15A

Analysis of faults detected by DGA method

in GETCO

Mr.R.D.Naik

Chief Engineer (TR)

Ms. A S Shah

Deputy Engineer (TR)

Dissolve Gas Analysis (DGA)

• Most acceptable predictive maintenance tool for oil filled

transformer.

• The gases are formed due to breakdown of insulating material

under fault condition and dissolve in oil.

• The distribution of gases has been related to type of fault.

• The rate of gas generation has been related to severity of

fault.

Advantages of DGA

• Non intrusive means of condition monitoring

• Advance warning of developing faults

• Prevent unexpected outage

• Status checks on new and repaired units

• Convenient scheduling of repairs

• Reduce risk

Dissolve Gas Analysis (DGA)

• The principal cause of gas generation in transformer are

1.Thermal Energy Dissipation

Decomposition of insulation due to copper loss

Chemical Reaction

2.Electrical Disturbance

Discharge of energy in oil

Arcing

• The distribution of gases has been related to type of fault.

• The rate of gas generation has been related to severity of

fault.

Gases analyzed by DGA

5

• Methane CH4

• Ethane C2H6

• Ethylene C2H4

• Acetylene C2H2

• Hydrogen H2

• Carbon Monoxide CO

• Carbon Dioxide CO2

• Nitrogen N2 -Non fault gas

• Oxygen O2 –Non fault gas

• some of these gases will be formed in larger or smaller

quantities depending on the energy content of the fault.

•Key Gas Method

•Rogers Ratio Method

•Duval triangle

Methods for interpretation of DGA

DGA Diagnostic Tools

[1] Dissolved Gases acceptable limits as per IEEE C57.104

• >10% increase per month above typical levels = active fault

• >50% per week or evolving faults of higher energy = serious

Conditi

on H2 CH4 C2H2 C2H4 C2H6 C0 Co2 TDCG

Lower Upper Lower Upper Lower Upper Lower Upper Lower Upper Lower Upper Lower Upper Lower Upper

1 0 100 0 120 0 25 0 50 0 65 0 350 0 2500 0 720

2 101 700 121 400 36 50 51 100 66 100 351 570 2500 4000 721 1920

3 701 1800 401 1000 51 80 1010 200 101 150 571 1400 4001 10000 1921 4630

4 1800 inf 1000 inf 81 inf 201 inf 151 inf 1401 inf 10000 inf 4631 inf

1 Normal

2

Above normal value. Additional investigation needed to establish

trend. Fault(s) may be present.

3

High level of decomposition. Additional invetigation needed to stablish

trend. Fault(s) are probably present.

4

Excessive decomposition. Continued operation could result

to failure of the transformer.


[2] Key Gases Method

Individual value of gases are taken in to consideration for evaluation.


[3] Roger’s Ratio-based method Ratio method has to be employed in the incipient fault diagnosis. It has to be employed also

in conjunction with key gas method to check the severity of the fault.

Code for examining analysis of gas dissolved in Mineral oil

C2H2 CH4 C2H4

C2H4 H2 C2H6

Code

<0.1 0 1 0

0.1-1 1 0 0

1--3 1 2 1

>3 2 2 2

Analyses 0 0 0 NO FAULT

1 1 0 LOW ENERGY PD

2 1 0 HIGH ENERGY PD

1--2 0 1--2 LOW ENERGY SPARKING ARCING

1 0 2 HIGH ENERGY SPARKING ARCING

0 0 1 THIRMAL FAULT <150C

0 2 0 THIRMAL FAULT 150C-300C

0 2 1 THIRMAL FAULT 300C-700C

0 2 2 THIRMAL FAULT >700C


[4] Duval Triangle

• The method is based on 3

gases (i.e.CH4, C2H4,

C2H2)

• The triangle plots the

relatively percentage of

three gases on each side.

• The figure depicts 6 main

zone of faults .

• Each DGA test report gives

one point in triangle , which

indicate the fault in the

respectively zone.

Type of Faults

•IEC 60599 has defined 6 basic types of faults detectable by DGA.

1. Partial discharges of the corona-type (PD).

-Typical examples: Discharges in gas bubbles or voids trapped in paper, as a

result of poor drying or poor oil-impregnation.

2. Discharges of low energy (D1)

-Typical examples: Partial discharges of the sparking-type, inducing carbonized

punctures in paper.

-or low-energy arcing, inducing surface tracking of paper and

carbon particles in oil.

3. Discharges of high energy (D2)

-Typical examples: High energy arcing, flashovers and short circuits with power

follow through, resulting in extensive damage to paper, large

formation of carbon particles in oil, metal fusion.

