winding insullation resistance
winding insullation resistance
(OP)
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,
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,






RE: winding insullation resistance
Regards,
Aditya
RE: winding insullation resistance
Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.
RE: winding insullation resistance
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
RE: winding insullation resistance
RE: winding insullation resistance
@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
RE: winding insullation resistance
I suggest you get these test standards
www.netaworld.org
RE: winding insullation resistance
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.
RE: winding insullation resistance
RE: winding insullation resistance
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.
RE: winding insullation resistance
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.
RE: winding insullation resistance
Muthu
www.edison.co.in
RE: winding insullation resistance
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.
RE: winding insullation resistance
RE: winding insullation resistance
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.
RE: winding insullation resistance
And that NETA table is only a representative value, not even recommended value.
Muthu
www.edison.co.in
RE: winding insullation resistance
NETA also has DAR/PI limits, if you care.
"And that NETA table is only a representative value, not even recommended value. "
Huh????
RE: winding insullation resistance
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.
RE: winding insullation resistance
And the IR levels mentioned in neta are quite impractible in my opinion.
Muthu
www.edison.co.in
RE: winding insullation resistance