Checking Ct Circuits; Checking Vt Circuits - ABB RELION REX640 Operation Manual

Commissioning
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7.2.2

Checking CT circuits

The CTs must be connected in accordance with the connection diagram of
the project, both with regards to phases and polarity. The following tests are
recommended for every primary CT or CT core connected to the protection relay.
• Primary injection test to verify the current ratio of the CT, the correct wiring up
to the protection relay and correct phase sequence connection (that is L1, L2, L3.)
• Polarity check to prove that the predicted direction of the secondary current
flow is correct for a given direction of the primary current flow. This is an
essential test for the proper operation of the directional function, protection
or measurement in the protection relay.
• 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 protection relay. 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 protection relay.
• 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.
• Earthing check 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.
7.2.3

Checking VT circuits

Check that the wiring is in strict accordance with the supplied connection diagram.
Test the circuitry.
112
The polarity must be correct before connecting auxiliary power supply.
Check that the wiring is in strict accordance with the supplied
connection diagram.
Both the primary and the secondary sides must be disconnected
from the line and the protection relay 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 cause damage to the
insulation. The re-energizing of the CT primary should be prohibited as
long as the CT secondary is open or unearthed.
Correct possible errors before continuing to test the circuitry.
1MRS759118 D
REX640
Operation Manual