4. Thermal faults of temperatures < 300 C (T1).

Typical examples: Overloading, blocked oil ducts paper turning brown.

5. Thermal faults of temperatures between 300 and 700 C (T2)

Typical examples: Defective contacts, defective welds, circulating

currents.

Carbonization of paper or formation of carbon

particles in oil.

6. Thermal faults of temperatures > 700 C (T3).

Typical examples: Large circulating currents in tank and core, short

circuits in laminations. Sparking/burning of joints

formation of carbon particles in oil

Type of Faults

Population of Power Transformers in GETCO

> 30 Yrs

16%

21 - 30 Yrs

17%

0 - 10 Yrs

32%

11 - 20 Yrs

35%

Sr. No. Voltage

Class

Total

Nos.

Aging of Power X’mers in Years

0 - 10 11 - 20 21 - 30 > 30

1 400 KV 20 08 08 04 00

2 220 KV 195 60 85 31 19

3 132 KV 162 14 68 27 53

4 66 KV 1885 641 635 329 280

Total 2262 723 796 391 352

Practices Adopted by GETCO

• DGA for 132KV & above class transformers once in a year.

However, frequency depends on rate of rise of individual

gas.

• Table top assessment for normal & abnormal unit.

• For abnormal unit, analysis of DGA done by Key gas,

Rogers ratio & Duval triangle through our own software

• For abnormal unit, further low voltage testing & rectification

has been planned.

Summary of results of typical cases of DGA

(found in GETCO)

Sr.

NoS/S MVA

Key Gas

Method

Ratio Method

Analyses

Duval

Triangl

e

Fault observed

1 Asoj 167

Thermal

Fault

>500C

Not defined T3

Magnetic shunt on inner

tank body found shorted

with multiple earthing

2 Chorania 167

Thermal

Fault

>500C

Not defined T3

Tertiary winding connections

loose. Other loose points

observed at HV

3 Vadavi 315

Thermal

Fault

>500C

Core and Tank

circulating

current,

Overloaded

Joints

T3No abnormality observed

during internal inspection

•Voltage Ratio 400/220 KV


(found in GETCO)

Sr.

No.S/S MVA

Key Gas

Method

Ratio Method

Analyses

Duval

TriangleFault observed

1 Shivlakha 100Thermal

Fault >500C

Core and tank

circulating

current,over

loaded joints

T3

In Y phase switch many

contacts found eroded. In

R phase and B phase

also contacts of pre-

selector found eroded.

2 Kim1 100Thermal

Fault >500CNot defined T3

OLTC lead connection to

winding loose, sparked

and bolt thread damaged

3 Khanpur 100Thermal

Fault >500C

Core and Tank

circulating

current,

Overloaded

Joints

T3

Nut found at the bottom

touching the core and

also water found at

bottom.Multiple earthing

of core



(found in GETCO)

Sr.N

o.S/S MVA

Key Gas

Method

Ratio Method

Analyses

Duval

Triangl

e

Fault observed

4 Viramgam 50 High arcing Not defined T3OLTC selector and

diverter damaged

5 Sachin 100

Thermal

Fault

>500C

Circulating

currents in

winding

T3

OLTC Tap contacts

and terminals found

overheated

6 Kim2 100

Thermal

Fault<>500

C

Not defined T2

OLTC Y phase lead

connection to winding

loose with missing cap

nut found at bottom.

Copper wire also

found at bottom



(found in GETCO)

Sr.N

o.S/S MVA

Key Gas

Method

Ratio Method

Analyses

Duval

Triangl

e

Fault observed

7 Ranavav 50All within

limit

Partial

Discharge -

corona with

tracking

PD

OLTC diverter switch B

phase main lead bolt

at the common ring

loose, sparked with

insulation damaged

8 Ichchhapur 100

High

energy

discharge

Arc-- with

power flow

through

D1

66 KV Y ph.lead found

opened.2 bolts of

lower portion of jumper

found melted.



(found in GETCO)

Sr.N

o.S/S MVA

Key Gas

Method

Ratio Method

Analyses

Duval

Triangl

e

Fault observed

1 Jambuva 50

Thermal

Fault

>500C

Not defined T3

Yoke and Core near

HV R phase found

overheated and

damaged

2 Idar 50

Thermal

Fault

>500C

Not defined T3OLTC polarity switch

damaged

3 Bhestan1 50

Thermal

Fault

>500C

Core and Tank

circulating

current,

Overloaded

Joints

T3

OLTC diverter switch

lead to drum loose at

drum end

4 Bhestan2 50Overheatin

g ,oilNot defined T2

Oil leakage from OLTC

to main tank due to O

ring damaged and

loose contact.



