Transformer Shortcircuit

[aishu]

Can we put the power transformer into service after short circuit testing per IEC 60076-5 (i.e.after the transformer sucessfully passing all the tests after the short circuit tests) in to our 132kV Grid. There are some objection from operation Dept. of our company. Please share your experience.
Thanks you

 

[dpc]

Either it is going to be put back into service or it is not. I would advise energizing at no-load for a period of time, if that it feasible. Then you put it in service and take your chances, just as with any other transformer.

 

[rbulsara]

What are the objections?

Rafiq Bulsara

http://www.srengineersct.com

 

[jghrist]

Why would you do a test if you would not put the equipment in service after it passed the test???

 

[burnt2x]

Wait a minute! You said "short-circuit test" and then "after the transformer sucessfully passing all the tests after the short circuit tests"!
If this is a new transformer, an acceptance testing should be done. You will have to do a lot more testing, and perhaps that includes a "soak test" as mentioned in the early post.

 

[prc]

As per National standards, a short circuit test on a transformer will be considered successfull and complete only when it passes all repeat routine tests (including dielectric tests at 100 %) and type tests after the short circuit test.So you need not have any apprehension regarding putting such transformers in to service.In India we have put hundreds of such trfs in to aervice after successful SC test( rating 10 kVA to 780,000 kVA bank)SC test is not a destructive test,but definitely it put mechanical stress in windings and the extent of change (or damage) can be seen from the extent of change in impedance after SC test. In Large trfs permissible change in impedance is 1 % after test and if the reported change is 0.9% I will feel uneasy eventhough I cannot reject the transformer.

 

[davidbeach]

I don't know IEC standards, but if you are referring to a test in which one side of the transformer is short-circuited and a reduced voltage is applied to another winding to produce full rated current, then there is no harm to the transformer from the test. I would expect that any power transformer would be subjected to that test at the factory. Wouldn't want a transformer that hadn't been properly tested, wouldn't know what to put into the system model without all the transformer parameters that can only come from this test.

 

[prc]

David, when we say SC test it is meant shorting the secondary with rated primary voltage for 0.25-0.5 sec whether IEC or ANSI.Duration will be more as per ANSI. The switching moment on voltgae wave will be taken so as to create maximum assymetry in the current wave.This test cannot be done in factory but only at high power labs like KEMA or test stations where power will be taken from the grid.

In the above test. windings will be stretched to the maximum limit and hence the apprehension of the suitability for regular use.Many years back, it was a special design or development test on the prototype.But today many utilities are considering this as a type test to be done on the first unit.40 years back,when transformers were ordered, utility wanted an impulse test certificate from tests done on a similar transformer ie he did not want impulse test to be done on his transformer. Today, thanks to the progress in fault detection techniques,impulse test is a routine test, at least for EHV units.

The test you mentioned is not short circuit testing,but " impedance voltage and load loss measurement" as per ANSI or "Measurement of short circuit impedance and load loss " as per IEC.This is a routine test and will always be done in factories.

 

[aishu]

Thanks for the replies. But I would like to have replies whether any body successfully put the transformer into service after the test.

Mrrbulsara
The objection is obvious. As per IEC after the shortcircuit tests,all routine tests and impulse tests (if agreed) shall be conducted. However the point is these tests alone may not prove the integrity of transformer. Naturally Operation will have their objection.

jgrist
There are more technical papers on the present performance of transformers during short circuit. Some of the technical papers mention as many as 30% transformers failed during short circuit.Margin in the present transformer design is very less.Hence our company want to carry out short circuit test and not just rely on calculations only.

Just by passing the test or routine testing the transformer after short circuit as suggested by IEC, we can not conclude the transformer is 100% healthy.That is why I would like to have member experience in this.How many of you used the transformer successfully after short circuit test in network

burnt2X
Please check IEC 60076-5.

