ABB REL670 Installation And Commissioning Manual page 180

3.2.6
3.2.7
3.2.8
172
Periodic maintenance tests
Trip circuit check
When the protection IED undergoes an operational check, a tripping pulse is normally obtained
on one or more of the output contacts and preferably on the test switch. The healthy circuit is of
utmost importance for the protection operation. If the circuit is not provided with a continuous
trip circuit supervision it is possible to check that circuit is really closed when the test-plug han-
dle has been removed by using a high-ohmic voltmeter and measuring between the plus and the
trip output on the panel. The measurement is then done through the tripping magnet of the circuit
breaker (note! Breaker must be closed) and therefore the complete tripping circuit is checked.
Please observe that the test system does not have its built-in security during this test. If the in-
strument should be set on Amp instead of Volts, the circuit breaker naturally is tripped, there-
fore, greatest care is necessary.
Trip circuit from trip relays to circuit breaker is often supervised by trip circuit supervision relay.
It can then be checked that a circuit is healthy by opening tripping output terminals in the cubi-
cle. When terminal is opened an alarm shall be achieved on the signal system after a delay of
some seconds. However, remember to close the circuit directly after the test and tighten the ter-
minal carefully!
Measurement of service currents
After a maintenance test it is recommended to measure the service currents and service voltages
recorded by the protection IED. The service values are checked on the IED670 local HMI or
with the PCM600 tool. 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 a transformer differential protection the achieved differential current value is dependent on
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 differ-
ential current.
The zero-sequence current to earth-fault protection relays should be measured. The current
amounts normally very small but it is normally possible to see if the current circuit is "alive".
The neutral-point voltage to an earth-fault relay is checked. The voltage is normally 0.1 to 1V
secondary. However, voltage can be considerably higher due to harmonics normally a CVT sec-
ondary 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 etc.
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.
Commissioning and maintenance of
the fault clearing system
Chapter 12