Detection Of External Ground Faults - ABB Relion Technical Manual

Section 6
Differential protection
6.2.7.4
126
The bias current is the highest current of all separate input currents to REFPDIF (87N),
that is, of current in phase A, phase B, phase C, and the current in the neutral point
(designated as IN in figure
If there are two feeders included in the zone of protection of REFPDIF (87N), then the
respective bias current is found as the relatively highest of the following currents, that is,
those which are connected in an application:
current[1] = max (I3PW1CT1)
EQUATION1526 V1 EN
current[3] = max (I3PW2CT1)
EQUATION1528 V1 EN
current[5] = IN
EQUATION1530 V1 EN
The bias current is thus generally equal to none of the input currents. If all primary ratings
of the CTs were equal to IBase, then the bias current would be equal to the highest current
in Amperes. IBase shall be set equal to the rated current of the protected winding where
REFPDIF (87N) function is applied.

Detection of external ground faults

External faults are more common than internal ground faults for which the restricted
ground fault protection should operate. It is important that the restricted ground fault
protection remains stable during heavy external ground and phase-to-phase faults, and
also when such a heavy external fault is cleared by some other protection such as
overcurrent, or ground fault protection. The conditions during a heavy external fault, and
particularly immediately after the clearing of such a fault may be complex. The circuit
breaker's poles may not open exactly at the same moment, some of the CTs may still be
highly saturated, and so on.
The detection of external ground faults is based on the fact that for such a fault a high
neutral current appears first, while a false differential current only appears if one or more
current transformers saturate.
For an internal ground fault, a true differential current develops immediately, while for an
external fault it only develops if a CT saturates. If a trip request comes first, before an
external fault could be positively established, then it must be an internal fault.
If an external ground fault has been detected, then the REFPDIF (87N) is temporarily
desensitized.
44 and in figure 45).
1
×
CTFactorPri1
1
×
CTFactorSec1
1MRK 502 048-UUS A
(Equation 27)
(Equation 28)
(Equation 29)
Technical manual