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1MRK 506 331-UUS A
Commissioning Manual
Commissioning and maintenance of the fault clearing system
HMI or in PCM600. Ensure that the correct values and angles between voltages and
currents are recorded. Also check the direction of directional functions such as
Distance and directional overcurrent functions.
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.
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.
Section 7
147
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