Transformer Commissioning at site

 Transformer Commissioning Procedure

transformer commissioning at site 

After the satisfactory completion of installation, the following pre-commissioning checks, and tests on instruments must be performed before putting the transformer into service.

 COMMISSIONING TESTS

1) Insulation Resistance (IR)

1a) Before starting this test all the power terminal bushings should be thoroughly cleaned with a dry clean piece of cloth.

1b) At all the tap positions, IR values of windings to earth & between windings shall be measured with designated insulation tester of suitable ratings and readings noted.

Between HV Winding and Earth (Megger) use 5000V or 2500V Insulation Tester,

Between HV and LV Winding (Megger) use 5000V or 2500V Insulation Tester,

Between LV Winding and Earth (Megger) use 1000V or 500V Insulation Tester,

 

1c) IR values obtained should be similar to those indicated in the manufacturers’ test report, furnished with the handing –over documents. In humid weather, IR values obtained may be lower due to condensation on the terminal bushings

1d) If IR values are very low and unacceptable, and then it may be necessary to filter the oil / dry-out the winding till the insulation reaches satisfactory values.

2) Break-Down Voltage (BDV) Test:

2a) Oil samples from tank bottom, tank top, radiator, etc. shall be carefully taken and tested for BDV value, as per clause of manual.

2b) BDV value of oil should be more than 50kV (rms) for 1 minute in the standard test cell

2c) If BDV value is very low and unacceptable (30kV(rms) or less for 1 minute, then it may be necessary to dry-out & clean the oil till the insulation reaches satisfactory values.

It is recommended to contact suppliers or manufacturer for suitable recovery procedure based on available facilities at site

3) Voltage Ratio Test

3a) Apply 3-Phase, 433V AC supply on the HV side and the Voltage Ratio at all tap positions can be derived using suitable precision voltmeter connected to the LV side. A ratio meter, if available can be used for a more accurate measurement

3b) The Ratio value obtained should be similar to those indicated in the manufacturers’ test report, furnished with the handing-over documents.

4) Winding Resistance Measurement Test

4a) Winding Resistance of every phase of each winding should be measured using a suitable DC Resistive Bridge or similar.

4b) Winding Resistance values obtained should be similar to those indicated in the manufacturers’ test report, furnished with handing-over documents

5) Marshalling Box Scheme Check

5a) All the auxiliary wiring from various accessories to marshalling box shall be checked with marshalling box scheme drawing furnished with the handing-over documents.

5b) During testing of accessories like Buchholz relay, etc., an operation of all the alarm/trip contacts shall be checked at marshalling box terminal blocks ensuring both operation and wiring are checked correctly.

6) Buchholz Relay Test

6a) Relay operation for alarm and trip contact shall be checked by injecting air inside the relay through test petcock. Injected air collected inside the relay allows the alarm float/flap & trip float/flap to fall thereby operating their respective switch

7) Temperature Indicator Test

7a) Indicators operation for alarm and trip contact shall be checked by manual stimulation

8)  Off-Circuit Tap Selector (OCTS)

8a) During shipment, generally, the OCTS has not been separated from the transformer so it is not necessary to recheck the internal connections of tapping an internal mechanism

8b) Means of protecting the OCTS from unauthorized operation is provided by using a padlocking arrangement at the designated tap position

 

9)  Before switching, it is to ensure that

9a) All the Oil Shut-off Valves are OPEN and Draw-Off Valves are CLOSED.

9b) All Thermometer Pockets are near filled (85%) with oil.

9c) Oil is at the correct level in the Bushings, Conservator, etc

 9d) In general, colour in breather is blue for silica-gel or yellow/orange for envirogel.

9e) Earthing Connection of Main Tank, Neutral Bushing, Marshalling Box, Control Gear Box, Cable Box, Arcing-Horn, etc., are correctly made.

9f) Bushing arcing horn gap is set correctly

9g) All CT Secondary Circuits are closed

9h) All Air-Release Plugs of Main Tank, Radiator, Conservator, Buchholz Relay, Bushings, etc., are free of air pocket/bubbles.

9i) It is recommended that after Oil-Filling/filtration or before Commissioning, at least 10-12 9j) hrs. Should be allowed for the oil to settle-down and the air is released from all points at 2 hourly intervals.

9k) It is recommended that the transformer is initially energized at NO-LOAD only and checked for any abnormalities for the next 6 - 8 hours

9l) AFTER SWITCHING on no-load, if the Primary side Circuit Breaker is tripped investigate the cause thoroughly and re-energize the transformer ONLY AFTER ensuring that the fault is properly cleared.

9m) If satisfactory (transformer on No-Load) then apply load gradually and observe for any abnormalities for the next 6- 8 hours

9n) If the transformer is satisfactory on-load up to 50% for the first 4-8 hours, shut-down the transformer and ensure that all air-release plugs of the tank, radiator, conservator, Buchholz relay, bushing, etc., are free of air pockets which might have developed during initial loading.

Credit: rajasthanpower.in