Checking Ct Circuits - ABB rec670 Commissioning Manual

1MRK 511 360-UEN A
4.7
Bay control REC670 2.1 IEC
Commissioning manual

Checking CT circuits

Check that the wiring is in strict accordance with the supplied
connection diagram.
The CTs must be connected in accordance with the circuit diagram provided with the
IED, both with regards to phases and polarity. The following tests shall be performed
on every primary CT 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 differential function and directional protection
functions.
• CT secondary loop resistance measurement to confirm that the current
transformer secondary loop DC resistance is within the specification for the
connected protection functions. When the measured loop resistance is near the
calculated value for maximum DC resistance, perform a complete burden test.
• CT excitation test in order to confirm that the current transformer is of the correct
accuracy rating and that there are no shorted turns in the current transformer
windings. Manufacturer's design curves must be available for the current
transformer 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.
While the CT primary is energized, the secondary circuit shall never
be open circuited because extremely dangerous high voltages may
arise.
Both the primary and the secondary sides must be disconnected from
the line and the IED when plotting the excitation characteristics.
If the CT secondary circuit earth connection is removed without the
current transformer primary being de-energized, dangerous voltages
may result in the secondary CT circuits.
Section 4
Starting up
49