Current Transformer Introduction & Testing

 CURRENT TRANSFORMER

Function of a Current Transformer

1. Measurement

Transform the primary signal into a secondary signal which can be measured by secondary equipment.

> CT 1000/5A Ip = 1000 A → Is = 1 A

2. Isolation

Isolation of the secondary circuit and equipment from the high voltage of the primary circuit.

   

Relevant CT Standards and Field Testing Guides

1. IEC 60044-1 (old): Current transformers

2. IEC 60044-6 (old): Requirements for protective current transformers for transient performance

3. IEC 61869-2: new, replacing 60044-1 & 60044-6, Additional requirements for current transformers

Metering CT requirements

Protection CT requirements

IEC Knee Point Definition

Differences between Metering and Protection CT

Current Transformer Specification (IEC61869)

1. Protection CT

2. Example Protection CT: CT for OCEF, 600/5A, 10P20, 15VA

3. Metering CT

4. Current Transformer –Typical name plate

5. Current Transformer Testing typical site testing includes:

a. Resistance measurement

b. Polarity

c. Ratio

d. Excitation / Magnetizing Curve

e. Insulation resistance

Current Transformer Testing

1. Resistance Test

- Performed for assessing possible electrical damages in windings or contact problems.

- Shorted turns could change the winding resistance and endanger CT operation.

2. Polarity Test

- Verify the correct orientation of the primary and secondary windings of the CT as per name plate/drawing.

3. Ratio Test

- Compares measured ratio of CTs to nameplate specifications and factory measurement results

- Deviations from specifications may indicate internal faults (e.g. open circuits)

- Ratio deviations may cause malfunction of protection and false interpretation of the system current

4. Excitation / Magnetizing Curve Test

- Knee point voltage important for a proper operation of the protection device

- e.g. for Class X CT.

- Indication of: shorted turns & core faults

5. Insulation Test

- Ensure CT insulation is in good condition.

Current Transformer Testing: Typical test result

Note refer from: Ir. Jamil Ahmad Radziyan (Warna RS Sdn Bhd)