CAPACITIVE VOLTAGE TRANSFORMER

PRE-COMMISSIONING TEST CAPACITIVE VOLTAGE TRANSFORMER

PRE-COMMISSIONING TEST PROCEDURE FOR CAPACITIVE VOLTAGE TRANSFORMER

CAPACITIVE VOLTAGE TRANSFORMER Purpose

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

Scope

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

  1. Continuity test
  2. Secondary Winding resistance test
  3. Insulation Resistance Test
  4. Polarity test
  5. Ratio test
  6. Tan delta test

Pre-Testing Checks

  1. Check out the working condition of ELCB(30 mA)
  2. Check the insulation of power cable for equipment
  3. Ensure proper earthing and rubber mats available at the work location.
  4. Calibrated Testing equipment should be used and calibration reports available with engineers for the same
  5. Check out the insulates hand tools available with the technician
  6. Ensure the Healthiness of the test leads (Check for any insulation Damages)
  7. Ensure appropriate PPE’s available at sites

Resources – Tools and Equipment Required

  1. For Continuity Test
    1. Multimeter with leads
  1. For Insulation Resistance measurement Test (IR Test):
    1. Test Leads
    2. 500 V, 5000 V Insulation Tester
    3. Insulated Hand Tools
  1. Winding resistance test
    1. Multimeter with leads
  1. Polarity test
    1. 9V battery
    2. Galvano meter
    3. Connecting leads
  1. Ratio test
    1. Single phase Variac 28 A / auto transformer
    2. Step-up transformer 230/5000 V
    3. Multimeter
    4. Connecting leads
    5. Single phase AC supply
  1. Tan delta test
    1. Tan delta kit
    2. Connecting leads

Operational Sequence of work(Test Procedure)

  1. Obtain the work permit from the client
  2. Ensure zero energy in the CVT
  3. Ensure the external wires are removed from CVT
  4. Ensure the all connections are in tight before testing

Insulation Resistance(IR) Test

 

  • Connect the 5000 VIR kit earth lead with earth and connect Positive lead with CVT primary side
  • Connect 5000 V IR kit the positive lead to primary and earth lead to secondary each core
  • Connect 5000 V IR kit the positive lead to core and earth lead to earth
  • Connect the positive lead to any core and earth lead to another core if applicable
  • Select the required voltage range in IR kit and apply the voltage to the CVT
  • Note the IR values from IR kit
  • Discharge the voltage from the CVT with respect to earth.
DO’S DON’TS
  • Remove the CVT primary earth
  • Check all the CVT primary and secondary wires are isolated
  • Check the calibration due of the test equipment
  • 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 CVT 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 remove the cables without discharging.
  • Don’t test with damaged cables and improper clips
  • Don’t work in wet (rainy) conditions
  • Don’t lose your concentration while testing.

 

Polarity test

  • Connect the galvanometer to secondary terminal –a1/a2/a3 (positive)
  • Connect the galvanometer to earth –( negative )
  • Apply pulse voltage of 9 V battery to primary + positive and earth negative
  • Check the deflection in the galvanometer
  • It must be in clock-wise direction
  • Above mentioned test to repeated remaining cores
DO’S DONT’S
  • Check the connections as per the diagram.
  • Check the CVT secondary cable continuity from CVT secondary to CVT 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

  • Connect the single phase variac 28 A and step-up transformer230/5000 V
  • Connect the step-up transformer across the primary terminals(P1) and earth
  • Connect the variac in A.C supply extension board with ELCB rated with 30 mA
  • Inject the voltage across the primary terminal and earth
  • Note down the corresponding secondary voltage at various cores between phase and earth
  • Switch off the supply to the primary injection kit
DO’S DONT’S
  • Apply the voltage in the primary terminals only
  • Secondary side should be kept open
  • Don’t apply voltage across the secondary terminals

 

Winding resistance test

winding resistance test
winding resistance test
  • Connect the multimeter leads to the CVT 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 CVT secondary terminals.
  • Don’t measure the terminals with dust in CVT secondary terminals.

 

Continuity test

  • Connect the multimeter leads to the CVT secondary winding Terminals
  • Observe the beep sound in multimeter
  • Repeat the above procedure for remaining cores

Tan delta test

Top and middle stacks:

  • Apply 10 KV between flanges of Top/Bottom stacks
  • Carry out measurements in UST mode at 10 KV

Bottom stack connected to EMU PT

  • Connect HV of the test kit at the top flange of bottom stack HF point to be grounded Earth connection of the neutral of the PT to be opened/isolated from ground
  • Top of CVT to be guarded LV lead of the kit to be connected at the top of the CVT guarding
  • Carry out measurements in GST mode at 10 KV
  • Repeat the test with neutral of PT connected to ground
  • In case Tan delta value negative/erratic only capacitance value are to be monitored

Control Measures

  1. OCP for IR and Ratio test for CVT is available with testing engineers
  2. Ensure PTW system in place for the particular bay
  3. Ensure all supply to the equipment is dead
  4. Unauthorized persons should not be allowed to enter Testing area
  5. Barricade the testing area with warning tape
  6. Proper supervision, conduct job specific toolbox meeting and barricading the entire area
  7. Calibrated equipment shall be used
  8. Insulated hand tools shall be used
  9. Avoid joints for power cable and supply has to be taken via ELCB
  10. Avoid Lone working
  11. Use appropriate insulated hand gloves

 

 

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