-;~9°'~
COINCIDENCE PULSES
J']~"'-~J--L [-[ N [] rLjI-! ( a ) OUTPUT OF"OR'GATE
t
(b)
Fig. 4. (a) Mode o f comparison in the two-stage phase-comparison system for a trip angle o f 90 ° in the m h o characteristic distance relay: criterion for blocking. S A = --VA; S B = - - V A + IAZr. (b) Block diagram of the two-stage phase comparator. PFC 1 = stage-1 pulse-forming circuit; PFC 2 = stage-2 pulseforming circuit; P.C. = phase c o m p a r a t o r ( A N D gate); ST 45 = 45 ° pulse-stretcher circuit; BPF 50 = 50 Hz centre f r e q u e n c y bandpass filter; A - B = A1, A2, A3, A 4 - B1, B2, B3, B4; A ' - B' = A I ' , A2', A3', A4' - B I ' , B2' , B3' , B4'.
parison carrier system, while Fig. 6 shows the block diagram of a three-stage phase comparator, in which the maximum operating time can be reduced to a value corresponding to one-third of the trip angle of the system. It should be noted that a three-stage phase-corn-
.-----!..%
ii
!
.---..
~ 60",-- ;,
STAGE-I COINCIDENCE
PULSES
60° I-- ',i 1! ,"I, ~ n n STAGE-2 COINCIDENCE PULSES
t
OUTPUT OF'OR" GATE
Fi~. 5. Mode o f comparison in the two-stage phasecomparison system for a trip angle o f 60 ° in t h e current phase-comparison carrier system. SA = IA = localend current signal; S B = IB = r e m o t e - e n d signal detected at the local end.
150
,qur,~t
I~
~ U:4~.LL--I t-AE-Y
The filter n e t w o r k used in the p r o t o t y p e c o m p a r a t o r is a bandpass filter with a 50 Hz c e n t r e f r e q u e n c y , a passband gain o f 42 dB, and Q = 5.4. T h e phase delay p r o d u c e d b y the filter n e t w o r k is 4 × 10 - 4 degree in the passb a n d . The filter n e t w o r k is s h o w n in Fig. 7. •
L
.fi
T;,,
g'
Fig. 6. Block diagram of the three-stage phase comparator system for a trip angle of 90 °. PFC 1,2,3 are stage-l, -2, and-3 pulse-forming circuits, respectively; P.C. = phase comparator; ST 30 = 30 ° pulse-stretcher circuit. parison process can be achieved in t h e same way b y generating t h r e e sets o f c o m p a r i s o n periods displaced b y o n e - t h i r d o f t h e trip angle f r o m each o t h e r , and in such a case the m a x i m u m o p e r a t i n g time is r e d u c e d t o onet h i r d o f t h a t in the single c o m p a r i s o n m e t h o d . FILTER NETWORK Since the phase c o m p a r a t o r u n d e r discussion is i n h e r e n t l y very fast, it follows t h a t the rejection of non-power-frequency compon e n t s ( h i g h - f r e q u e n c y as well as DC o f f s e t c o m p o n e n t s ) present in t h e relaying signals is essential. T h e a p p e a r a n c e o f h i g h - f r e q u e n c y comp o n e n t s is a c c o u n t e d for b y the fact t h a t at the initial m o m e n t o f an e a r t h fault t h e capacitance o f t h e e a r t h e d phase loses its charge, while t h e capacitances o f t h e o t h e r t w o phases acquire an a d d i t i o n a l charge, because t h e i r voltages relative t o e a r t h rise t o a phaseto-phase voltage value. This process o f discharging and a d d i t i o n a l charging o f the phase capacitances takes the f o r m o f periodic d a m p e d oscillations. In p o w e r circuits with large zero s e q u e n c e resistance and in t h e e v e n t o f distant faults associated with an increase in resistance o f the oscillatory circuit, the process o f discharge and recharge m a y app r o x i m a t e t h e aperiodic f o r m . T h e f r e q u e n c y o f these t r a n s i e n t c u r r e n t s changes with t h e p a r a m e t e r s o f the p o w e r circuit f r o m 200 t o 3 0 0 0 Hz, while t h e d a m p i n g t i m e is very small and varies f r o m 0.01 t o 0 . 0 2 5 s. T h e a p p e a r a n c e o f a DC o f f s e t c o m p o n e n t at t h e instant o f fault i n c e p t i o n is a c c o u n t e d for b y the e f f e c t o f the reactive c o m p o n e n t s o f t h e s y s t e m impedances.
fl
cJ A '&l
Fig. 7. Filter network used in the prototype comparator, f0 (centre frequency) = 50 Hz;H 0 (passband gain) = 42 dB; Q (f0!~f2 -- fl)) = 5.4; phase delay in passband = 4 × 10- degree; type of integrated circuit = Fairchild ~A 741.
