14
For large machines, the machine itself can be used to provide the fault current to
check the primary fault setting as shown in Figure 5. The machine should be run up
to speed with no excitation. The excitation should then be increased until the relays
have all operated.
The primary current, relay current and relay plus stabilising resistor voltage, should
be noted as each relay operates.
2.5.5
Through fault stability
With any form of unbalanced protection it is necessary to check that the current
transformers are correctly connected. For this purpose with a restricted earth fault
scheme the circuit shown in Figure 3 may be used. During this test it is necessary to
measure the spill current in the relay circuit and short out the relay and stabilising
resistor (if fitted). The current is increased up to as near full load as possible and the
spill current noted. The spill current should be very low, only a few milliamps if the
connections are correct. A high reading (twice the injected current, referred through
the current transformer ratio) indicates that one of the current transformers is reversed.
Injection should be carried out through each phase to neutral.
Where primary injection is not practicable in the case of restricted earth fault
protection on a transformer it may be possible to check stability by means of back
energising the transformer from a low voltage (415 volt) supply as shown in
Figure 4.
In the case of machine protection, similar stability tests must be carried out by
injecting into one and out of another current transformer connected on the same
phase.
For large machines, the machine itself can be used to provide the fault current, but
the short circuit must now be fitted as shown in Figure 6. The machine should be run
up to normal speed and the excitation increased until the primary current is
approximately full load, when the spill current should be checked.
All other types of balanced protection should be tested in a similar manner.
At the conclusion of the tests ensure that all connections are correctly restored and
any shorting connections removed.
Section 3. MAINTENANCE
Periodic maintenance is not necessary. However, periodic inspection and test is
recommended. This should be carried out every 12 months or more often if the relay
is operated frequently or is mounted in poor environmental conditions.
Repeat secondary injection tests 2.5.2 to prove operation, with emphasis on contact
wear and condition. Mechanical settings may be checked against those shown in
Section 4.
