GE MFAC 14, Руководство пользователя, Страница: 14 / 25

2.2.5 Primary injection testing
It is essential that primary injection testing is carried out to prove the correct
polarity of current transformers.
Before commencing any primary injection testing it is essential to ensure that the
circuit is dead, isolated from the remainder of the system and that only those
earth connections associated with the primary test equipment are in position.
2.2.6 Primary fault setting
The primary fault setting of any balanced scheme can be checked using the circuit
shown in Figure 2. The primary current is injected into each current transformer in
turn and increased until the relay operates. The voltage at which the relay
operates should be within
±
10% of the relay setting voltage V
R
. The primary
current for operation and relay current should be noted.
In the case of machine protection similar tests must be carried out by injecting
first into each current transformer in turn to determine the primary fault setting.
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 voltage
should be noted as each relay operates.
2.2.7 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 netural.
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 (415V) 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.
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