STUDIES OF RELAY APPLICATION Studies o f the relay response w h e n applied t o a t y p i c a l 40 km, 132 kV, 12.4 / 70 ° transp o s e d single-circuit line with the configurat i o n s h o w n in Fig. 8 have b e e n p e r f o r m e d . 291. m
183+ 183~m i
13~m
Fig. 8. 132 kV single-circuit line configuration. All strands are of 0.28 cm diameter. Phase conductors are 30/7 steel-scored aluminium. Neutral conductor is 12/7 steel-cored aluminium. The n o m i n a l shape o f the characteristic is the m h o - t y p e distance relay which is prod u c e d b y means o f the t w o relaying signals SA and SB : S A =
_
VA
S B =
_
VA
+
ZrI A
w h e r e VA is the phase voltage r e f e r r e d to the s e c o n d a r y side o f the voltage t r a n s f o r m e r at the local end, IA is the line c u r r e n t r e f e r r e d to the s e c o n d a r y side o f c u r r e n t t r a n s f o r m e r at the local end, and Z~ is the replica i m p e d a n c e . T h e t w o relaying signals SA and SB are derived with the help o f a t r a n s a c t o r , an auxiliary c u r r e n t t r a n s f o r m e r and an auxiliary p o t e n t i a l t r a n s f o r m e r .
151 TESTS CARRIED
OUT
A hardware unit was built to realise a mho characteristic distance relay for protection of the model transmission line under consideration. The p r o t o t y p e circuits were designed to use readily available components and to demonstrate the design operations. The principle integrated circuits used were the Fairchild (~A 741) series transistor-transistorlogic circuits. With the arrangement illustrated in Fig. 4, tests were performed on the model transmission line. Voltage and current signals were derived from a single-phase test circuit having a source impedance of 0.5/75 °, and a line impedance of 1 . 0 / 7 0 °. A variable fault resistance was connected in series with a point-on-wave switch which controlled the instant of fault incidence. The voltage and current measured at the junction of the source and line impedance were transformed and mixed to produce signals SA and SB. Z~ was set at 1.0 p.u.[ 77 °.
(a) (b)
(c)
Fig. 10. Comparison periods A 3 and A 1 of relaying signal S A. (a) Zero-crossing signal of SA; (b) comparison period A3; (c) comparison period A 1.
(a)
(h) (c) (a)
(b) (c)
Fig. 9. Comparison periods A 2 and A 4 of relaying signal S A. Ca) Zero-crossing signal of SA; (b) comparison period A2; (c) comparison period A 4.
Fig. 11. Comparison periods A 3' and A 1' of relaying signal S A. (a) Zero-crossing signal of SA; (b) comparison period (inverted) A3'; (c) comparison period (inverted) A 1'.
(a) (b)
Some steady~state sample oscillograms are reproduced in Figs. 9 - 14. The performance o f the relay during critical system conditions was tested. The operating times for a number of fault impedances were measured for faults at zero voltage and at maximum voltage. The latter gives the maximum overshoot which was found to be o f the order of 6%, at which the maximum operating time was found to be 2.65 ms.
(c) (d)
Fig. 12. Output of stage-1 phase comparator at 55 ° phase displacement between S A and SB. (a) Zerocrossing signal of SA; (b) coincidence period of A 2 and B2; (c) coincidence period o f A 1 and B1; (d) output of stage-1 phase comparator.
152
(a)
(b) (c) (d) L
Fig. 13. Output of stage-2 phase comparator at 55 ° phase displacement between S A and S s. (a) Zerocrossing signal of SA; (b) coincidence period of A 3 and B3; (c) coincidence period of'A 1 and B1; (d) output of stage-2 phase comparator.
