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Commissioning and Maintenance P746/EN CM/F21
MiCOM P746 (CM) 10-29
7. ON-LOAD CHECKS
The objectives of the on-load checks are to:
−
Confirm the external wiring to the current inputs is correct.
−
Ensure the on-load differential current is well below the relay setting.
CM
However, these checks can only be carried out if there are no restrictions preventing the
energisation of the plant being protected and the other P746 relays in the group have been
commissioned.
Remove all test leads, temporary shorting leads, etc. and replace any external wiring that
has been removed to allow testing.
If it has been necessary to disconnect any of the external wiring from the relay in order to
perform any of the foregoing tests, it should be ensured that all connections are replaced in
accordance with the relevant external connection or scheme diagram.
Confirm current transformer wiring:
Measure the current transformer secondary values for each input using a multimeter
connected in series with the corresponding relay current input.
Check that the current transformer polarities are correct.
Ensure the current flowing in the neutral circuit of the current transformers is negligible.
Compare the values of the secondary phase currents with the relay’s measured values,
which can be found in the MEASUREMENTS 1 menu column.
Note: Under normal load conditions the earth fault function will measure
little, if any, current. It is therefore necessary to simulate a phase to
neutral fault. This can be achieved by temporarily disconnecting one
or two of the line current transformer connections to the relay and
shorting the terminals of these current transformer secondary
windings.
If cell [MEASURE’T SETUP, Local Values] is set to ‘Secondary’, the currents displayed on
the LCD or a portable PC connected to the front EIA(RS)232 communication port of the relay
should be equal to the applied secondary current. The values should be within 5% of the
applied secondary currents. However, an additional allowance must be made for the
accuracy of the test equipment being used.
If cell [MEASURE’T SETUP, Local Values] is set to ‘Primary’, the currents displayed on the
relay should be equal to the applied secondary current multiplied by the corresponding
current transformer ratio set in ‘CT & VT RATIOS’ menu column (see Table 5). Again the
values should be within 5% of the expected value, plus an additional allowance for the
accuracy of the test equipment being used.
Note: If a single dedicated current transformer is used for the earth fault
function, it is not possible to check the relay’s measured values.