Transformer insulation resistance

CR4 Home | Electrical Engineering | Electrical Engineering

5 comments

Rate

achawla77 Active Contributor Join Date: Feb 2008 Location: Faridabad India Posts: 20	Transformer insulation resistance 12/28/2008 6:57 AM
	Can some body tell me what could be the possible reason of low insulation resistance of power transformer. Is it Oil, the dielectric strength of oil is 50kV but the water content in the oil are approx 30ppm. or is it due to winding problem.
	Register to Reply

Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Join CR4, The Engineer's Place for News and Discussion!

Good Answers:

These comments received enough positive votes to make them "good answers".

#1 "Re: Transformer insulation resistance" by Siswanto on 12/28/2008 11:58 AM (score 3)

"Almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, vote them!

#4 "Re: Transformer insulation resistance" by KRW43 on 02/17/2009 6:02 PM (score 1)

Siswanto

Commentator

Join Date: Mar 2008

Location: JAKARTA. INDONESIA

Posts: 73

Good Answers: 15

Re: Transformer insulation resistance

12/28/2008 11:58 AM

Dear friend

HAPPY NICE HOLIDAY AND NEW YEAR 2009

Its not so simple to answer this questions, because you do not inform the detail to analyzed, could you inform what test that has been done to this x-mer, and what is the IR that has been measured, phase - phase, phase to ground ? and what is the PI and DAR (dielectric absorption ratio) status, did you test "step voltage" ?

SFRA (sweep frequency response analyzer) is good to analysis for the winding deformation and its position caused by mechanical and thermal effect.............

Following is table that taken from many literatures may can helping you to analysis.

Suggested test limit for service – aged oil by voltage class

IEEE Insulating Fluids Committee, Projects 637

Property	Test Limit
Voltage Class	≤ 69 KV	69 - 288 kV	≥ 345 KV	ASTM Test
Dielectric Breakdown Voltage 60 Hz, KV Minimum	26	26	26	D – 877
Dielectric breakdown voltage KV Minimum [0,040 inch gap thin ]	23	26	26	D – 1816
Neutralization Number mg KOH / g, oil maximum	0,2	0,2	0,1	D – 974
Interfacial Tension Test Dynes/cm, minimum	24	26	30	D – 971
PF, 60 Hz. 25 ^oC Max. Percent	0,65	0,39	0,31	D – 924
Moisture Contents PPM Maximum	35	25	20	D – 1533

RECOMMENDED TEST VALUES CONTINUED SERVICE OF TRANSFORMER INSULATION OIL

Authority	NN mg KOH/g Maximum	IFT Dynes/Cm Minimum	Dielectric D-877 KV Min.	P,F %at 25^oC Max	Moisture Content PPM. Max
Westinghouse	0.15	21	28	1.0
GE	0,20	24	26	0,65
Kemper	0,36	21	24	1,0	25
Factory Mutual	0,25	18	23,5	-
American Nuclear Insures	0,20	22	23,5-31	-	50
EPRI Utility Survey	0,32	23	27	0,80	34
NFPA	0,40	-	22	-	-
IEC	0,50	15	30-50	-	20-30 mg/l

SUGGESTED PROPERTY REQUIRMENTS OF RECLAMED OIL FOR TRANSFORMER References : IEEE Insulating Fluids Subcommittee (proposed, April, 1980] Project 637.

PROPERTY / TEST	LIMIT	ASTM TEST METHOD
Physical
Flash point, minimum, ^oC	140 ^oC	D – 92
Specific gravity 15 ⁰C / 15^oC, Maximum	0,91	D – 1298
Viscosity Maximum At 40 ^oC [mm2 / sec ]	12,0	D – 88 or D - 445
Visual examination	Clear	D – 1524
IFT minimum [dyne / cm]	35	D – 971
Electrical
Dielectric Breakdown Voltage 60 Hz, Min. KV	30	D – 877
Power Factor at 60 Hz, 100 ^oCMax. %	1	D – 924
Chemical
Neutralization Number [NN]Max. mg KOH/g	0,05	D – 974
Oxidation InhibitorMax. % by weight	0,3	D – 2668
Water Maximum, ppm	35	D - 1533