(a) (b) (c) (d)
Fig. 14. Output of phase comparator at 55 ° phase displacement between S A and S B. (a) Zero-crossing signal of SA; (b) coincidence~ period of A 1 and B1; (c) coincidence period of A 1' and BI'; (d) output of phase comparator. CONCLUSION
The application of 'phase-comparison' concepts in protective relaying has been presented. The main goal of the relaying sys-
tern is to reduce fault clearing times. The laboratory test results prove that the system has consistent high-speed performance. Speed varies from instantaneous to a time corresponding to half the trip angle of the system in the case of a two-stage phase-comparison process, and to a time corresponding to onethird the trip angle in the case of three-stage phase-comparison process. The relay performance has been examined during critical system conditions and shows quite satisfactory results.
REFERENCES 1 C. Adamson and E. A. Talkhan, The application of transistors to phase-comparison carrier protection, Proc. Inst. Electr. Eng., 107 (1960) 37 - 47. 2 W. D. Humpage and S. P. Sabberwal, Development in phase-comparison techniques for distance protection, Proc. Inst. Electr. Eng., 112 (1965) 1383 1394. 3 L. Jackson, J. B. Patrickson and L. M. Wedepohl, Distance protection: Optimum dynamic design of static relay comparators, Proc. Inst. Electr. Eng., 115 (1968) 280 - 287. 4 A. T. Johns, Generalised phase comparator techniques for distance protection -- basis of their operation and design, Proc. Inst. Electr. Eng., 119 (1972) 833 - 841. 5 P. G. McLaren and M. A. Redfern, Hybrid phase comparator applied to distance protection, Proc. Inst. Electr. Eng., 122 (1975) 1294 - 1300. 6 S. M. El-Sobki, A. A. E1-Alaily and A. S. El-Din, Microwave techniques adopted for the primary and secondary protection of EHV lines. -- Part I. Fundamental studies and scheme outlines, Electr. Power Syst. Res., 2 (1979) 221 - 226. 7 S. M. E1-Sobki, A. A. E1-Alaily and A. S. El-Din, Microwave techniques adopted for the primary and secondary protection of EHV lines. Part II. Performance under different working conditions, Electr. Power Syst. Res., 2 (1979) 227 - 232.
147
A High-Speed Two-Stage Phase Comparator A. A. EL-ALAILY and OMAR A. S. EL-DIN*
Faculty of Engineering, Cairo University, Cairo (Egypt) (Received November 10, 1980)
SUMMARY The paper reports further work on phasecomparison techniques by the same authors. The technique reported here describes the operation o f a double-input phase comparator based on a two-stage phase-comparison process between two relaying signals used to measure the phase difference between them, and hence it can be applied to either the phase-comparison carrier or distance protection systems. The comparator reduces the maximum operating time to a value corresponding to half the trip angle o f the system. An example is given o f its application to a mho characteristic distance relay when applied to protect a typical 40 km, 132 k V single-circuit transposed line.
INTRODUCTION The phase-comparison carrier system in its simplest form may be regarded as a singlephase comparison of two signals which are functions of the line currents at each end of the protected section [1], whereas, in distance-type protection schemes, the conventional double-input phase comparator measures the phase difference between the two relaying signals which are functions of the line current and phase voltage at one end of the protected line section [2]. Although numerous techniques have been reported [3 - 5] for developing the phasecomparison process, most of them are susceptible to system transients and other spurious signals by virtue of their nearly instantaneous operation or a higher measuring
*Present address: Electrical Consultants Group, 8 EI-Heussain Ebn-Ali St., West Heliopolis, Cairo, Egypt.
accuracy may be obtained b y making sacrifices in the operating speed (block average comparator), in which case the comparator may either fail to trip or will give rise to unacceptably long operating times at the threshold conditions. The phase comparator described here reduces the maximum operating time to a value corresponding to half the trip angle of the system, irrespective of the value of the prefault or postfault phase difference between the relaying signals. The hardware necessary to realise the comparator consists of readily available and well-established small-scale and medium-scale integrated circuits. Results are given for off-line tests on a model transmission line for the comparator used in a mho characteristic application.
