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 ground-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 ground-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.
14.2.2.8 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.
1MRK 504 165-UUS - Section 14
Commissioning and maintenance of the fault clearing system
Transformer protection RET670 2.2 ANSI 301
Commissioning manual
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