(found in GETCO)

Sr.

No.S/S MVA

Key Gas

Method

Ratio Method

Analyses

Duval

Triangl

e

Fault observed

1 Gotri 12.5Partial

Discharge

Partial

Discharge -

corona with

tracking

PD

LV B phase winding

insulation at lead

insulation failed at

joint


Analysis & classification of faults based on

inspection

• 4 main type faults are found (i.e. T2,T3, PD, D1)

Key gas method Identify the general faults

Ratio & Triangle

method

Further qualitatively analyzed the fault which

enabled us taking decision regarding continuation

in service or immediate repair

T3Hot spots in oil with overheated joint in oil or circulating

current in core & winding

T1 & T2Thermal faults in oil & paper with heating and browning of

paper.

D1 & D2

Electrical discharge with power follow through with lesser

or higher degree & may lead to failure of paper insulation

& winding

Case Study-1 : 220/66KV, 100 MVA (220 KV Sardargadh)

[1] Year of Mfg. :- 1997

(2) Year of Comm. :- 1997

[2] Problem observed :- In April 2009, during annual routine monitoring of DGA,

the higher concentration of key gases were detected.

DGA Test Results

Date H2 CH4 C2H4 C2H6 C2H2 CO2 CO

17.04.08 10 8 4 3 0 2505 431

17.04.09 28 19 23 4 39 3625 330

25.06.09 32 21 23 4 37 3983 477

21.07.09 35 20 21 4 24 3629 410

28.08.09 19 16 20 4 19 3386 333

30.09.09 38 22 23 5 11 3937 446

16.11.09 0 0 0 0 0 79 13

17.03.10 1 0 0 0 0 430 40

03.08.10 6 3 2 1 0 1328 121

23

1.By Key Gas :-High energy fault

2.By Ratio method :-Arc with power follow through

3.By Duval Triangle :-D2- Discharge of high energy. High energy arcing, flashovers

and short circuits with power follow through, metal fusion are

some of the example of D2.

DGA Analyses


•The Trend of gas concentration and further testing:-

• The test was repeated after one month, there was no increase in

concentration of dissolved gasses.

• The low voltage test including winding resistance indicates no fault.

• The trend as per triangle indicates there was either one time arc

between joint, which has subsequently stopped or there may be mixing

of Tap changer oil.

• In September 2009 --- increasing trend of Hydrogen.

• The internal inspection of transformer through inspection windows was

arranged.


• 80 % of Lead wire of B phase bushing found burnt at the top joint

• The rectification done at site and transformer put in service

• Catastrophic failure averted

Case Study-2 : 220/66KV, 100 MVA (220 KV Chhatral)

[1] Year of Mfg. :- 1996

(2) Year of Comm. :-1996

[2] Problem observed :- In August-2009, during annual routine monitoring of

DGA, the higher concentration of key gases were

detected.


06.08.06 8 6 9 1 0 922 123

20.06.07 0 3 0 0 0 814 91

14.07.08 5 4 3 1 0 2556 266

12.08.09 44 207 186 50 0 1399 99

24.09.09 85 732 642 211 0 6619 586

16.10.09 1 1 1 0 0 123 24

17.11.09 13 7 5 1 0 442 36

10.03.10 13 7 3 1 0 1175 78

25.06.10 13 13 5 3 0 1507 92

27

1. By Key Gas Method :-Thermal fault -Oil

2. By Ratio : Not defined

3. By Triangle Method : Thermal faults of temperatures between 300 and 700 C

(T2).

Carbonization of paper, formation of carbon particles in oil.

Typical examples: lead / paper involvement problem.



• Periodical test results indicates rise in concentration of H2, CH4,C2H4 , C2H6 and

CO gases.

• Low voltage test including winding resistance indicates no fault.

• SFRA testing was carried out, no abnormality observed.

• The increasing trend of key gases became faster over a period of time and DGA

trend indicates persistence thermal fault.

• Due to increasing trend of key gases, it was planned to carry out internal

inspection at site.


•Internal Inspection

Internal inspection of transformer was done on dtd.26.09.09.

During internal inspection, equipotential link between two top frames were found burnt.

Also frame earthing link found burnt.

The rectification done at site and transformer put in service.

Catastrophic failure averted

Case Study-3 : 220/66KV, 100 MVA (220 KV Kim)

[1] Year of Mfg. :- 2001


[2] Problem observed :- In July 2008, the high concentration of key gases were

observed during annual testing.

