Ground Fault Location (In Isolated Systems) - Siemens siprotec SJ62 User Manual

2.12.5 Ground Fault Location (in isolated systems)

Application
Example
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015
Pickup of the definite time elements can be stabilized by setting the dropout time 3121
50Ns T DROP-OUT. This time is started and maintains the pickup condition if the
current falls below the threshold. The function thus does not drop out instantaneously.
The trip delay time continues in the meantime. After the dropout delay time has
elapsed, the pickup is reported OFF and the trip delay time is reset unless the thresh-
old has been violated again. If the threshold is exceeded again while the dropout delay
time is still running, it will be cancelled. The trip delay time continues however. If the
threshold is still exceeded after the time has elapsed, a trip will be initiated immediate-
ly. If the threshold violation then no longer exists, there will be no response. If the
threshold is exceeded again after the trip command delay time has elapsed and while
the dropout delay time is still running, a trip will be initiated at once.
Directional determination can often be used to locate ground faults. In radial systems,
locating the ground fault is relatively simple. Since all feeders from a common busbar
(Figure 2-73) deliver a capacitive charging current, nearly the total ground fault current
of the system is available at the measuring point on the faulty line in the isolated
system. In resonant-grounded system it is the residual wattmetric current of the Pe-
tersen Coil that flows via the measuring point. Therefore, on the faulty cables a clear
"forward" decision is made whereas in other feeders either "reverse" direction is sent
back or no measurement is carried out in case ground current is too low. Definitely the
faulty line can be determined clearly.
Figure 2-73 Location of ground faults in a radial network
In meshed or ring systems, the measuring points of the faulty line also may detect the
maximum ground fault current (residual current). Only in this line, "forward" direction
is signaled at both ends (Figure 2-74). However, also the rest of the direction indica-
tions in the system may be useful for ground fault detection. Some indications may not
be output when ground current is too low.
2.12 Ground Fault Protection 64, 67N(s), 50N(s), 51N(s)
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