8.2.2
Checking CT circuits
The CTs must be connected in accordance with the terminal diagram provided with
the IED, both with regards to phases and polarity. The following tests are
recommended for every primary CT or CT core connected to the IED:
•
Primary injection test to verify the current ratio of the CT, the correct wiring up
to the protection IED and correct phase sequence connection (that is L1, L2,
L3.)
•
Polarity check to prove that the predicted direction of secondary current flow is
correct for a given direction of primary current flow. This is an essential test for
the proper operation of the directional function, protection or measurement in
the IED.
•
CT secondary loop resistance measurement to confirm that the current
transformer secondary loop dc resistance is within specification and that there
are no high resistance joints in the CT winding or wiring.
•
CT excitation test to ensure that the correct core in the CT is connected to the
IED. Normally only a few points along the excitation curve are checked to ensure
that there are no wiring errors in the system, for example due to a mistake in
connecting the CT's measurement core to the IED.
•
CT excitation test to ensure that the CT is of the correct accuracy rating and that
there are no short circuited turns in the CT windings. Manufacturer's design
curves should be available for the CT to compare the actual results.
•
Check the earthing of the individual CT secondary circuits to verify that each
three-phase set of main CTs is properly connected to the station earth and only
at one electrical point.
•
Insulation resistance check.
•
Phase identification of CT shall be made.
Both primary and secondary sides must be disconnected from the line
and IED when plotting the excitation characteristics.
If the CT secondary circuit is opened or its earth connection is missing
or removed without the CT primary being de-energized first,
dangerous voltages may be produced. This can be lethal and damage,
for example, insulation. The re-energizing of the CT primary should
be inhibited as long as the CT secondary is open or unearthed.
Section 8
1MRS756499 A
Commissioning
84
RED615
Operation Manual
