Check Of Ct Circuits - ABB REL670 Installation And Commissioning Manual

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Check of CT circuits

Check of CT circuits
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 REx670 IED:
• primary injection test to verify the current ratio of the CT and the correct wiring
up to the protection IED
• 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.
• CT secondary loop resistance measurement in order 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 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 should be available for the current trans-
former in order 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.
• Insulator resistance check.
Note!
Both primary and secondary sides must be disconnected from the line and IED when plotting
the excitation characteristics.
Note!
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.
Checking the external optical and
electrical connections
Chapter 6