BASIC PRINCIPLE OF OPERATION It has been shown in a previous paper [2] that the operating criterion for double-input symmetrical phase comparator is expressed by
where ¢ is the phase difference between the two relaying signals and a is the limit of comparison (trip angle or the phase-angle setting of the system). The double-input symmetrical phase comparator has been widely used in phase-comparison carrier systems and distance-type protective schemes to produce certain characteristics,as shown in Fig. 1. In a previous paper [6], a new technique of phase comparison was presented, in which comparison periods equal to the trip angle of the system and related to the positive-tonegative and negative-to-positive zero crossings are assigned to each relaying signal in such a way that
148 j.X ZL
__. SA
-|.-'X~ IA
ZL
27-.I A.~ [~IS.-'" I i
(a)
I(oad
SA - ~ [ - - " ~
SB~q
t
IB
.- _..su e/
-,
/
t rj p
~ I
SA
I
', )
OUTPUT OF COMPARATOR -:=
( a
t
)
TRIP ZONE "
//-
SB ~
(b)
Fig. 1. D o u b l e - i n p u t s y m m e t r i c a l , p h a s e c o m p a r a t o r . (a) Simplified p o w e r s y s t e m . P.C. -- p h a s e c o m p a r a t o r . ( b ) P h a s e - c o m p a r i s o n carrier s y s t e m . S A = I A = locale n d c u r r e n t signal; S B = I B = r e m o t e - e n d c u r r e n t signal d e t e c t e d a t t h e local end. (c) M h o characteristic d i s t a n c e relay. SA -- --VA = local-end p h a s e voltage; S B = - - V A + I A Z r w h e r e Z r is t h e replica i m p e d a n c e ; a 2 -- 90 ° = trip angle o f t h e s y s t e m .
n = 180/~ where a is the width of the comparison period, equal to the trip-angle of the system, and n is the number of comparison periods, assigned per half cycle of the relaying signal (integral number). This is shown in Fig. 2 for a trip angle o f 90 °, i.e. t w o comparison periods per half cycle of the relaying signal. The comparison process is performed in such a way that each two corresponding comparison periods are compared in a separate coincidence gate and the o u t p u t s of all coincidence gates are combined in an OR gate delivering the o u t p u t of the phase comparator [6, 7]. This method will be defined b y the single-stage phase~comparison m e t h o d in which the maximum operating time is 5 ms (corresponding to the width of the 90 ° trip angle). It should be mentioned that there are certain specified trip angles which satisfy the operating criterion; these are indicated in Table 1. It can be concluded that for large trip angles the maximum operating time may reach a large value, as in mho characteristic distance relays where the trip angle is 90 °, and in phase-comparison carrier systems incorporating EHV transmission lines where the trip angle may reach 60 °. In such cases it is desirable to reduce the operating time to a minimum.
]
-.COMPARISON ut R[OD
B4 I1."B, ~ B2"~" B~ I /"
~
/
. 90%.-
J1
t
B4/ / 17" J
COINCIDENCE PERIOD
HzR (b)
R
r,
Fig. 2. M o d e o f c o m p a r i s o n in the single-phase comparison s y s t e m for a t r i p angle o f 90 °. (a) C r i t e r i o n
f o r tripping. ( b ) C r i t e r i o n for blocking.
The two-stage phase-comparison technique which is described here is considered to be two single-stage phase-comparison processes in which another set of comparison periods is generated in precisely the same way as that of the single-stage technique but displaced b y half the trip angle, as shown in Fig. 3 for the relaying signal SA at a trip angle of 90 °. For such a trip angle, the comparison periods are as follows. (1) Stage-1 comparison periods: A 1 , A 2 , A 3 and A4 related to the positive-tonegative and negative-to-positive zero crossings. (2) Stage-2 comparison periods: A 1 ' , A 2 ', A 3 ' and A4' displaced by half the trip angle (45 °) from the stage-1 comparison periods. In the two-stage comparison process, the comparison periods of the restraining signal SB are generated in the same way as those of TABLE 1 No. o f c o m p a r i s o n p e r i o d s assigned/ h a l f cycle
T r i p angle (deg)
Max. o p e r a t i n g t i m e (ms)
1 2 3 4 5 6 8
180 90 60 45 36 30 22.5
10 5 3.33 2.5 2 1.66 1.25
149 __.SA
STAGE-I C O M P A R I S O N 5A .......-., PERIODS
--I,,,'k Ii / i
.,
" -I
': .__.%:
~'/~5'% 90"--~ "",.
SA L--" ....
