PRE COMMISSIONING TEST FOR CURRENT TRANSFORMER

Purpose

This procedure is prepared to carry out various pre commissioning for current transfromer tests to be conducted in systematic manner for CT to ensure the healthiness and performance of the equipment

Scope

This procedure covers the Pre commissioning test for CT as for the following

Insulation Resistance Test
Secondary Winding resistance test
Polarity test
Ratio test
Magnetizing Curve test
Capacitance and Tan Delta Measurement Test

Pre-Testing Checks

- Check out the working condition of ELCB(30 mA)
- Check the insulation of power cable for equipment
- Ensure proper earthing and rubber mats available at the work
- Calibrated Testing equipment should be used and calibration reports available with engineers for the same
- Check out the insulates hand tools available with the technician
- Ensure the Healthiness of the test leads (Check for any insulation Damages)
- Ensure appropriate PPE’s available at sites
- Unused spare secondary cores terminals should be shorted in CT JB
- Physically check for CT for any damage, oil leakage

Resources – Tools and Equipment Required

Continuity Test

Multimeter with leads

2. For Insulation Resistance measurement Test(IR Test)

Test Leads
500 V, 5000 V Insulation Tester
Insulated Hand Tools

3. Winding resistance test

Multimeter with leads

4. Polarity test

9V battery
Galvano meter
Connecting leads

5. Ratio test

Variac / auto transformer
Loading transformer
Welding cable
Clamp meter
Current leakage tester
Connecting leads

6. Magnetizing curve test

Single phase variac 0-230 V,28 A
Step-up transformer(230 V/5000 V/2 KVA)
Multimeter
Current leakage tester
Connecting leads

7. Capacitance and Tan Delta Measurement Test

Tan Delta kit
Connecting leads

Pre-Testing

Insulation Resistance (IR) Test

Primary to Earth:

Connect the Earth lead of the 5000 V IR kit to the earth Terminal
Connect HV lead of the 5000 V IR kit to either P1 or P2 of the current Transformer
Switch “ON” the 5000 V IR kit and Select the required voltage range in IR kit
press the Test button take down the readings
Switch “OFF” the IR kit

Primary to Secondary:

Connect HV lead of the5000V IR kit to either P1 or P2 of the current Transformer
Connect Earth lead of the 5000V IR kit to either S1 or S2 of the current Transformer
Switch “ON” the 5000V IR kit and Select the required voltage range in IR kit
press the Test button take down the readings
Switch “OFF” the IR kit

Secondary to Earth:

Connect Earth lead of the IR kit to the earth terminal
Connect HV lead of the IR kit to either S1 or S2 of the current Transformer
Switch “ON” the IR kit and Select the required voltage range in IR kit
press the Test button take down the readings
Switch “OFF” the IR kit

Between Secondary Cores:

Connect HV lead of the 500 V IR kit to either S1 or S2 of the current Transformer
Connect Earth lead of the 500 V IR kit to either S1 or S2 of the other core of the current Transformer
Switch “ON” the 500 V IR kit and Select the required voltage range in IR kit
press the Test button take down the readings
Switch “OFF” the 500 V IR kit
Discharge the voltage from the CT with respect to earth

DO’S

DON’TS

Check all the CT secondary wires are isolated
Check the calibration due of the test equipment
Breaker should be in open condition and in test position.
Ensure zero resistance when line and earth are shorted and switched on to know the zero value for the proper working of kit.
Keep some distance while doing the test.
After Insulation resistance test completion, note down the values and discharge the CT terminals and remove the test cables
Ambient temperature must be noted.

Don’t touch any cables and the kit when it is in running condition.
Don’t touch any bushing while doing the tests
Don’t keep the breaker in service and on position
Don’t test with damaged cables and improper clips
Don’t work in wet (rainy) conditions
Don’t lose your concentration while testing.

Winding resistance test

Secondary resistance test is to verify the CT secondary winding resistance with specified one and no discontinuity in the winding. This value can be used in other calculations.

Operational sequence:

Connect the multimeter leads to the CT secondary winding Terminals
Note down the winding resistance value
Repeat the above procedure for remaining cores

DO’S	DON’Ts
Measure with calibrated multimeters Note the ambient temperature Remove the secondary terminals to isolate the CT Clean the secondary terminals and measure the resistance.	Measure the CT winding resistance for individual cores Primary winding resistance should not be measured (Only the secondary) Loose connections should be avoided in the CT secondary terminals. Don’t measure the terminals with dust in CT secondary terminals

Polarity test

Description

Polarity test is to confirm the polarity marking on the CT primary and secondary and verify it is matching with drawing. More ever it is giving an idea, how to connect the secondary’s to make the protection (like directional, differential) and metering function properly.

Operational Sequence:

Connect the secondary S1 terminal to galvanometer positive terminal and S2 to the negative terminal
Touch the battery positive terminal to P1 in and negative terminal to the P2 terminal at the CT primary winding Terminals
Check the deflection in the galvanometer
It must be in clock-wise direction

DO’S	DONT’S
Check the connections as per the diagram. Check the CT secondary cable continuity from CT secondary to CT terminal block. Check whether the battery is charged or not Verify the galvanometer in good condition	Don’t use low batteries for testing Don’t apply the charge continuously (apply single pulse).

