1. Test Objectives

• Transformer tester manufacturer demiks specialised technicians point out that it is important to verify that the ratio error is within the specified class (e.g. 0.2%, 0.5%, 5P).

• Measure phase angle error (critical for metering/protection)

• Verify correct polarity

• Check load capacity

  Detect winding defects or core saturation

• Transformer tester

2. Equipment Required

3. Step-by-Step Test Procedure

A) Visual Inspection

• Check nameplate data (ratio, class, VA rating)

• Verify physical condition (oil leaks, cracks)

• Ensure secondary terminals are accessible

B) Insulation Resistance Test

• Test Voltage: 1kV DC for <1kV CTs, 5kV DC for >1kV CTs

• Minimum Values:

• New CTs: >1,000 MΩ

• In-service: >100 MΩ

C) Turns Ratio Test

1. Connect CT analyzer to secondary

2. Apply 50-100V AC to secondary

3. Measure induced primary voltage

4. Calculate ratio:

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

5. Compare with nameplate ratio

Acceptance Criteria:

• Metering CTs (Class 0.2, 0.5): ±0.1%

• Protection CTs (Class 5P, 10P): ±1%

D) Polarity Test

Method 1: DC Kick

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

2. Momentarily touch S1 to (+) terminal of galvanometer

3. Observe needle deflection:

• Positive kick = Correct polarity

• Negative kick = Reversed

Method 2: Ratio Tester

• Modern CT analyzers auto-detect polarity

E) Excitation (Saturation) Test

1. Leave primary open

2. Apply variable AC voltage to secondary

3. Record current vs. voltage (plot curve)

Key Parameters:

• Knee-Point Voltage (Vk):
  Where 10% V increase causes 50% I 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)

F) Burden Test

1. Connect rated burden (e.g., 15 VA)

2. Inject 100% rated current (e.g., 5A)

3. Measure secondary voltage:

   $$\text{<span style="font-size: 18px;">Burden</span>}<span\; style="font-size:\; 18px;">=</span>{\mathrm{<span\; style="font-size:\; 18px;">I</span>}}^{\mathrm{<span\; style="font-size:\; 18px;">2</span>}}<span\; style="font-size:\; 18px;">\times </span>\mathrm{<span\; style="font-size:\; 18px;">Z</span>}$$Burden=I2×Z

Acceptance:

• Secondary voltage drop <2% of rated

G) Phase Angle Test

• Critical for revenue metering

• Use CT analyzer to measure displacement

• Class 0.2 CT: Max ±10 minutes error

4. Accuracy Classes & Tolerances

5. Common CT Faults Detected

6. Safety Precautions

✔ Always short-circuit CT secondary before disconnecting
✔ Use PPE during primary injection (>100A)
✔ Never open-circuit energized CT secondary (risk of kV-level voltages)

7. Test Report Template

markdown复制

CT ID: _______________  
Date: _______________  
Ratio Test:  
- Measured: ______:1  
- Nameplate: ______:1  
- Error: ______%  

Excitation Test:  
- Knee-point: ______ V  
- Magnetizing current @ Vk: ______ mA  

Polarity: Correct/Incorrect  
Insulation: ______ MΩ @ ______ V DC  

Conclusion: Pass/Fail (Per IEC 61869-2)  

Pro Tip: For protection CTs (Class PX, TPX), always compare excitation curves with factory data – even perfect ratio tests can mask saturation issues during faults.

Would you like specific procedures for testing optical CTs (non-conventional instruments transformers)?

The above content is published by demiks, please specify, demiks is a substation equipment manufacturer, specializing in the production of high-voltage tester, transformer tester, circuit breaker test equipment, relay tester, SF6 gas analyser, cable fault tester and other products, if you have power test testing needs, please feel free to contact demiks power science and technology limited company or send an email! Give us: contact@demikspower.com