ABB REQ650 1.3 IEC Commissioning Manual page 108

Section 8
Commissioning and maintenance of the fault clearing system
For transformer differential protection, the achieved differential current value is dependent on
the tap changer position and can vary between less than 1% up to perhaps 10% of rated
current. For line differential functions, the capacitive charging currents can normally be
recorded as a differential current.
The zero-sequence current to earth-fault protection IEDs should be measured. The current
amounts normally very small but normally it is possible to see if the current circuit is "alive".
The neutral-point voltage to an earth-fault protection IED is checked. The voltage is normally
0.1 to 1V secondary. However, voltage can be considerably higher due to harmonics. Normally a
CVT secondary can have around 2.5 - 3% third-harmonic voltage.
Restoring
Maintenance is very important to improve the availability of the protection system by
detecting failures before the protection is required to operate. There is however little point in
testing healthy equipment and then putting it back into service with an open terminal, with a
removed fuse or open miniature circuit breaker with an open connection, wrong setting, and
so on.
Thus a list should be prepared of all items disturbed during test so that all can be put back into
service quickly and without overlooking something. It should be put back into service item by
item and signed by the responsible engineer.
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1MRK 505 293-UEN B
SEMOD56528-36 v2
Breaker protection REQ650
Commissioning manual