Answer / satya dev

electrical test to be performed are as under.

1- Magnetic balance test.
2- Megnetising current test.
3- Turn to turn ratio test.
4- Vector group test
5- Knee point test
6- Winding ressistance test( Phase - Phase - Eatrh)
7- Meggering of bushing.
8- Tan Delta test.

All test must be performed after disconned to the
transformer from the Input & output supply.

Answer / k.prakashchandra

You can conduct tests with the instruments available with
you.If you entrust it to a private firm to conduct all the
tests ,they will charge you about Rs 20,000/-.As you are not
supposed to open the Tr and apply any substantial
voltage.The following tests will be sufficient.1. Insulation
2.continuity 3.bdv of oil 4. winding resistance(with
sensitive ohm meter) .These together with relay operations
will give some idea to the manufacturer.You can conduct
ratio and magnetic balance tests with great care if IR and
bd values are satisfactory.

Answer / satya dev

Dear Mr. Praksh I am agree with you but all the above test
details which given by me is for high rating power
transformer & it must be carried out and Rs 20 K does not
matter with respect to cost of transfromers.

Answer / k.prakashchandra

Test No. 1 and 8 belong to life evaluation tests.As the tr
has been in use, tests No. 4 and 5 may not be necessary.For
Inter-turn short, test No .2 may prove useful if the
previous values are available and magnetic balance test (
No.1 )is really useful.Oil test is just for knowing the
present condition.If the tr has failed within the guarantee
period , we have to take extra care while furnishing the
test results. The firm may put the blame on you,for the
damages for applying the voltages to test.Simple tests and
relay operations are sufficient.It is more difficult to
prove that a tr is healthy by all means.

Answer / k.prakashchandra

Let me cite an example.We have one 220/110 KV 100 MVA Auto
transformer.After 5 years of service ,it tripped with a
sound, on the following relay
indications.Diff,OS,Bucholtz,OC -R ph and EF (REFR not
provided).On meggering without test charging ,the results
were OK with LT R -body being higher.Then we resorted to
continuity test ,which indicated that LT R phase bushing
has been open circuited.On examination it was found that
the joint in the lead from R-phase winding to bushing was
severed due to bad workmanship.We did not conduct any other
tests.

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