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Functions
2.2 Overcurrent Protection 50, 51, 50N, 51N
2.2.9
Two-phase Overcurrent Protection (Only Non-Directional)
The 2-phase overcurrent protection functionality is used in isolated or grounded systems where interaction with
existing 2-phase protection equipment is required. As an isolated or grounded system remains operational with
a 1-phase ground fault, this protection serves to detect double ground faults with high ground fault currents.
The respective feeder must be switched off only in this case. A 2-phase measurement is sufficient for this pur-
pose. In order to ensure selectivity of the protection in this section of the system, only phases A and C are mon-
itored.
If 250 50/51 2-ph prot (to be set under P.System Data 1) is set to ON, I
comparison. If the fault is a simple ground fault in B, no tripping occurs. Only in the case of a tripping on A or
C, a double ground fault is assumed. This leads to a pickup and after expiry of the time delay to a tripping.
Note
With inrush detection 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 current elements can be blocked via binary inputs. 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 fast busbar protection to be applied in star systems or open ring
systems by applying "reverse interlocking". This principle is often used, for example, in distribution systems,
auxiliary systems of power plants and similar systems, 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 2-18).
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 2-18). 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.
76
is not used for threshold value
B
SIPROTEC, 7SK80, Manual
E50417-G1140-C344-A5, Release date 11.2012
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