Currnt Transformer Testing Procedure

CURRENT TRANSFORMER


List of testing to be carried out in Current transformer. • • • • • •

Physical Verification Insulation Resistance Test Polarity Test Ratio Test Winding Resistance Test Magnetizing Characteristics Test

Testing Procedure: 1. PHYSICAL VERIFICATION i. Check for Installation of Current Transformer as per Manufacturers Recommendation, Approved Drawings. ii. Check for Physical Damage iii. Check for Oil leakage, Cleanliness, Earthing – Equipment/Structure Earthing. iv. Check the Interconnection between Mechanical Cubical and Control Cubical

2. INSULATION RESISTANCE TEST: 1.

Primary to Secondary and Primary to Earth: Measure Insulation resistance value using Insulation Tester between the Primary terminal and Secondary terminal (All Secondary terminals shorted and connected to earth).

The minimum Insulation Resistance value should be: Up to 12 KV Up to 36 KV Up to 145 KV 2.

-

100 Mega Ohms 500 Mega Ohms 1000 Mega Ohms

Between Secondary:

Measure Insulation resistance value using Insulation Tester between the Secondary terminals (if the CT Secondary is multicore), The minimum Insulation Resistance value should be: 5.0 Mega Ohms.

3.

Secondary to Earth:

Measure Insulation resistance value using Insulation Tester between the Secondary terminal and Earth. The minimum Insulation Resistance value should be: 5.0 Mega Ohms. Insulation Tester used for Corresponding Voltage level
If the winding is 433V or 1000V use 500V Insulation Tester
If the winding is 6.6KV use 2500V Insulation Tester
If the winding is more than 11 KV use 5000V Insulation Tester 3. POLARITY TEST: Polarity refers to the instantaneous direction of the primary current with respect to the secondary current. 1.

Connect the Analog multimeter between the secondary terminals of S1 and S2.

2.

Select the micro amps or milli amps selection in the analog multimeter.

3.

Apply 1.5Volt DC to the primary terminal of P1 and P2 instantly. (P1 should be +ve, P2 should be –ve)

4.

At the time of applying instant DC voltage, the meter will deflect clockwise position if the polarity is normal. If the polarity is abnormal the meter will deflect anti-clockwise direction.

5.

If the polarity is abnormal change the Secondary terminal connection of the CT. After changing the secondary terminals again do the polarity test.

4. RATIO TEST: 1.

Short all the secondary cores of CT tightly.

2.

Connect the primary injection kit between the terminals P1 and P2.

3.

Check initially the current injection unit is zero, and then injects 20% current of actual ratio; by using tong tester check the primary current value and note down the reading.

4.

By using leakage tester check the secondary current value and note down the reading.

5.

After tabulating the primary and secondary current values, calculate the ratio.

6.

The calculated ratio value should be the actual ratio value of CT.

7.

Repeat the procedure for 50%, 75% and 100% of Rated CT current.

5. WINDING RESISTANCE TEST: 1. Check the Secondary Winding is free from external connection. 2. Connect the winding resistance kit or Multimeter to CT’S Secondary terminals ( S1,S2 or 1S1-1S2 etc.,) 3. If secondary having more than one core, check the other core also by connecting (1S1-1S3, 1S1-1S4 etc.,) 4. Tabulate the reading; check the value is similar for all the three phases.

6. MAGNETISING CHARACTERISTICS TEST: 1. Connect the High voltage injection kit to the secondary terminals of the Current Transformer. ( S1,S2 or 1S1-1S2 etc.,) that is for Ps class only 2. Inject the voltage from lower value till the voltage mentioned and note the mille amps current. 3. Knee point voltage is that for 10% rise in voltage there will be 50% rise in current

7. REVIEW TEST RESULTS WITH RESULTS

RESPECT TO FACTORY TEST