Gas Qualitative and Quantitative Interpretation

TYPES OF PROBABLE FAULTS
Detected Gas	Interpretation
Nitrogen + 5 % or less of Oxygen	Normal operation of sealed transformer
Nitrogen, Carbon dioxide, or Carbon monoxide, or all	Transformer overloaded or operating hot, causing some cellulose breakdown, check operating conditions
Nitrogen and Hydrogen	Corona discharge, electrolysis of water, or rusting
Nitrogen, Hydrogen, Carbon dioxide, and carbon monoxide	Corona discharge involving cellulose or severe overloading of transformer
Nitrogen, Hydrogen, Methane, with small amount of ethane, and ethylene	Sparking or other minor fault causing some breakdown of oil
Nitrogen, Hydrogen, methane, with carbon dioxide, carbon monoxide, and small amount of other hydrocarbons, acetylene is usually not present	Sparking or other minor fault in presence of cellulose
Nitrogen with high hydrogen and other hydrocarbons including acetylene	High energy arc causing rapid deterioration of oil
Nitrogen with high hydrogen methane, high ethylene, and some acetylene	High temperature arcing of oil but in confined area, poor connection or turn to turn shorts

INSULATION RESISTANCE

R = Insulation Resistance of Winding to Ground at 20^oC

C = 0.8 for oil filled transformer at 20^oC

C = 16 for dry compound filled or un tanked oil filled transformer

E = Voltage rating of winding under test

KVA = Rated Capacity of winding under test

Good luck

Rgds

Sis

__________________
LOVE FORUM AND DISCUSSION

No more Good Answers. Go to first "Almost" Good Answer

Anonymous Poster	#5 In reply to #1 Re: Transformer insulation resistance 02/25/2009 8:33 AM
	wht ia the basis of this formulla
	Register to Reply

BertHickman Commentator Join Date: Mar 2007 Location: Chicago suburbs, USA Posts: 73 Good Answers: 21	#2 Re: Transformer insulation resistance 12/29/2008 12:26 AM
	The problem sounds like it may be high moisture content, or insulation degradation (aging or perhaps high temperature damage). A power transformer's associated cellulose-based insulation readily absorbs moisture, lowering insulation resistance. Although it may be possible to use vacuum treatment on the oil to remove moisture, if this does not correct the problem transformer rebuilding or replacement may be the only other option. __________________ Stoneridge Engineering - Wreaking Havoc with Electrons for over 40 Years!
	Register to Reply

biswanath.das Power-User Join Date: Oct 2006 Location: Kolkata, India Posts: 447	#3 In reply to #2 Re: Transformer insulation resistance 01/06/2009 5:54 AM
	Do Formaldehyde content test, it will guide how much insulating papers are healthy. __________________ BNDas
	Register to Reply

KRW43 Associate Join Date: Feb 2009 Posts: 34 Good Answers: 4	#4 Re: Transformer insulation resistance 02/17/2009 6:02 PM
	This is a common occurance that confuses many people. Simply put. If you have a high moisture (water) content in the total insulation system (cellulose + oil) you will get that water migrating between the paper & the oil during the heating and cooling cycles the transformer goes through. as the transformer heats up water migrates to the oil & conversely when it cools the water goes back to the cellulose insulation. If you take an oil sample & get 30ppm it will depend on what temperature you took the sample at. As well the dielectric strength can still be 50KV with that level of moisture but not be a problem. If the IR is low you need to assess that at the temperature it was taken. So if you took the oil sample at high temperatures then did IR at a cool temperature then the moisture will be high in both as you are measuring some of the moisture twice because it has been migrating. You need to try & do both tests at similar temperatures that way you will see the true level of moisture. If the same readings are obtained then I would consider looking at drying out the transformer to remove as much moisture as possible. I hope this helps __________________ Murphy was an optimist....
	Register to Reply Score 1 for Good Answer

No more "Almost" Good Answers.