Current Transformer Testing

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Testing Procedures

• Insulation Resistance test.
• CT Resistance, Excitation curve, Ratio & Polarity test.
• Current Transformer Loop test.
• Primary Injection Test.

Download: CT Test Format

Equipment Required

1. Insulation Resistance tester: Megger mit1020. 
2. CT Analyzer
3. Multimeter.
4. Primary Injection Kit: ODEN AT

Insulation Tester

CT Analyzer

Multimeter

Insulation Resistance Test

• Use Megger mit1020 to measure insulation resistance.
• Insulation Resistance Tests to be done:
  • Primary winding - Ground      (Apply 5000V for 1 minute)
  • Primary winding - Secondary winding      (Apply 2500V for 1 minute)
  • Secondary winding - Secondary winding      (Apply 2500V for 1 minute)
  • Secondary - Ground      (Apply 1000V for 1 minute)

CT Characteristics Test 

• Use CT Analyzer to test CT Characteristics.
• ​Connection on CT Analyzer will be same as the shown figure.
• Repeat the test for the other phases and the other secondary cores.
• Refer to CT Analyzer User Manual to find how to use CT Analyzer & how to enter CT Data.

Connection Circuit

CT Loop Resistance Test

• Measure CT resistance from CT terminal blocks (CT Side).
• Measure Loop resistance from CT terminal blocks (Load Side).
• Measure the Total Loop resistance (CT resistance + Load resistance + wiring resistance). 

CT Primary Injection Test

• Core Identification Test:
  • In this test we confirm the CT secondary connection.
  • Inject low current (for example: 50A).
  • Use small jumper to short the CT secondary from CT box, the measurement will be gone at panel.
  • Repeat this test for all phases and cores.
• Short & Isolate Facility Test:
  • In this test we confirm the CT short & isolate facility for correct connection.
  • Inject low current (for example: 50A).
  • Short CT links then Isolate the external wiring to confirm CT circuit will not be opened.
• Single Earth Point Test:
  • For all CT Loops, each loop must have only one earth point, otherwise secondary current will be divided.
  • Open all earth point in the loop, check with buzzer that there is no earth link is closed, then close only one point.
  • For all protection & measurement CTs, Close the earth point in the nearest point to the CT.
  • For High Impedance Differential Protection, Close the earth point in the nearest point to the Relay.
• Primary Injection Test:
  • After confirming all previous steps, CT is ready now to the primary injection test.
  • Use the ODEN AT kit to inject high current (for example: 500A).
  • Measure the secondary current in all loop up to the end of this loop.

ODEN AT

CT Terminal Block