Functions
2.2 Overcurrent Protection 50, 51, 50N, 51N
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
81
2.2.9
Two-phase Overcurrent Protection (Only Non-directional)
The two-phase overcurrent protection functionality is used in grounded or compensated systems where inter-
action with existing two-phase protection equipment is required. As an isolated or resonant-grounded system
remains operational with a single-phase ground fault, this protection serves the purpose of detecting double
ground faults with high ground fault currents. Only then must the respective feeder be switched off. A two-phase
measurement is sufficient for this purpose. In order to ensure selectivity of the protection in this section of the
system, only phases A and C are monitored.
If
250
50/51 2-ph prot
(settable in
P.System Data 1
) is set to
ON
,
I
B
is not used 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 cross blocking will take place in the other phases.
On the other hand, if inrush with cross blocking is activated on A or C, B will also be blocked.
2.2.10
Fast Busbar Protection Using Reverse Interlocking
Application Example
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-energized) mode. This allows, for example, the busbar protection to take immediate
effect in star systems or looped systems which are open on one side, utilizing "reverse interlocking". This prin-
ciple is often used, for example, 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 2-16).
The reverse interlocking principle is based on the following: Time overcurrent protection of the busbar feeder
trips with a short time delay T 50-2 independent of the grading times of the feeders, unless the pickup of the
next load-side overcurrent protection element blocks the busbar protection (Figure 2-16). Always the protection
element nearest to the fault will trip with the short time delay since this element cannot be blocked by a protec-
tion element located behind the fault. Time elements T 50-1 or T51 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.
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