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Do Not Energize Oil-Filled Transformer Without PerformingThese 15 Tests and Checks!

Do Not Energize Oil-Filled Transformer Without Performing These 15 Tests and Checks! (on photo 3-core type oil-immersed rectifiertransformer 380kV; via electric-power-transformers.com)

Obligatory ChecklistThe following tests and checks should be performed at a minimum to ensure that the transformer is ready to beenergized. Do not energize the transformer without performing these tests and checks.

If any of below explained tests fail and transformer is energized anyway, it may lead to the serious hazard, andthats not good Thats not what we want.

Lets see these 15 tests

1. Insulating Fluid TestDraw a fluid sample and test its dielectric strength. Dielectric strength of new fluid should be 26 kv or greater.

2. Pressure TestCheck the transformer tank for leaks by pressurizing the tank with dry air or dry nitrogen through the pressure

The breakdown voltage indicates how well insulating oil can withstand an electrical load and istherefore decisive for the operational efficiency of a transformer. The breakdown voltage is measured

according to VDE 0370 part 5 (IEC 60156); photo credit: SIEMENS

Repairing transformer oil-tank for leakage (photo credit: khia.belzona.com)

test fitting to a pressure of 3 to 4 PSIG. let the tank stand under pressure for one to two hours, then examine thetank and fittings for leaks. leaksabove the fluid level can bedetected by applying soapsolution to all welds, joints, pipefittings, and cable connections.

Upon completion of this test,reduce the internal pressure to1 or 2 PSIG.

3. Insulation Megger TestPerform a 1000-volt Meggertest and a power factor test toensure that none of the windingsis grounded. See the completeprocedure here.

4. Ratio TestPerform a ratio test at each tapposition to ensure that transformer coil ratiosand tap changer connections are correct. Readthe complete procedure of transformer turnsratio test step by step.

5. Continuity Test and Resistance TestPerform a continuity check on eachwinding. Measure the winding resistance ofeach winding and compare results to factorytest values.

An increase of more than 10% could indicatea loose internal connection.

6. Line ConnectionsIn preparation for making line connections, check to make sure that all mating connector surfaces are clean andsmooth. Connections must be tightened appropriately to prevent overheating and possible subsequent failure ofthe connection.

Connections to should be made with care to avoid placing undue stress on the bushings.

7. Tap Changer SettingCheck the tap changer setting to ensure it is set to the proper position for the required voltage. See an example oftransformer tap-changer correct adjustment.

Regular insulation testing is one of the most cost effective methods of identifying

aging of transformer

Oil-filled transformer bushings (photo credit: khia.belzona.com)

8. Delta/Wye and Series/MultipleSwitch SettingsCheck delta/wye and series/multiple switchsettings to make sure they are set correctly.

If these connections are made using aninternal terminal board, check to ensure thatthese connections are made properlyaccording to the chart on the transformernameplate.

9. GroundingCheck to ensure that the transformer tank ispermanently and effectively grounded. Thetransformer tank ground pad is located nearthe bottom of the tank.

10. WiringCheck wiring of control and alarm circuits (ifprovided) to make sure there areno loose connections and nodamage to insulation.

11. Fluid levelCheck to make sure the fluidlevel as indicated by the fluidlevel gauge is as follows:

The fluid level gauge indicatesthe fluid level inside the tank. Thetransformer tank is filled with fluidat the factory to the normal levelat a reference temperature of25C.

12. Tank FinishCheck all painted surfaces tomake sure that there is no damage or corrosion .

13. Bolted ConnectionsCheck all bolted connections for tightness.

14. Tools

On-Line Oil Filtration for Load Tap Changer Compartment (photo

credit: velcon.com)

GE distribution transformer nameplate (photo credit: tucsontransformer.com)

Oil-filled transformer terminal box

Check to make sure that all tools and equipment areaccounted for and have been removed from thetransformer.

15. Fluid TemperatureRead the fluid temperature gauge and make sure thetemperature is no lower than minus 20C before the unitis energized.

Reference: Installation, Operation, and Maintenance ofMedium Power Substation Transformers HowardIndustries

Figure 1 Liquid Level Gauge

Oil transformer bolted connections (photo credit: polywater.com)

Figure 2 Liquid Temperature gauge

Do Not Energize Oil-Filled Transformer Without Performing These 15 Tests and Checks!Obligatory Checklist1. Insulating Fluid Test2. Pressure Test3. Insulation Megger Test4. Ratio Test5. Continuity Test and Resistance Test6. Line Connections7. Tap Changer Setting8. Delta/Wye and Series/Multiple Switch Settings9. Grounding10. Wiring11. Fluid level12. Tank Finish13. Bolted Connections14. Tools15. Fluid Temperature