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2.5
Commissioning tests
If the relay is wired through an MMLG test block it is recommended that all
secondary injection tests should be carried out using this block.
Ensure that the main system current transformers are shorted before isolating the relay
from the current transformers in preparation for secondary injection tests.
DANGER
DO NOT OPEN CIRCUIT THE SECONDARY CIRCUIT OF A CURRENT
TRANSFORMER SINCE THE HIGH VOLTAGE PRODUCED MAY BE LETHAL AND
COULD DAMAGE INSULATION.
When type MMLG test block facilities are installed, it is important that the sockets in
the type MMLB 01 test plug, which correspond to the current transformer secondary
windings, are LINKED BEFORE THE TEST PLUG IS INSERTED INTO THE TEST BLOCK.
Similarly, a MMLB 02 single finger test plug must be terminated with an ammeter
BEFORE IT IS INSERTED to monitor CT secondary currents.
It is assumed that the initial preliminary checks have been carried out.
2.5.1
Relay CT shorting switches
With the relay removed from its case, check electrically that the CT shorting switch is
closed.
2.5.2
Secondary injection testing
Connect the circuit as shown in Figure 1 and ensure that the current transformer
primary is open circuit and that if any earthing connections are fitted, they do not
short out the primaries of any current transformers.
Increase the current until the relay just operates.
Note the current in the relay (this can be done using the miniature split plug inserted
into the appropriate position of the plug bridge connected to an ammeter).
Note also the voltage (V
R
) at which the relay operates which should correspond to
the setting V
R
of the relay plus stabilising resistor, with a tolerance of +10%.
The total secondary current for operation will be given on ammeter A1. This test
should be repeated for each pole of the relay.
2.5.3
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.
2.5.4
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 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.
