Alstom MiCOM P54 Series, Technical Manual

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Application Notes P54x/EN AP/La4
MiCOM P543, P544, P545 & P546
(AP) 6-39
AP
Cab le g l an d
Cab le g l an d /s he at h
e a r t h co nn ec ti o n
Cab le b o x
SEF
SEF
N o o p er ati o n
SEF
Op er ati o n
“In correc t”
“C orrec t”
P0112ENa
Figure 10 Positioning of core balance current transformers
As can be seen from the diagram, if the cable sheath is terminated at the cable gland and
earthed directly at that point, a cable fault (from phase to sheath) will not result in any
unbalance current in the core balance CT. Hence, prior to earthing, the connection must be
brought back through the CBCT and earthed on the feeder side. This then ensures correct
relay operation during earth fault conditions.
2.9 Negative sequence overcurrent protection (NPS)
The following section describes how negative phase sequence overcurrent protection may
be applied in conjunction with standard overcurrent and earth fault protection in order to
alleviate some less common application difficulties:

Negative phase sequence overcurrent elements give greater sensitivity to resistive
phase-to-phase faults, where phase overcurrent elements may not operate.

In certain applications, residual current may not be detected by an earth fault relay due
to the system configuration. For example, an earth fault relay applied on the delta side
of a Dy (delta-wye) transformer is unable to detect earth faults on the star (wye) side.
However, negative sequence current will be present on both sides of the transformer for
any fault condition, irrespective of the transformer configuration. Therefore, a negative
phase sequence overcurrent element may be employed to provide time-delayed back-up
protection for any uncleared asymmetrical faults downstream.

It may be required to simply alarm for the presence of negative phase sequence currents
on the system. Operators may then investigate the cause of the unbalance.