,~.4"1 ~:1 I ' " # ~ 1
,-"
"..... A'31
,~ ' '
"'STAGE-2
COMPARISON PERIODS
"" .... ""
Fig. 3. Comparison periods of relaying signal S A in the two-stage phase-comparison system for a trip angle o f 90 °.
the polarising signal SA, as shown in Fig. 4. These are as follows. (1)Stage-1 comparison periods: B1, B2, Bs and B4 related to the positive-tonegative and negative-to-positive zero crossings. (2) Stage-2 comparison periods: BI', B2', B~' and B4' displaced by half the trip angle in the same direction as the trip angle for the comparison periods of SA. The stage-1 phase comparator performs the comparison process between the stage-1 comparison periods of the two relaying signals SA and SB in such a way that each two corresponding comparison periods are compared in a separate coincidence gate as follows: (A1, B1), ( A 2 , B 2 ) , ( A s , B s ) and ( A 4 , B 4 ) , the outputs of the four coincidence AND gates, are combined in an OR gate, delivering a train of pulses in the blocking region of period 90 ° or a zero DC level in the trip region. The stage-2 phase comparator performs the comparison process between stage-2 comparison periods of the two relaying signals SA and SB in the same way as that performed by the stage-1 comparator as follows: (AI', BI'), (A2', B2'), ( A 3 ', B3 '} and ( A 4 ' , B 4 ' ) . The output of the stage-1 phase comparator is combined with that of the stage-2 phase comparator by an OR gate, delivering a train of pulses in the blocking region having a period equal to half the trip angle of the sys. tern {45°), and a width depending on the prefault phase angle between the two relaying signals or a zero DC level in the trip region. The o u t p u t of the OR gate is applied to an a/2 pulse stretcher which acts either on the negative going edge of the pulses or on the positive going edge. The m a x i m u m operating time in this m e t h o d is reduced to half the value o f that in the single-stage process. Figure 5 illustrates the operating criterion for a trip angle of 60 ° of a typical phase-com-
ISTAGB]
"~
', I k.-90"~4 COINCI;?ENCE
i-
STAGE-2
i--i
PULSES
~-;~9°'~
COINCIDENCE PULSES
J']~"'-~J--L [-[ N [] rLjI-! ( a ) OUTPUT OF"OR'GATE
t
(b)
Fig. 4. (a) Mode o f comparison in the two-stage phase-comparison system for a trip angle o f 90 ° in the m h o characteristic distance relay: criterion for blocking. S A = --VA; S B = - - V A + IAZr. (b) Block diagram of the two-stage phase comparator. PFC 1 = stage-1 pulse-forming circuit; PFC 2 = stage-2 pulseforming circuit; P.C. = phase c o m p a r a t o r ( A N D gate); ST 45 = 45 ° pulse-stretcher circuit; BPF 50 = 50 Hz centre f r e q u e n c y bandpass filter; A - B = A1, A2, A3, A 4 - B1, B2, B3, B4; A ' - B' = A I ' , A2', A3', A4' - B I ' , B2' , B3' , B4'.
parison carrier system, while Fig. 6 shows the block diagram of a three-stage phase comparator, in which the maximum operating time can be reduced to a value corresponding to one-third of the trip angle of the system. It should be noted that a three-stage phase-corn-
.-----!..%
ii
!
.---..
~ 60",-- ;,
STAGE-I COINCIDENCE
PULSES
60° I-- ',i 1! ,"I, ~ n n STAGE-2 COINCIDENCE PULSES
t
OUTPUT OF'OR" GATE
Fi~. 5. Mode o f comparison in the two-stage phasecomparison system for a trip angle o f 60 ° in t h e current phase-comparison carrier system. SA = IA = localend current signal; S B = IB = r e m o t e - e n d signal detected at the local end.
