TESTING of 1250kVA TRANSFORMER

[arnsam]

Hi engineers
I have a 1250kVA 11/0.4kV DYN11, oil cooled, distribution transformer at stock kept idle for about 7-8 years without being connected to power.
I want to test weather is still in good condition. May you please give me the Testing procedures I should follow.
regards
arnsam

 

[dewone]

If it was properly stored. Oil sample, megger, turns ratio, and test any devices on the unit ie. pressure vacuum, oil level, temp gauge fan start, alarm, trip... Should give you a good indication of weather to energize. Had it been just shoved aside. The above plus Doble testing.

 

[smur]

I will do tha following regarding this transformer:

1. Meggar the transformer with an appropriate meggar. This will have the correct voltage. Note the insulation resistance. If more than 200Mohms, we can start.
2. Test the oil. Take a sample and do a dielectric test.
3. Clean all the insulators
4. Check all other devices on the transformer
5. Take a 480V autotransformer and apply 480 volts to the input slowly. You can rent one ( 50kva ). Note the voltage on the output,\. Before you do this, you will have to isolate the transformer, keep both breakers open.
6 Make sure that it follows the turns ratio
7. If every thing is OK, you can apply full voltage to the primary. It will be better if you reduce the current setting on the primary winding relay to minimum say cross the inrush
8. Note the voltages on both primary and secondary
9. If OK, then close the secondary breaker to the bus
10. Keep on 1 load breaker say for 100 amps. Let the transformer warm up for 2 hours.
11. If OK, then you can increase the load slowly.
12. Monitor the transformer for the whole day.

 

[oldfieldguy]

arnsam--

Attend to the physical things: Clean insulators, pull and have an oil sample analyzed (physical properties as well as dissolved gas analysis), check external controls and devices. If it has a nitrogen blanket, some people do an oxygen analysis on the nitrogen. I don't consider that necessary at this size of transformer.

Electrical tests: turns ratio, winding resistance, insulation resistance (megger), insulation power factor and excitation tests.

If the transformer shows good results for these tests, energize it with confidence. Some clients want to let it "percolate" overnight before putting on load, but many more don't.

Do monitor temperature after the transformer is loaded, and after two to four weeks in service, again pull an oil sample for dissolved gas analysis. Compare these results with the first one and act on any changes as needed.

If the dissolved gases are still good, you're going be in good shape with the transformer.

old field guy