Prc
IEC advises 2% change in reactance value acceptable criteria for transformer 2.5MVA to 100MVA and for above 100MVA it is 1 to 2%. For values above this, detail examination required after dismantling. Is your transformer manufacturer guarantee the performance or you accepted without guarantee. We would like to know the risk in accepting the transformer.

 

[prc]

aishu,As per IEC limit of reactance change for Category 3 trfs is 1 %.There is a provision that up to 2 % variation can be accepted subject to agreement between purchaser and manufacturer.Rarely any one will agree for more than 1 % change for large units.My experience with SC testing on large units 270 MVA- 780 MVA bank( total 6 units) is reactance change was in the range of 0.1-0.3 %.
As regards,using short circuit tested trfs in service I confirmed that in India there are hundreds of transformers in service which were SC tested. Manufacturers give performance gurantee for such units.Risk is not for utility but for the manufacturer.My personal experiene is from 1984 when a 12.5 MVA traction feed transformer was Sc tested and then it was in service for many years.

 

[aishu]

Auxiliary Question for Transformer Short Circuit


Suppose I would like to purchase 60MVA 132/13.8kV YNyn0d1 transformer. I would like to accept the transformer without short circuit and on the basis of calculation only. Calculation can :
a. comparison with transformer (reference transformer) which passed short circuit test or
b. Manufacturer design rules.
We would like to select calculation based on transformer per (a) i.e transformer passed short circuit test.As per IEC for transformer with absorbed power (rated power/per unit short circuit impedance) 30% to 130% relating to reference transformer (which per IEC this is known as similar transformer) can be accepted with calculated forces not exceeding 120% times of reference transformer.

This seems to be large margin.My question is can we just go per IEC or suppose I require 100MVA power transformer using some thump rule I insists the manufacturer should have conducted short circuit test for 40 to 120 MVA and then per calculation I can accept 100 MVA power transformer.

As I already mentioned we prefer manufacturer who conducted short circuit tests near the rating I require then accept the new transformer and not just calculation only.

Please share you experience. Sorry for reference to IEC as in our part of the world (Middle East) IEC is normal standard reference. Presume ANSI requirements aresimilar

Thanks

 

[rbulsara]

aishu:

NO it is not obvious yet what the objection is. Your OP implies you concluded the short circuit test as well as other tests and yet operations will not allow putting it back in service.

While your last post suggests that you want to accept the transformer without the short circuit test (and perhaps other tests) and operations has objection to test.

Which is correct? To me sounds like your operations are in the right. No test prove all the integrity of a transformer, but they do provide a reasonable assurance. But you do need to do some testing. When there are standards recommending them, it is only wise to follow them.



Rafiq Bulsara

http://www.srengineersct.com

 

[edison123]

I'm confused too. The trafo passed the required tests but now the operation people won't accept it because these tests could have damaged it ?

Muthu

http://www.edison.co.in

 

[jghrist]

It is true that a power transformer can be damaged during a short circuit test. This is why the test is normally done only as a type test and routine tests are required after the short circuit test. To require the test and then not use the transformer after it successfully completes the test, however, seems like a very expensive approach unless a very large number of transformers of identical design are being purchased. This might be the case for small distribution transformers but not for power transformers. I assume that you are not objecting to paying for the transformer after it is tested. I don't think you could get away with this unless it was clearly spelled out in the purchase contract.

Ask the operations department this:

If you had a power transformer in service and there was a fault on the secondary bus, would you scrap the transformer if there were no indication that it was damaged by the fault?

If not, why scrap a new transformer that was subject to a similar fault?

 

[prc]

jghrist,your logic is interesting,But only one difference-The severity (peak value of current) of SC test current is much more than the current from any normal secondray line faults

 

[burnt2x]

IMHO, power transformers are made one at a time and it's not some kind of "molded" production process where you only test a few units and if the sample passes, the whole production run is accepted! Each unit has to be tested to assure reliability when operating them. I think operations call the shots and not the statisticians cranking math machines.