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

Various electrical laboratory test and purpose

Equipment name- LT & MV Power Cables, Go Switches, AB Switches, porcelain Insulator, Current Transformer, Voltage transformer. Test Name 

  Partial discharge test – Partial discharge measurement of cables is very important as it gives an indication of expected life of the cable. When a cable is subjected to high voltage and if there is a void in the cable, the void breaks down and a discharge takes place. 

Impulse withstands voltage test- The test samples are subjected to positive and negative impulse voltage of magnitude as specified in specification; the sample should withstand without any damage. 

Power frequency withstand voltage test (HVT)- electrical products are tested for power frequency A.C. and D.C. voltages. The test voltage is applied on the sample for specified time. the sample is said to have withstood the test if no insulation failure/ flashover /puncture takes place during specified time duration. Dielectric power factor test (Tan Delta Test) -the healthiness of electrical samples can be determined by the ratio of the resistive component to the capacitive component. For good samples, this ratio would be quite low. This ratio is commonly known as tan δ or tan delta 

Hot Set Test - A Measure to Determine the Amount of Cross-Linking Achieved, for Cured Materials such as XLPE and Elastomers Hot Air Aging Test- The hot air aging test is one of the main test methods for evaluating polymer materials and studying the aging resistance of polymer materials. It is usually carried out in a constant temperature blast drying test chamber.

 Electrical Resistance- Electrical Resistance is an electrical quantity that measures how the device or material reduces the electric current flow through it. The resistance is measured in units of ohms (Ω). Tensile Strength: Tensile strength is usually measured as the amount of force in newtons per square mm or megapascals (MPa) required to pull a specimen to the point of material failure. This test is accomplished by placing a dumbbell shaped specimen into the grips, or jaws. 

Elongation – Elongation is measured by applying tensile force, or stretching the material in the same manner as described previously, and determining the change in length from original. Elongation is expressed as a percentage of the original length. Ultimate elongation is the percentage change in length from original to rupture.

 Thermal stability – tests are conducted on PVC materials in order to determine the rate of thermal degradation at elevated temperatures. 

Power Frequency Wet Voltage Withstand Test -The test voltage applied to the samples is the specified value of the wet power frequency withstand voltage adjusted for atmospheric condition at the time of test.

 Water Absorption – 

 a) This test is done to ascertain the quantity of water absorbed when the insulation or sheath of cable laid underground comes into contact with water. 

 b) Loss of mass is the evaluation of loss of plasticisers and lubricants due to volatilisation during heating. 

Shrinkage Test – Cable testing for shrinkage is designed to check if the insulation material will shrink back whilst in service Heat Shock Test – Replicates the stress of bending and heating on the cable to check for any failure as evidenced by cracks appearing in the materials Hot deformation test – Thermoplastics tend to deform under high temperatures and small damage during movement or under installation to cable, can cause a puncture or cut of insulation Cold bend and cold impact testing - This method determines the resistance to impact or shock of the thermoplastic and elastomeric insulation or sheath at low temp 

Insulation Resistance (IR) test- insulation resistance test measures the total resistance between any two points separated by electrical insulation 

Bending test - bending tensile test) is a method of testing materials for their bending strength. Flammability test - Flammability testing determines how easily a material or finished product will ignite or burn when exposed to or used near fire or heat.

electrical laboratory test

Heat cycle test - The test to confirm the effect on power supply caused by the repetition of temperature change. 

ASTM G154 _ (UV Test) ASTM G154 - testing uses fluorescent light sources that can simulate UVA or UVB. This technique is used to evaluate the impact of UV exposure on a product’s mechanical properties 

ASTM G53 (UV Test)- This practice covers the basic principles and operating procedures for using fluorescent ultraviolet (UV) and condensation apparatus to simulate the deterioration caused by sunlight and water as rain or dew. This test covers the technique to confirm the thickness, diameter of sample, height, width & length conform to relative specification requirements. 

Test Method for Density of Smoke or ASTM D2843-22- This test method serves to determine the extent to which plastic materials are likely to smoke under conditions of active burning and decomposition in the presence of flame. 

Halogen Acid Gas Evolution- The test is to determine the amount of halogen acid gases that are created when cable insulation and sheathing compounds are burned 

Oxygen Index with Temperature Index - To determine the minimum percentage of Oxygen required to support flame combustion of a material at Room Temperature or at a Higher Temperature. (Up to 400 deg. C) 

 Torsion test -The purpose of a torsion test is to determine the behaviour a material or test sample exhibits when twisted or under torsional forces as a result of applied. Melt Flow Index, (MFI) is a measure of the ease of flow of the melt of a thermoplastic polymer. 

Carbon Black Content -To determine the average of carbon black content especially in polyethylene materials by pyrolysis method, like plastic pipes, films, sheets, cables Wrapping Test– The wrapping test is a ductility test which is employed to evaluate the ability of metallic wire to undergo plastic deformation during wrapping. 

Carbon Black Dispersion- This test method is used to determine the average carbon black dispersion in materials like Films, Plastic Pipes, Cables, Sheets. 

Stress strain Test Stress-Strain Diagram expresses a relationship between a load applied to a material and the deformation of the material. 

Source: Ghaziabad Testing Laboratories Pvt. Ltd