Ratio test

Description:

This test is to ensure the turn’s ratio of CT at all taps. The circuit connection shall be made as shown Figure 2. The primary current of minimum of 25% rate primary current to be injected on primary side of CT with secondary’s shorted and the secondary current can be measured and recorded for all cores

Operational sequence:

Connect the variac and loading transformer
Connect the loading transformer across the primary terminals(P1,P2)
Connect the variac in A.C supply extension board with ELCB rated with 30 mA
Inject the current across the primary terminals.
Short all secondary windings core by wire at CT junction box
Note down the corresponding secondary current each cores at 20 % and 40% of rated primary current
Switch off the supply to the primary injection kit

DO’S

DONT’S

Check the current leakage testers are calibrated
Check the input voltage levels for avoid voltage variation, fluctuations
Connect the welding cables with full tightness in the loading transformer and CT primary terminals.
80% of the rated current should be applied in the CT primary and measure the secondary current values.
Measure the CT secondary current in meters and relays end points.
Single point earthing should be taken in the CT secondary circuits.
Make the variac slowly to Zero position and remove the cables

Don’t leave any CT secondary terminals in open condition during the test.
Multiple earthing should not in the CT secondary circuits
No loose connections should be in the CT secondary circuits.
Don’t leave the spare core in open condition.

Excitation Current

Purpose of the Test

The magnetization test is conducted in order to see the condition of the turns of the secondary. This test gives the indications regarding the shorting of turns CT secondary winding and to establish CT characteristics as well as capability of CT.

Procedure of the test

Apply AC voltage to the secondary winding of the CT with primary open circuit
Vary the applied voltage from 25% of Vk to 110 % of Vk
Measure the current drawn by the winding at each selected value is recorded
Verify that, exciting current is less than specified at Vk/2
This test should not be performed for metering core
If Knee Point Voltage is not mentioned then Knee Point Current may be taken into Consideration.

Test Equipment

Make	Type	Sr. no.	Range	Cal. Due date

Acceptance Criteria

Excitation current should not be more than specified on name plate.
10 % increase in the voltage from 100 % to 110 %, increase in current should not be more than 50%

DO’S	DONT’S
Do the Magnetizing curve test for PS class only. Keep the CT primary in open Apply voltage in the CT secondary coils only. Isolate the CT secondary wires. Do the test for protection cores only. Measure the knee point voltage and exciting current.	Don’t apply the voltage above the limit Or else the CT will be saturated. Don’t apply more voltage in CT secondary terminals. Don’t touch the primary and secondary terminals while doing the test Don’t take more time to conduct the test Don’t short the primary and the secondary terminals during this test.

Capacitance and Tan Delta Measurement Test

Purpose of the test:

Dissipation factor / loss factor/ Tan delta is defined as the ratio of resistive components to that Of capacitive current flowing in an insulating material.
Dissipation factor (tan delta) and capacitance measurement of bushing/winding provides an Indication of the quality and soundness of the insulation in the bushing/winding. Changes in the normal capacitance of an insulator indicate abnormal conditions such as the Presence of moisture layer, short -circuits or open circuits in the capacitance network.

Principle of the test:

The capacitance and dissipation/loss factor (Tan δ / Cos φ) measurement are made to determine the insulating condition of the Current transformer’s both winding to earth and between the windings, and to form a reference for future measurements during operating the current transformer.

Equipment for the test:

10 KV or 12 KV fully automatic Capacitance and Tan delta test kit to be used for accurate measurement and repeatability of test results

Procedure for the test

Primary to secondary

These values of capacitance and their respective values of insulation factor (tan delta) are to be measured.
HV line terminals connect in the primary & LV line terminals connected tan delta point of the CT
Disconnect the earthing of the tan delta point
These measurements are usually made at voltage of 10 kV or less, at power frequency.

For tan delta between windings, connections are to be carried out in UST

Acceptance Criteria

Tan δ for bushing (C1):

Type of bushing	Tan δ (C1)		Capacitance
	% Limit	% Change/annum	% Change/annum
Resin impregnated paper insulated	0.85	+0.04 / -0.04	+1.0 / -1.0
Oil impregnated paper insulated	0.40	+0.02 / -0.06	+1.0 / -1.0

DO’S

DONT’S

Ensure all the primary and secondary terminals should be isolated
Apply 2KV& 10KV only for testing using tandelta kit.
Ensure the kit is grounded properly
Disconnect the grounded wires in the tandelta point
After complete the test discharge the CT terminals
Use proper safety shoes while conducting the test
Follow the instructions & manual how to operate the kit
Take away the unwanted cables, power sources from the working station

Don’t use unskilled helpers for connecting leads to the terminals
Don’t forget to connect the grounding cable
Don’t use the damaged wires or cables for the test
Don’t drink and smoke while testing
Don’t allow the unskilled persons to operate the kit
Don’t work without work permit
Don’t work with sick & illness

Control Measures

OCP for all test for CT is available with testing engineers
Ensure PTW system in place for the particular bay
Ensure all supply to the equipment is dead
Unauthorized persons should not be allowed to enter Testing area
More than 2Mtr height shall be used Proper working plat form or man lifter and suggested for complying with fall arrester or Full body harness
Less than 2Mtr height shall be used standard insulated ladder
Inspect the ladders prior to work, keep both feet below 3rd rung from top, keep one hand on ladder minimum at all times and suggested for complying with full body harness
Barricade the testing area with warning tape
Proper supervision, conduct job specific toolbox meeting and barricading the entire area
Calibrated equipment shall be used
Insulated hand tools shall be used
Avoid joints for power cable and supply has to be taken via ELCB
Avoid Lone working
Use appropriate insulated hand gloves
CT secondary side don’t keep on open condition

PRE COMMISSIONING TEST PROCEDURE FOR CURRENT TRANSFORMER