Page 383 - Offshore Electrical Engineering Manual

370 CHAPTER 4 Protection and Control

this representing the motor starting current (about 600% running current).
Measuring the time for the relay to operate at about 115% running current will
enable the hot characteristic time to be checked.
4. Check the operation of the relay at different tap positions by varying the injec-
tion current. This should confirm that the operation of each bimetal phase ele-
ment is balanced with that of its neighbour.
5. Check that if the heater of each element is injected in turn, representing a single
phasing situation, the relay becomes unbalanced and trips.
6. Test any attracted armature elements (instantaneous overcurrent, earth fault, flag
auxiliary, etc.), if fitted, for operation at the correct settings.

Primary Injection Tests
Primary injection tests are carried out on the relays following the completion of all
commissioning tests on CTs and VTs and relay calibration tests by secondary injec-
tion. The purpose of the primary injection test is to check the stability of the sys-
tem for external through-faults and to check the entire system, including the circuit
breaker control circuitry, for satisfactory operation when subjected to a fault current
to which it is designed to react.
Transporting primary injection test sets offshore is difficult, as they tend to be too
heavy to be flown out to the installation and have to be transported by sea. Having
seen the treatment such equipment can be subjected to when lifted off a supply boat
in heavy weather, the author advises that great care should be taken in packing the
equipment to avoid damage and hence delays to the commissioning programme.
As switchgear, cabling and protection system designs can vary greatly, design
documentation and the manufacturer’s manual will need to be carefully considered
for each part of the system to which primary injection currents are applied.
The most common types of protection are overcurrent, earth fault (residual) and
restricted earth fault, for which typical test circuits are given in Figs 7.4.8–7.4.10.

Overcurrent Relays
The effective setting for overcurrent relays can be checked using the circuit in
Fig. 7.4.8. The method shown checks the operating time of each element, and the
residual current flowing in the common CT connection represents the imbalance
between the phases under test. This residual current should be very low, normally
less than 1 mA.

Residual Earth Fault Relays
The sensitivity of residually connected earth fault relays can be tested as shown in
Fig. 7.4.9. Relays connected in this manner can be either instantaneous current oper-
ated elements or induction disc elements. The effective setting of the relay for pri-
mary faults is checked by single-phase injection.

Restricted Earth Fault Relays
For restricted earth fault relays, both sensitivity and stability tests are necessary, as
these relay schemes may be subject to through-fault currents owing to faults in the