Operation P54x/EN OP/La4
MiCOM P543, P544, P545 & P546
(OP) 5-25
OP
1.1.7 Stub bus protection
The P54x relays include a facility to provide stub bus protection. When the line isolator is
open, an auxiliary contact from the isolator can energize an input on the relay to enable this
protection. When stub bus mode is enabled, all current values transmitted to the remote
relays, and all those received from remote relays, are set to zero. The protection will
now provide differential protection for the stub zone.
The local biased protection will operate as normal using the local currents with the exception
that no differential, or permissive intertrips will be sent to the remote relay(s) and all such
intertrips received will be ignored and also any trip will be 3 pole only. Direct Intertrips still
operate as normal.
Figure 11 Stub bus protection
For an internal fault the relay will operate, tripping the two local circuit breakers. When in
stub bus mode, the relay will not send a differential intertrip signal.
In P544 and P546 with 2 sets in Stub bus mode the currents from the two sets of CTs are
used for differential protection.
However in P543 or P545 when in stub bus mode there is only 1 set of CT which effectively
means if the current is above Is1 the relay will trip as there is no effective bias.
In stub bus mode if the communication fails then the differential protection is lost. A possible
way to provide protection then would be to apply a high set overcurrent protection which can
be enabled by settings when communication fails and in Stub bus mode.
1.1.8 The minimum operating current
The minimu
m operating current is related, but not equal to, the Is1 setting.
Consider a single end fed fault with no load but fault current, I:-
|
diff| =
|bias| = ½
Assuming |bias| < s2, then, using the equations from section 1.1, the relay will operate if:-
|
diff| > k1.| bias| + s1 or
> k1.½ + s1 or
> s1 / (1 - 0.5 k1)
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