Detection Of External Ground Faults - ABB RELION 670 Series Technical Manual

1MRK 502 066-UUS B
current[2] = max (I3PW1CT2)
EQUATION1527 V1 EN-US
current[3] = max (I3PW2CT1)
EQUATION1528 V1 EN-US
current[4] = max (I3PW2CT2)
EQUATION1529 V1 EN-US
current[5] = IN
EQUATION1530 V1 EN-US
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.
6.4.7.4

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 restrictedground 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.
An external fault is thus assumed to have occurred when a high neutral current suddenly appears,
while at the same time the differential current Idiff remains low, at least for a while. This condition
must be detected before a trip request is placed within REFPDIF. Any search for external fault is
aborted if a trip request has been placed. A condition for a successful detection is that it takes not
less than 4ms for the first CT to saturate.
For an internal 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 detected, then it must be an internal fault.
If an external fault has been detected, then the REFPDIF is temporarily desensitized.
Directional criterion
The directional criterion is applied in order to positively distinguish between internal and external
ground faults. This check is an additional criterion, which should prevent malfunctions at heavy
external ground faults, and during the disconnection of such faults by other protections. Ground
faults on lines connecting the power transformer occur much more often than ground faults on a
power transformer winding. It is important therefore that the Restricted ground fault protection,
Technical manual
1
×
CTFactorPri2
1
×
CTFactorSec1
1
×
CTFactorSec2
Section 6
Differential protection
(Equation 31)
(Equation 32)
(Equation 33)
(Equation 34)
M5447-75 v11
M5447-110 v11
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