winding insullation resistance 2

[martense]

hi every one,

i need help on my transformer, we have measured the value winding insullation resistance of secondary-ground of our transformator and the value was 0.1 MegOhm,

the transformator is 1600 KVA, primer 20 KV to secunder 400 Volt,

my question is , is it possible to have 0.1 MegOhm reading, the transformator is still running just fine (not short)?is it not suppose tobe short? when the reading 0.1 MegOhm of secunder-Ground winding insullation resistance?

thank you

regards,

 

[Addy71]

The 0.1 megohms is on the LV side, so machine can run. The winding IR has weakened, but the winding is not grounded. Oil filtration & dry-out can improve the IR value as required.

Regards,

Aditya

 

[alehman]

That seems very low to me. Is it oil insulated or dry?

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[pwrengrds]

Yes it is possible, are you sure of what you were testing?

Is this a Y or delta winding? Was the secondary ungrounded during the test? Was the X0 cleared of intentional grounds, were lightning arrestors cleared from the low side? Was the load cleared from the secondary? (maybe Y connected PT's on the low side)

Seems a little low to me also, but if I saw that low of reading I would get my ohm meter out and find out how low it really was.

Good Luck

 

[Zogzog]

Did you disconnect the Nuetral?

 

[martense]

@alehman : its oil transformator
@pwrengrds : the connection was Dyn5, the winding insullation just tested
@zogzog: yes we have disconnect the neutral.

for the winding insullation resistance 400 Volt how many voltage i should inject to know the winding insullation value?
does anyone have the link for this?

thank you guys,

regards
Marten

 

[Zogzog]

400V winding should be tested at 1000V for 10 minutes recording results at 30 sec, 60 sec, and 10 minute. Calulate DAR and PI ratios (However it sounds like your wont matter at such a low reading).

I suggest you get these test standards

http://www.netaworld.org

 

[prc]

Zogzog-AC 400 V winding is to be tested with 500 V DC(max)megger.( see Megger "A guide to diagnostic insulation testing above 1 kV)The test you mentioned is for checking the wetting of inside insulation(Polarisation Index)

More than absolute values of IR, check with earlier bench marks.Any drastic change should be a cause for concern and to be investigated.Years back, I found a working 10 MVA 66 KV star connected transformer with zero megger value for 66 kV winding.I found the neutral bushing (66 Kv condenser bushing )was full of water -through a damaged gasket on expansion chamber of bushing!

IR of transformer is influenced very much with the resistivity of oil used.Modern oils are highly refined, hydrocracked oils with very little conducting polar compounds.Hence with these oils one will get IR of Hv transformers in the order of 10,000 Mohms,while 20years back 500 Mohms was very good value.

 

[martense]

@zogzog and @prc : thank you

 

[Zogzog]

prc,

Not sure where you are getting your information but the minimum DC megger test value for transformer windings rated at 0-600V is 1000VDC per ANSI and NETA MTS standards. Table 100.5.

The minimum acceptable result for oil filled transfomrer is 500 Megohms. Per the same standards.

Your publication by Megger you quoted is not a recognized standard and the version you are quoting is dated.

 

[prc]

Zogzog- I stand corrected. Thank you for the same.I am not familiar with NETA standard.I believe it is testing association standard. Can you give the ANSI std number and title?

The booklet I mentioned is not outdated. It is by Megger (the company that invented insulation testers in early 1900s)last revised in 2002.IEEE std 43-2000 Insulation measurement for rotating machinery also recommend maximum 500 V DC megger for winding rating less than 600 V.( Table 1)

When we apply 1 KV DC on a 400 V Ac winding,we are applying 4.3 times rated working voltage for 10 minutes.As transformer manufacturer I would prefer to avoid it.

For minimum IR values there were many thumb rule formulae ( see earlier editions of Transformer Maintenance by SD Meyers)The major change that happened recently was sudden increase in IR values due to better purified transfornmer oils used today.

 

[edison123]

zogzog - Do DAR & PI matter in a liquid filled trafo ? I have never done it and am just curious.

Muthu

http://www.edison.co.in

 

[Zogzog]

Are you refering to "A stich in time"? New revision in 2006.

The NETA testing standards are ANSI standards. ANSI/NETA MTS-2007.

The test voltages and minimum values in the ANSI/NETA Std (Table 100.5) are derived from IEEE Std C57.12.90-2006
IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers

IEEE std 43 is more for motors and generators.

 

[Zogzog]

See table 100.5 attached

 

[prc]

Zogzog: Thank you for NETA table.But ANSI C57.90 & C57.00 are not giving the minimum IR values or test voltage for rated voltage of 400 V winding. In our country applied voltage test for 400 V winding is 3 kV Ac for 1 minute.ANSI C57.90 do say that the Dc voltage applied shall not exceed applied test level.That way 1 Kv megger for i minute is Ok.But if you are applying for 10 minutes (for PI) wont it be safer to go for 500 V?

I was referring to megger publication MEG-116 of 10/2002 "A Guide to Diagnostic Insulation Testing above 1 kV" Probably they might have taken from IEEE 43 and may be general for application.But my above argument is still valid and amy be 500 V may be a better bench mark.

edison,it is true PI may be sometimes misleading for oil filled insulation,but unfortunately many users stick to it and IEEE standards recommends minimum values for oil filled trfs.

 

[edison123]

Thanks prc. Is there a recommended DAR and PI for oil filled trafos ?

And that NETA table is only a representative value, not even recommended value.

Muthu

http://www.edison.co.in

 

[Zogzog]

So you guys both would rather follow a piece of literature by a company selling test equipment than following an internationally recognized standard?

NETA also has DAR/PI limits, if you care.

"And that NETA table is only a representative value, not even recommended value. "

Huh????

 

[prc]

Dear zogzog, Dont get offended.In the beginning itself, I said I stand corrected.I dont know v-t characteristics of 400v transformer insulation.I only doubted about applying 4.33 times overvoltage for 10 minutes.


IEEE 62-1995(R2005) "Guide for Diagnostic Field Testing of Oil filled Power Transformers" recommends following regarding minimum IR &PI ( Clause 6.1.5.1 &6.1.5.2)
1)No specific absolute values of acceptable IR can be given .However reference should be made to previous test history to establish a trend.

2) For small trfs,the PI will be equal to 1 or slightly higher.Larger trfs may exhibit a PI of 1.1 -1.3.In general, a high value of PI indicates that the insulation system is in good condition.A PI <1 indicates that immediate correction is required.

But in India there are utilities who insist for a minimum PI of 1.5.Earlier days, with poor quality oils it was easier to demonstrate such high PI values.But with todays highly purified oils ( which give extremely high IRs) it is almost impossible to get such high PI, pointing out more and more to the irrelevance of absolute values for these parameters.In 1970s, Indian National standards ( other standard too )had minimum IR value graphs.These were removed in later editions.

 

[edison123]

well zozgog. We guys are not in the neta world and are quite confident about our testing processes based on our performance over two decades. As far as I know, there are no Indian standards that insist on minimum IR.

And the IR levels mentioned in neta are quite impractible in my opinion.

Muthu

http://www.edison.co.in

 

[Zogzog]

Well opinions and internationally recognized standards are 2 very different things.