DGA Test Results


18.07.08 10 92 125 15 0 2888 201

25.11.08 18 107 146 20 0 2928 143

09.01.09 19 115 151 23 0 3226 153

19.02.09 14 131 181 25 0 3452 153

12.03.09 22 144 190 28 0 3826 172

18.03.09 0 10 21 4 0 452 14

15.06.09 1 5 20 3 0 476 15

06.10.09 4 6 16 3 0 1023 28

07.11.09 4 7 19 3 0 1290 57

32

1. By Key Gas Method :-Thermal fault -Oil

2. By Ratio : Thermal Fault >700 C

3. By Triangle Method : Thermal faults of temperatures > 700 C (T3).

T3 Typical examples: large circulating currents in tank and core, short circuits in

laminations. Sparking/burning of joints


1

2

DUVAL TRIANGLE

0 100

0

100 0

C2H2

CH4 C2H4

100

D1

D2

DT

T1

T2

T3



• Periodical test results indicates rise in concentration of CH4,C2H4 and

C2H6 gases.

•Routine test of above transformer (Magnetic balance test, magnetic

current test, voltage ratio test, Short ckt. test, measurement of winding

resistance) was carried out on 20.1.09.

•All test results were found in order except winding resistance test.

Winding resistance of LV Y ph for tap no.14 was found high compared to R

&B phase.

•OLTC inspection by draining oil was carried out in presence of OEM on

dtd.24.2.09, No abnormality has been observed. Again winding resistance

measured on all tap, but on tap no.14 winding resistance found higher.

•For investigation of fault, transformer detailed inspection after draining of

oil and opening of all inspection covers was carried out from dtd . 12.3.09

to 15.3.09.

During detailed inspection, BV6 connectionon OLTC switch ( i.e. BV6 No. connectionused for Y-Ph tap no 14 connection 2-6 ) issparked and melted due to looseconnection.

The bolt thread found damaged up to 3threads. Same was replaced by new onewith two washers.

Minor pitting was also noticed onconnection bush and Lug. Same wascleaned and smoothen by the emery paper.

Also all other connection checked fortightness. No other abnormally was foundinside the transformer.

After refilling of oil and necessary testingtransformer was put in service.


Case Study-4 : 132/66KV, 50 MVA (220 KV Jambuva)

[1] Year of Mfg. :- 2008


[2] Problem observed :- In Nov’10, during annual routine monitoring of DGA,

the higher concentration of key gases were detected.

DGA Test Results


02.06.08 4 0 0 0 0 187 28

23.06.08 5 0 0 0 0 168 21

25.06.09 13 1 2 0 0 1714 110

23.11.10 29 63 151 17 0 1918 115

36

1.By Key Gas :-High energy fault

2.By Ratio method :-Core & tank circulating current, over loaded joints

3.By Duval Triangle :-Thermal faults of temperatures > 700 C (T3).

T3 Typical examples: large circulating currents in tank and

core, short circuits in laminations. Sparking/burning of joints

DGA Analyses



• The low voltage test including winding resistance carried out, under

which

• In turn ratio test,%Deviation not available at Tap Nos 1,5,13 for B

Ph.

• Winding resistance not available for Tap No.1,3,5,7 & 13 for LV b

Ph.

• For further investigation, the internal inspection of transformer through

inspection window was arranged.


• Fixed & moving contacts of B phase OLTC was found damaged.

• The contacts replaced at site and transformer put in service

• Catastrophic failure averted

Benefits realized

• Total 28 nos of transformers are observed problematic in

DGA in last three years and all are attended at site

before catastrophic failure.

• By attending the transformer at site, huge amount of

procurement cost or its repairing cost saved.

• Reduction in failure

Transformer failure

0

1

2

3

4

5

6

7

8

9

failu

res in

No

s

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Failu

re in

%

400 KV 220 KV 132 KV % failure

400 KV 0 0 1 0

220 KV 6 7 2 0

132 KV 8 2 3 5

% failure 3.93 2.51 1.63 1.33

2006-07 2007-08 2008-09 2009-10

Popul

ation354 358 369 377

Conclusion

• From DGA test , according to key gases value , Criticaltransformers are identified & further rectification actions taken.

• Four main types of faults namely T2,T3,PD and D1 wereidentified .By co relating three methods of DGA ,we were able totake corrective actions before the faults become severe i.e. D2.While the Key Gas method helped to identify general fault, Ratioand Triangle methods further qualitatively analyzed the faultwhich enabled us taking decision regarding continuation inservice or immediate repair.

• We have diagnosed the fault at site and attended it timelybefore permanent failure.

• By SFRA , Accoustic Emission, tan delta & capacitancemeasurement also provide information to locate fault once thegassing in transformer starts.

15A

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