150
,qur,~t
I~
~ U:4~.LL--I t-AE-Y
The filter n e t w o r k used in the p r o t o t y p e c o m p a r a t o r is a bandpass filter with a 50 Hz c e n t r e f r e q u e n c y , a passband gain o f 42 dB, and Q = 5.4. T h e phase delay p r o d u c e d b y the filter n e t w o r k is 4 × 10 - 4 degree in the passb a n d . The filter n e t w o r k is s h o w n in Fig. 7. •
L
.fi
T;,,
g'
Fig. 6. Block diagram of the three-stage phase comparator system for a trip angle of 90 °. PFC 1,2,3 are stage-l, -2, and-3 pulse-forming circuits, respectively; P.C. = phase comparator; ST 30 = 30 ° pulse-stretcher circuit. parison process can be achieved in t h e same way b y generating t h r e e sets o f c o m p a r i s o n periods displaced b y o n e - t h i r d o f t h e trip angle f r o m each o t h e r , and in such a case the m a x i m u m o p e r a t i n g time is r e d u c e d t o onet h i r d o f t h a t in the single c o m p a r i s o n m e t h o d . FILTER NETWORK Since the phase c o m p a r a t o r u n d e r discussion is i n h e r e n t l y very fast, it follows t h a t the rejection of non-power-frequency compon e n t s ( h i g h - f r e q u e n c y as well as DC o f f s e t c o m p o n e n t s ) present in t h e relaying signals is essential. T h e a p p e a r a n c e o f h i g h - f r e q u e n c y comp o n e n t s is a c c o u n t e d for b y the fact t h a t at the initial m o m e n t o f an e a r t h fault t h e capacitance o f t h e e a r t h e d phase loses its charge, while t h e capacitances o f t h e o t h e r t w o phases acquire an a d d i t i o n a l charge, because t h e i r voltages relative t o e a r t h rise t o a phaseto-phase voltage value. This process o f discharging and a d d i t i o n a l charging o f the phase capacitances takes the f o r m o f periodic d a m p e d oscillations. In p o w e r circuits with large zero s e q u e n c e resistance and in t h e e v e n t o f distant faults associated with an increase in resistance o f the oscillatory circuit, the process o f discharge and recharge m a y app r o x i m a t e t h e aperiodic f o r m . T h e f r e q u e n c y o f these t r a n s i e n t c u r r e n t s changes with t h e p a r a m e t e r s o f the p o w e r circuit f r o m 200 t o 3 0 0 0 Hz, while t h e d a m p i n g t i m e is very small and varies f r o m 0.01 t o 0 . 0 2 5 s. T h e a p p e a r a n c e o f a DC o f f s e t c o m p o n e n t at t h e instant o f fault i n c e p t i o n is a c c o u n t e d for b y the e f f e c t o f the reactive c o m p o n e n t s o f t h e s y s t e m impedances.
fl
cJ A '&l
Fig. 7. Filter network used in the prototype comparator, f0 (centre frequency) = 50 Hz;H 0 (passband gain) = 42 dB; Q (f0!~f2 -- fl)) = 5.4; phase delay in passband = 4 × 10- degree; type of integrated circuit = Fairchild ~A 741.
STUDIES OF RELAY APPLICATION Studies o f the relay response w h e n applied t o a t y p i c a l 40 km, 132 kV, 12.4 / 70 ° transp o s e d single-circuit line with the configurat i o n s h o w n in Fig. 8 have b e e n p e r f o r m e d . 291. m
183+ 183~m i
13~m
Fig. 8. 132 kV single-circuit line configuration. All strands are of 0.28 cm diameter. Phase conductors are 30/7 steel-scored aluminium. Neutral conductor is 12/7 steel-cored aluminium. The n o m i n a l shape o f the characteristic is the m h o - t y p e distance relay which is prod u c e d b y means o f the t w o relaying signals SA and SB : S A =
_
VA
S B =
_
VA
+
ZrI A
w h e r e VA is the phase voltage r e f e r r e d to the s e c o n d a r y side o f the voltage t r a n s f o r m e r at the local end, IA is the line c u r r e n t r e f e r r e d to the s e c o n d a r y side o f c u r r e n t t r a n s f o r m e r at the local end, and Z~ is the replica i m p e d a n c e . T h e t w o relaying signals SA and SB are derived with the help o f a t r a n s a c t o r , an auxiliary c u r r e n t t r a n s f o r m e r and an auxiliary p o t e n t i a l t r a n s f o r m e r .
