Protection Application Handbook (англ. яз.)
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SYNCHRO CHECK DEVICE
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- a moderate operation time, typically less than 0.2-0.5 seconds.
6.3 HIGH-SPEED AUTO RECLOSING WITH SYN-
CHRONISM CHECK
In power systems where the stability is strained under periods of
high load, one would like to reclose quickly in order to improve the
stability. If the parallel interconnection is unreliable, for instance
in a system like fig. 29b or 29c, it can be risky to perform a
high-speed reclosing without synchronism check. One can then
be forced to refrain from reclosing, rely on single pole auto-re-
closing at single-phase faults or take a chance with delayed au-
to-reclosing.
In the latter case the Auto reclosing may be blocked at high loads,
which is unfortunate, especially if a high-speed reclosing could
have saved the stability. In such cases it would be preferable to
adopt high-speed reclosing with dead line check and synchro-
nism check.
When a parallel line is in service Auto-reclosing can then be per-
formed. On the other hand, when the parallel circuit is out of ser-
vice, the reclosing will be blocked as the synchronism is most
likely lost during the dead time.
Auto-reclosing with dead line and synchronism check can in
many cases be applied also for lines near a power station.
By choosing to energize the line from the remote end the strain
at reclosing into a persistent fault will be moderate. Because the
remote end is retripped at a persistent fault, the end near the
power station will not be reclosed. Only after a successful line re-
energizing the synchronism check conditions will be met and the
line reclosing completed.
At a controlled high-speed Auto reclosing the dead line check is
made during the dead interval of 0.4-1.0 seconds and a time for
dead-line check of 0.2 seconds can be allowed. The synchronism
check is made after line reenergizing and the check should be
quick to avoid unnecessary delay of the reclosing. It must howev-
er wait about 0.1 seconds for a possible retripping of the remote
end. Assuming a dead interval of 0.4 seconds, a synchronism
check time of 0.2 seconds and time for circuit-breaker closing of
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0.1 seconds the open-circuit time will be 0. 4 + 0. 2 + 0.1 seconds
= 0.7 seconds.
One must expect a considerable power system swing at
high-speed reclosing of long transmission lines in a system with
threatened stability. Therefore, the synchronism check device
must not be too sensitive, but allow fairly high values of phase
shift and frequency difference. Values of 70° and 200 mHz are re-
alistic. The synchronism check shall be supplemented by a timer
to interrupt the auto-reclosing if the check conditions are not met
within a short time, e.g. 1 seconds
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6.4 ARRANGEMENT OF SYNCHRO-CHECK UNITS
FOR AUTO RECLOSING
In Fig. 30 is indicated Auto reclosing with dead line check at one
end and synchronism-check at the other end. It would thus be
sufficient with a live-bus/dead-line check unit in the first and a
synchronism check device in the other end. Complete syn-
chro-check devices with live bus/dead-line check facility are how-
ever usually installed at both ends. In this way the direction of line
reenergizing can easily be changed by change of settings.
Figure 30. Dead line check (DLC) and synchronism check (SC) at auto-re-
closing. 25 = DLC/SCdevice
Normally there are several lines connected to a substation. It is
practical to install a synchro-check device for each line cir-
cuit-breaker. In case of a simultaneous tripping of two lines the
Auto reclosers can then operate independent of each other. Si-
multaneous faults could occur at lightning strokes in towers com-
monly used for two lines. Furthermore, by use of one
synchro-check device per line breaker there will not be any need
for complicated selection in the voltage and trip circuits.
In stations of breaker- and -a-half, double breaker or ring busbar
type, two circuit-breakers per line end are tripped at line faults
and shall be reclosed.
The line is energized by one circuit-breaker and if the fault has
disappeared, then the other breaker is also reclosed. The order
is determined by a priority circuit between the reclosers. In Fig.
31 is shown successive reclosing with dead line check of the first
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CB closing (Al) and synchronism check of the second CB (A2).
Should however the first CB be out of service at the fault, no re-
closing is made with this CB.
In that case the second CB shall energize the line and the syn-
chro-check device shall be arranged to permit dead line charging.
