Maintenance and Testing of Instrument Transformers
The following general recommendations are offered to supplement the manufacturer ’s instructions.
General maintenance
• If instrument transformers are allowed to remain out of service for a long period of time, they should be dried before being put into service
• Instrument transformers containing oil or synthetic dielectrics should be tested periodically to determine the breakdown voltage remains at 26 kV or above
• The secondaries of CTs that are not connected to relays, meters, and the like, should have their secondaries shorted and grounded as mentioned in Section 9.2.8
• The secondary circuits and cases of all instrument transformers should be grounded with at least a number 12 AWG conductor as discussed earlier in Section 9.2.8
Testing
Routine testing as described below should be performed on instrument transformers.
General purpose instrument transformers
• Ratio and phase angle tests should be made on CTs for metering at 100% and 10% of rated primary current, when energized at rated frequency with maximum standard burden for which the transformer is rated
• The turns ratio and polarity tests should be made on CTs for relaying to ensure that they have the correct turns ratio and relaying accuracy
Revenue metering instrument transformer
• Calibrate all meters at 10%, 50%, and 90% of full scale. Instruments used for calibration of test equipment should have precision of no more than 50% of the instrument being tested.
• Calibrate watt-hour meters to 0.5%.
• Verify all instrument multipliers.