OM 7-3 Case Study on DGA Analysis of Oil in Transformer

Power Finance Corporation Ltd.

(A Govt. of India Undertaking)

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Best Practices in Distribution Systems Operation and Maintenance (O&M)Distribution Reform, Upgrades and Management (DRUM) Training Program

Case Study on Dissolved Gas Analysis of Oil in TransformerBy North Delhi Power Limited

Insulating materials within transformers and related equipment break down to liberate gases within the unit. The distribution of these gases can be related to the type of electrical fault and the rate of gas generation can indicate the severity of the fault. The causes of fault gases can be divided into three categories; Corona or partial discharge,Pyrolysis or thermalHeating, and arcing.

These three categories differ mainly in the intensity of energy that is dissipated per unit time per unit volume by the fault. The most severe intensity of energy dissipation occurs with arcing, less with heating, and least with corona.

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Gases are formed inside an oil-filled transformer, in that various gases begin forming at specific temperatures. From the Gas Generation Chart, we can see relative amounts of gas as well as approximate temperatures. Hydrogen and methane begin to form in small amounts around 150 °C. Notice from the chart that beyond maximum points, methane (CH4), ethane and ethylene production goes down as temperature increases. At about 250°C, production of ethane (C2H6) starts. At about 350 °C, production of ethylene (C2H4) begins. Acetylene (C2H2) starts between 500 °C and 700 °C. In the past, the presence of only trace amounts of acetylene (C2H2) was considered to indicate a temperature of at least 700 °C had occurred; however, recent discoveries have led to the conclusion that a thermal fault (hot spot) of 500 °C can produce trace amounts (a few ppm). Larger amounts of acetylene can only be produced above 700 °C by internal arcing. Notice that between 200 °C and 300 °C, the production of methane exceeds hydrogen. Starting about 275 °C and on up, the production of ethane exceeds methane. At about 450°C, hydrogen production exceeds all others until about 750 °C to 800 °C; then more acetylene is produced. It should be noted that small amounts of H2, CH4, and CO are produced by normal aging. Thermal decomposition of oil-impregnated cellulose produces CO, CO2 , H2, CH4, and O2. Decomposition of cellulose insulation begins at only about 100 °C or less. Therefore, operation of transformers at no more than 90 °C is imperative. Faults will produce internal “hot spots” of far higher temperatures than these, and the resultant gases show up in the DGA.

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Permissible concentration of dissolved gases in the oil of healthy transformer in ppm.

Limits <4 years <10 years>10years

CH4 70 150 300C2H4 150 200 400C2H6 50 150 1000C2H2 30 50 150H2 150 300 300CO 300 500 700CO2 3500 5000 12000

Few advantages that fault gas analyses can provide are as follows:1. Advance warning of developing faults2. Determining the improper use of units3. Status checks on new and repaired units4. Convenient scheduling of repairs5. Monitoring of units under overload

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The annexure 1 and 2 give the details of DGA analysis at NDPL

Annexure 1Power Transformer Failure Data sheet

Name of Grid Station Badli

Tx no: Tx 1

Capacity 20 Mva

Make NGEF s.no- 28000 56897

Year of manu 1999

Data of commissioning 2/17/2000 Date of failure : 7/14/2005

Major abnormalities in service life : Terminal Fault (monkey electrocution) on11kv side near bushing -on 08/01/05,Tx was Overhauled on 8th Dec 2004. It is not in load service. (Only kept charged)

Failure data :

1/21/2005 Tx Alarm " Main tank BZ Alarm" observed

Gas Accumulation observed in BZ

1) IR values indicated abnormality

2) HV mag balance indicated no abnormality

3) LV mag balance indicated no abnormality

4) Winding Resistance indicated no abnormality

DGA indicated high level of H2. NO acetelene was traced. Tx was taken back in service and kept charged on.Jan 26 05. However the Bz alarm was again observed on Jan 30 05.( BZ gas accumulation)

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Various Vendors were consulted.

SOS suggested to keep the tx charged till the full blown fault is developed.

CGL suggested the possibility of moisture (left during last O/H) resulting in PD and therefore high level of H2.CGL suggested to apply high vacuum and degassing and then again take tx in service

Feb 21 05: Tx was again charged after the degassing

Tx kept charged and under Observatin: ( DGA of oil on 23 Feb indicate no abnormality )

Tx taken on Load service on March 2 05

DGA after load service taken on :March 4th 2005

Tx under observation: DGA after 18 march 2005

Tx under observation: DGA after 18 April 2005

Tx Bz Alarm on 12 July 2005

Tx Bz Alarm on 14 July 2005 (DGA of Gas indicate -Partial discharge of low energy density : Discharge in Gas filled cavities resulting from incomplete impregnation or super saturation or caviation or high humidity but leading to tracking or perforation of solid insulation_

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DGA resultsAfter failure DGA Results

