Detection Of External Earth Faults - ABB RET650 Technical Manual

1MRK 504 135-UEN A
6.2.7.4
Technical manual
If there are two feeders included in the zone of protection of REFPDIF, 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 function is applied.

Detection of external earth faults

External faults are more common than internal earth faults for which the restricted
earth-fault protection should operate. It is important that the restricted earth-fault
protection remains stable during heavy external earth and phase-to-phase faults, and
also when such a heavy external fault is cleared by some other protection such as
overcurrent, or earth-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 earth 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 earth 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 earth 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 earth faults. This check is an additional criterion, which should prevent
malfunctions at heavy external earth faults, and during the disconnection of such
faults by other protections. Earth faults on lines connecting the power transformer
occur much more often than earth faults on a power transformer winding. It is
important therefore that the Restricted earth-fault protection, low impedance
1
×
CTFactorPri1
1
×
CTFactorSec1
Section 6
Differential protection
(Equation 27)
(Equation 28)
(Equation 29)
117