Two-Phase Time Overcurrent Protection (Non-Directional Only); Busbar Protection By Use Of Reverse Interlocking - Siemens siprotec SJ62 User Manual

2 Functions
2.2.8

Two-Phase Time Overcurrent Protection (non-directional only)

2.2.9

Busbar Protection by Use of Reverse Interlocking

Application
Example
73
Two-phase time overcurrent protection is used in isolated or resonant-grounded
systems where interaction with existing two-phase protection equipment is required.
Since an isolated or resonant-grounded system can still be operated with a ground
fault in one phase, this protection function detects double ground faults with high
ground fault currents. Only in the latter case, should a faulted feeder be shut down.
Measuring in two phases is sufficient to this end. Only phases A and C are monitored
in order to ensure selectivity of the protection in the network system.
If 250 50/51 2-ph prot (settable in P.System Data 1) is set to ON, I
for threshold comparison. If the fault is a simple ground fault in B, the element will not
pick up. Only after pickup on A or C a double ground fault is assumed, causing the
element to pick up and trip after the delay time has elapsed.
Note
With inrush recognition activated and inrush only on B, no crossblocking will take place
in the other phases. On the other hand, if inrush with crossblocking is activated on A
or C, B will also be blocked.
Each of the overcurrent elements can be blocked via binary inputs of the relay. A
setting parameter determines whether the binary input operates in the normally open
(i.e. actuated when energized) or the normally closed (i.e. actuated when de-ener-
gized) mode. This allows fast busbar protection to be applied in star systems or open
ring systems by utilizing "reverse interlocking". This principle is often used, for exam-
ple, in distribution systems, auxiliary systems of power plants, and the like, where a
station supply transformer supplied from the transmission grid serves internal loads of
the generation station via a medium voltage bus with multiple feeders (Figure 2-12).
The reverse interlocking principle is based on the following: time overcurrent protec-
tion of the busbar feeder trips with a short time delay 50-2 DELAY independent of the
grading times of the feeders, unless the pickup of the next load-side time overcurrent
protection element blocks the bus protection (Figure 2-12). Always the protection
element nearest to the fault will trip with the short time delay since this element cannot
be blocked by a protection element located behind the fault. Time elements 50-1
DELAY or 51 TIME DIAL are still effective as backup element. Pickup signals output
by the load-side protective relay are used as input message „>BLOCK 50-2" via a
binary input at the feeder-side protective relay.
is not used
B
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015