Dissolved gases Evolved BZ gas Oil Evolved BZ gas Oil

First time BZ alarm Second time BZ alarm

Date 21-Jan-05 21-Jan-05 30-Jan-05 30-Jan-05

Methane (CH4) 500 90 519 138

Ethylene (C2H4) 0 0 0 0

Ethane (C2H6) 0 6 0 6

Acetylene (C2H2) 0 0 0 0

Hydrogen (H2) 294858 3282 288297 5427

Carbon Monoxide(C02) 658 46 738 64

Carbon Dioxide(C02) 769 451 853 571

Dissolved gasesAfter High vacuum

filteration OilAfter load

service oil DGA

Under observation-under

load service

Under observation-under load

service

Transforemr is under load service

Date 23-Feb-05 4-Mar-05 18-Mar-05 22-Apr-05

Methane (CH4) 6 19 74 114

Ethylene (C2H4) 0 10 6 1

Ethane (C2H6) 0 2 0 11

Acetylene (C2H2) 0 0 0 0

Hydrogen (H2) 124 597 2443 3388

Carbon Monoxide(C02) 14 21 52 50

Carbon Dioxide(C02) 126 306 684 660

Dissolved gases Evolved BZ gas Oil Evolved BZ gas OilPrevious result

after O/M

Third time BZ alarm Fourth time BZ alarm main tank Oil

Date 12-Jul-05 12-Jul-05 14-Jul-05 14-Jul-05 8-Dec-04

Methane (CH4) 3 292 1164 298 4

Ethylene (C2H4) 0 1 0 0 1

Ethane (C2H6) 0 26 26 27 0

Acetylene (C2H2) 0 0 0 0 0

Hydrogen (H2) 0 5672 249640 5588 10

Carbon Monoxide(C02) 30 124 1235 120 24

Carbon Dioxide(C02) 2605 989 2074 1074 269

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Annxure 2

Observations1.0 6 Nos. of NGEF make ,66/11 kV , 20 MVA Transformers are in service, these transformer are installed at the

following Grids.It has been observed that this lot of transformers are showing an increasing trend in Dissolved gases.

Sr.No Site Area KV Rating MVA

Tx no

Make Sr.No Year Comm.Date Nos. of Tripping todate

DGA Observation

1 Badli NW 66/11 20 1 NGEF 2800056897 1999 2/17/2000 252

Xmer failed on 14/07/05, DGA of Buchholz indicate high volume of Methane & Hydrogen

2 DSIDC Narela -1 NW 66/11 20 1 NGEF 2800053810 1993 5/20/1994 406Dissolved gases are increasing day by day

3 DSIDC Narela -1 NW 66/11 20 2 NGEF 2800050875 1991 4/21/1992 442Dissolved gases are increasing day by day

4 Jahangirpuri NW 66/11 20 2 NGEF 2800055504 1995 2/5/1996 455

Dissolved gases are increasing and general conductor over heating & Thermal fault of low temperature <150 º C

5 Jahangirpuri NW 66/11 20 3 NGEF 2800056673 1998 8/27/1998 287

'Dissolved gases are increasing and Normal ageing & Low energy Partial discharge

6 Rohini sec 23 NW 66/11 20 2 NGEF 2800056676 1998 3/11/1999 593

'Dissolved gases are increasing and Normal ageing & Low energy Partial discharge

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2.0 NDPL has been repairing transformers from the following vendors1) Emco2) Set on Site3) Tarapur4) Voltamp5) Kotsons

It has been observed that the transformers repaired by M/S Emco are showing an increasing trend in DGA.The table below are indicative of the same.

a) 2 nos. Repaired by EMCO make ,33/11 kV Transformers are in service and these transformer are installed at following Grids.


Tx no


DGA Observation

1 Rani Baug N 33/11 16 2 BBL-R-EMCO 3579/10 2003 4/16/2004 33

Dissolved gases are increasing and DGA indicates Thermal fault of high Temp. > 700 º C

2 Pandav Nagar N 33/11 15 1 CGL-R-Emco 23840 2003 1/20/2004

DGA indicates no faults but Dissolved gases are increasing.

b) 2 nos. Repaired by Set on Site make ,66/11 kV Transformers are in service and these transformer are installed at following Grids.


Tx no


DGA Observation

1 Mangolpuri 2 N 66/11 20 1 BBL-R-Set On Site 3195/2 2003 8/24/2003 102Thermal fault of medium Temp. range 300 - 700 º C

2 PP-1 N 66/11 20 3 BHEL-R-Set On Site 2002693 2003 1/31/2005 49 No fault

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c) 2 nos. Repaired by Voltamp make ,66/11 kV Transformers are in service and these transformer are installed at following Grids.


Tx no Make Sr.No Year Comm.Date Nos. of Tripping todate

DGA Observation

1 PP-2 N 66/11 20 2 BBL-R-Voltamp RO-61/2 2004 5/29/2004 97 circulating currents/ overheated joints2 PoothKhurd N 66/11 20 2 BBL-R-Voltamp RO-61/1 2003 4/16/2005 No Fault

d) 2 nos. Repaired by Tarapur make ,66/11 kV Transformers are in service and these transformer are installed at following Grids.



DGA Observation

1 PP-2 N 66/11 20 1 BHEL-R-Tarapur 6004818 2003 6/18/2003 145 circulating currents/ overheated joints2 PP-3 N 66/11 20 2 BHEL-R-Tarapur 2002688 2003 10/18/2003 20 No fault

e) 3 nos. Repaired by Kotsons make ,33/11 kV Transformers are in service and these transformer are installed at following Grids.



DGA Observation

1 Indiravihar N 66/11 16 2 CGL-R-Kotsons XXX-1 2003 5/7/2004 53 No fault2 PUSA N 33/11 16 2 NGEF-R-Kotsons 2800054118 2003 9/27/2003 12 general conductor over heating3 Rohatak Road N 33/11 16 3 NGEF-R-Kotsons 2800056696 2003 4/10/2004 63 general conductor over heating

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OM 7-3 Case Study on DGA Analysis of Oil in Transformer

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