Current Transformer (CT) Accuracy Testing: Complete Procedure

1. Purpose of CT Accuracy Tests

• Verify ratio error and phase displacement meet specifications

• Ensure compliance with protection & metering class requirements

• Detect winding defects, saturation issues, or core damage

2. Key Standards

3. Test Equipment Required

• CT Analyzer (e.g., Omicron CTA, Megger TTR)

• Primary injection set (for high-current tests)

• Burden resistor (to simulate real load)

• Phase angle meter (for phase displacement)

4. Critical Tests & Procedures

A) Turns Ratio Test

Purpose: Verify actual vs. nameplate ratio
Method:

1. Apply low AC voltage (typically 50-100V) to secondary

2. Measure induced primary voltage

3. Calculate ratio:

   $$\text{<span style="font-size: 18px;">Ratio</span>}<span\; style="font-size:\; 18px;">=</span>\frac{{\mathrm{<span\; style="font-size:\; 18px;">V</span>}}_{\text{<span style="font-size: 18px;">primary</span>}}}{{\mathrm{<span\; style="font-size:\; 18px;">V</span>}}_{\text{<span style="font-size: 18px;">secondary</span>}}}$$Ratio=VsecondaryVprimary

Acceptance Criteria:

• ±0.5% for metering CTs (Class 0.2, 0.5)

• ±3% for protection CTs (Class 5P, 10P)

B) Polarity Test

Purpose: Ensure correct dot-marked polarity
Method:

1. Connect DC source (+) to P1, (-) to P2

2. Momentarily connect to secondary

3. Observe galvanometer deflection:

• Positive kick = Correct polarity

• Negative kick = Reversed polarity

C) Burden Test

Purpose: Verify CT can maintain accuracy under load
Test Setup:

• Connect rated burden resistor to secondary

• Inject 100% rated current
  Measure:

• Secondary voltage drop

• Phase angle shift
  Acceptance: Must stay within accuracy class limits

D) Excitation (Saturation) Test

Purpose: Find knee-point voltage (critical for protection CTs)
Procedure:

1. Leave primary open-circuited

2. Apply increasing voltage to secondary

3. Plot current vs. voltage curve
   Key Points:

• Knee-point (Vk): Voltage where 10% increase causes 50% current rise

• IEEE C57.13 Requirement:

  $${\mathrm{<span\; style="font-size:\; 18px;">V</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">k</span>}}<span\; style="font-size:\; 18px;">\ge </span>\mathrm{<span\; style="font-size:\; 18px;">2</span>}<span\; style="font-size:\; 18px;">\times </span>{\mathrm{<span\; style="font-size:\; 18px;">I</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">s</span>}\mathrm{<span\; style="font-size:\; 18px;">c</span>}}<span\; style="font-size:\; 18px;">\times </span><span\; style="font-size:\; 18px;">(</span>{\mathrm{<span\; style="font-size:\; 18px;">R</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">C</span>}\mathrm{<span\; style="font-size:\; 18px;">T</span>}}<span\; style="font-size:\; 18px;">+</span>{\mathrm{<span\; style="font-size:\; 18px;">R</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">b</span>}\mathrm{<span\; style="font-size:\; 18px;">u</span>}\mathrm{<span\; style="font-size:\; 18px;">r</span>}\mathrm{<span\; style="font-size:\; 18px;">d</span>}\mathrm{<span\; style="font-size:\; 18px;">e</span>}\mathrm{<span\; style="font-size:\; 18px;">n</span>}}<span\; style="font-size:\; 18px;">)</span>$$Vk≥2×Isc×(RCT+Rburden)

E) Insulation Resistance Test

Test Voltage:

• 1kV DC for <1kV CTs

• 5kV DC for >1kV CTs
  Minimum Values:

• >1,000 MΩ (new CTs)

• >100 MΩ (in-service)

F) Primary Injection Test

Purpose: Full-system verification
Method:

• Inject high current (up to CT rating) through primary

• Measure secondary output
  Critical For:

• Checking CT performance under fault conditions

• Validating entire circuit (wiring, relays)

5. Accuracy Classes & Tolerances

6. Common CT Problems Detected

7. Safety Precautions

✔ Always short-circuit CT secondary before disconnecting
✔ Use proper PPE for primary injection tests
✔ Never open-circuit a live CT secondary (risk of lethal voltages)

8. Test Report Template

CT ID: ___________  
Date: ___________  
Turns Ratio: ______ (Nameplate: ______)  
Polarity: Correct/Incorrect  
Knee-Point Voltage: ______ V  
Insulation Resistance: ______ MΩ @ ______ V  
Burden Test: ______ VA (Limit: ______ VA)  
Conclusion: Pass/Fail  

Pro Tip: For protection CTs (Class PX, 5P), always perform excitation tests – ratio tests alone are insufficient to guarantee performance during faults.

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