Page 373 - Offshore Electrical Engineering Manual

360 CHAPTER 4 Protection and Control

7. check the stability of the protective relays for external faults and determine
the effective current setting for internal faults, by means of primary injection
tests;
8. perform closing, tripping and alarm checks for the equipment associated with
protective relays.

Before commencing work on any equipment, the isolation and permit-to-work
procedures must be strictly adhered to. In particular, CTs and VTs should be de-
energised, isolated and discharged to earth before any commissioning operations are
carried out.

VOLTAGE TRANSFORMERS (REFER TO BS EN 61869 AND BS
EN 50482)

First the VT should be inspected both visually and mechanically as follows:

1. inspect for physical damage and for compliance with design documents,
2. check mechanical alignment, clearances and proper operation of any discon-
necting and earthing devices associated with the VT,
3. verify proper operation of grounding or shorting devices.

After the initial examination has been carried out, the following electrical tests
should be completed.

INSULATION RESISTANCE
1. Remove all deliberate earth connections from the VT windings.
2. Electrically isolate the VT primary and secondary windings by removing fuses,
links or connections.
3. Perform insulation resistance tests on secondary windings to earth and between
secondary windings at 500 V direct current (DC) for 1 min.
4. Having obtained approval from the VT manufacturer, perform a high-voltage
DC insulation test on the primary winding insulation.
Recommended test voltages are given in Table 7.4.1, which is for fully insulated
windings. This test is only to be used on new VTs which have yet to be placed in
service and is for 15 min duration.
Secondary windings should be shorted together and to earth for the high-voltage
DC test, so that the insulation is tested with a potential from primary to earth and
primary to (earthed) secondary. The test voltage should be gradually increased up
to the full value and maintained for the duration of the test.
5. An insulation test of 500 V DC should be applied to all windings to earth, and
between windings, for a duration of 1 min. This test should be performed both
before and after the high-voltage DC test described earlier and should result in a
minimum resistance reading of 100 MΩ.