151 TESTS CARRIED
OUT
A hardware unit was built to realise a mho characteristic distance relay for protection of the model transmission line under consideration. The p r o t o t y p e circuits were designed to use readily available components and to demonstrate the design operations. The principle integrated circuits used were the Fairchild (~A 741) series transistor-transistorlogic circuits. With the arrangement illustrated in Fig. 4, tests were performed on the model transmission line. Voltage and current signals were derived from a single-phase test circuit having a source impedance of 0.5/75 °, and a line impedance of 1 . 0 / 7 0 °. A variable fault resistance was connected in series with a point-on-wave switch which controlled the instant of fault incidence. The voltage and current measured at the junction of the source and line impedance were transformed and mixed to produce signals SA and SB. Z~ was set at 1.0 p.u.[ 77 °.
(a) (b)
(c)
Fig. 10. Comparison periods A 3 and A 1 of relaying signal S A. (a) Zero-crossing signal of SA; (b) comparison period A3; (c) comparison period A 1.
(a)
(h) (c) (a)
(b) (c)
Fig. 9. Comparison periods A 2 and A 4 of relaying signal S A. Ca) Zero-crossing signal of SA; (b) comparison period A2; (c) comparison period A 4.
Fig. 11. Comparison periods A 3' and A 1' of relaying signal S A. (a) Zero-crossing signal of SA; (b) comparison period (inverted) A3'; (c) comparison period (inverted) A 1'.
(a) (b)
Some steady~state sample oscillograms are reproduced in Figs. 9 - 14. The performance o f the relay during critical system conditions was tested. The operating times for a number of fault impedances were measured for faults at zero voltage and at maximum voltage. The latter gives the maximum overshoot which was found to be o f the order of 6%, at which the maximum operating time was found to be 2.65 ms.
(c) (d)
Fig. 12. Output of stage-1 phase comparator at 55 ° phase displacement between S A and SB. (a) Zerocrossing signal of SA; (b) coincidence period of A 2 and B2; (c) coincidence period o f A 1 and B1; (d) output of stage-1 phase comparator.
152
(a)
(b) (c) (d) L
Fig. 13. Output of stage-2 phase comparator at 55 ° phase displacement between S A and S s. (a) Zerocrossing signal of SA; (b) coincidence period of A 3 and B3; (c) coincidence period of'A 1 and B1; (d) output of stage-2 phase comparator.
(a) (b) (c) (d)
Fig. 14. Output of phase comparator at 55 ° phase displacement between S A and S B. (a) Zero-crossing signal of SA; (b) coincidence~ period of A 1 and B1; (c) coincidence period of A 1' and BI'; (d) output of phase comparator. CONCLUSION
The application of 'phase-comparison' concepts in protective relaying has been presented. The main goal of the relaying sys-
tern is to reduce fault clearing times. The laboratory test results prove that the system has consistent high-speed performance. Speed varies from instantaneous to a time corresponding to half the trip angle of the system in the case of a two-stage phase-comparison process, and to a time corresponding to onethird the trip angle in the case of three-stage phase-comparison process. The relay performance has been examined during critical system conditions and shows quite satisfactory results.
REFERENCES 1 C. Adamson and E. A. Talkhan, The application of transistors to phase-comparison carrier protection, Proc. Inst. Electr. Eng., 107 (1960) 37 - 47. 2 W. D. Humpage and S. P. Sabberwal, Development in phase-comparison techniques for distance protection, Proc. Inst. Electr. Eng., 112 (1965) 1383 1394. 3 L. Jackson, J. B. Patrickson and L. M. Wedepohl, Distance protection: Optimum dynamic design of static relay comparators, Proc. Inst. Electr. Eng., 115 (1968) 280 - 287. 4 A. T. Johns, Generalised phase comparator techniques for distance protection -- basis of their operation and design, Proc. Inst. Electr. Eng., 119 (1972) 833 - 841. 5 P. G. McLaren and M. A. Redfern, Hybrid phase comparator applied to distance protection, Proc. Inst. Electr. Eng., 122 (1975) 1294 - 1300. 6 S. M. El-Sobki, A. A. E1-Alaily and A. S. El-Din, Microwave techniques adopted for the primary and secondary protection of EHV lines. -- Part I. Fundamental studies and scheme outlines, Electr. Power Syst. Res., 2 (1979) 221 - 226. 7 S. M. E1-Sobki, A. A. E1-Alaily and A. S. El-Din, Microwave techniques adopted for the primary and secondary protection of EHV lines. Part II. Performance under different working conditions, Electr. Power Syst. Res., 2 (1979) 227 - 232.