In the other station sequential Auto reclosing of the CB’s is also
used but combined with synchronism check as indicated in
Fig. 31.
The four synchro-check devices are set in such manner, that A1
and A2 permit reclosing at dead line or at synchronism condi-
tions, but devices B1 and B2 require synchronism at reclosing.
Figure 31. Dead line check and synchronism check at auto-reclosing in a
One- and a half-breaker station and a Ring busbar station
6.5 SYNCHRO-CHECK RELAY/FUNCTION.
The synchro-check relay measures in a single phase mode the
voltage on both sides of the open circuit breaker and it checks:
- Voltage levels
- Difference in voltage level
- Phase angle difference
- Frequency difference
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When a voltage check function is used it can be set to allow en-
ergizing in one or the other, or both directions, or be blocked. To
suit different applications there are wide range of application se-
lections or/and settings, which differ with respect to speed and
frequency difference sensitivity.
The functions are made to suit the conditions at Quick and De-
layed Auto-reclosing as discussed above. It is then set for contin-
uous output signal as long as the conditions are fulfilled and it
releases the auto-recloser.
The main characteristics for a suitable Synchro check function
are as follows:
Synchro chec
k unit (paralleling check):
Limit for normal voltage U
H
: 45-80% of Un
Max voltage difference, ∆U = 5-75% of Un.
Max phase angle difference, ∆Ø=5 -75°.
Max frequency difference, ∆f = 20-200 mHz
Operation time: t = 0.2-0.5 seconds
V
oltage check unit (dead line check):
Limit for dead equipment, U
L
=30-80% of Un
Limit for live equipment, U
H
= 80% of Un
Operation time: t = 0.1-0.2 seconds.
6.6 7. EXAMPLE OF SYNCHRO CHECK FUNCTION
SETTINGS
Reference is made to Users Guide for the different functions/
relays.
Application: High-speed auto-reclosing.
On all four SC/VC functions in Fig. 29 above are set:
Nominal voltage U1, U2: Un =110/63.5 V
- Voltage difference ∆U: e.g. 15% of Un
- Phase angle difference ∆Ø: e.g. 45 °
The Synchro check function is set for continuous output signal
when conditions are fulfilled.
Unit A1 is set for line energizing: Dead Line - Live Bus DLLB
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Continuous output
No delay.
Dead line voltage:
The unit A2 can be set at energizing “BOTH” i. e. allowing ener-
gizing of one or the other direction. The other settings are as for
A1.
The B1 and B2 units are set with Dead line energizing blocked.
The other settings are as for A1.
7. LINE PROTECTION SYSTEM
7.1 GENERAL DESIGN
A block diagram for a EHV/HV line protection system is shown in
Appendix 1.
A redundant protection scheme with two Distance protection re-
lays is provided. The high resistive earth faults are detected by a
Directional earth fault relay (DEF), including communication with
remote end in a permissive or blocking scheme. The DEF is pro-
vided in one of the systems only as the consequences for the
power network are smaller due to the lower magnitude of fault
current and as the fault is in one phase only. These high ohmic
faults will also develop to a low ohmic fault if not cleared within a
reasonable time and fault clearance is thus secured in both sub
systems.
An Auto-Reclosing device is provided to restore the normal oper-
ation as quick as possible. To verify that Auto-Reclosing can take
place at multi-phase faults a Synchro check function is provided.
To measure the distance to the fault, thus simplifying the localiza-
tion, a Fault locator is supplied as an option to the Distance pro-
tection. This will much improve the time to repair for a fault on a
long transmission line.
A Fault recorder is included to register the analogue signal at the
faults, enabling a post fault analyze to check the behavior of the
protection system and by that improve the total reliability.
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A Breaker failure relay is provided to transfer the trip to the sur-
rounding breakers. This is only used for the case where the line
breaker fails to trip.
In Appendix 2 a MV/HV cable line protection based on a Differen-
tial relay and a back-up protection with directional short circuit
and Earth fault protection.
Appendix 1
The protection system Block diagram for a EHV/HV Line.
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Appendix 2
The protection system Block diagram for a HV/MV Cable.
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