Dissolved Gas Analysis

DISSOLVED GAS ANALYSIS OF TRANSFORMER OIL

Mrs. Harsha Shah

Insulation Division

DISSOLVED GAS ANALYSIS OF TRANSFORMER OIL

OIL PRODUCES GASES UNDER THERMAL / ELECTRICAL STRSS.

TYPES OF GASES AND RATE OF PRODUCTION DEPENDS ON NATURE AND EXTENT OF STRESS RESPECTIVELY.

PROVEN TO DETECT INCIPIENT FAULT. FACILITY AVAILABLE, NEEDS EXPERTISE

FOR INTERPRETATION OF RESULTS.

DGA

• One of the most important technique to indicate the health of a transformer.

• Analogous to blood testing of Humans to detect various Bio-Logical Abnormalities. However, unlike diagnosis of blood, rules are not clearly established for DGA.

• DGA, An Art & not A Science and interpretation of results much depends on one’s documented experience.

GAS EVOLUATION PROCESS

PRESENCE & QUANTITY OF GASES DEPENDS ON : TYPE /AREA OF FAULT SOLUBILITY & DEGREE OF SATURATION OF

VARIOUS GASES TYPES & RATE OF OIL CIRCULATION TYPE OF OIL PRESERVATION KINDS OF MATERIALS IN CONTACT WITH FAULT

Sampling And Labeling ProcedureSampling

• Dry Weather, avoid contamination• Clean, dry, leak proof glass or stainless

steel container.• Equipment operating normally• Take safety precautions.• Sample bottle must be full without any air

trap completely sealed and should be properly labeled.

LABELLING PROCEDURE

A] IDENTIFICATION OF TRANSFORMER

1. LOCATION

2. MAKE

3. SERIAL NUMBER / TRANSFORMER

NUMBER

4. TRANSFORMER RATING

5. VOLTAGE RATIO

B] HISTORICAL INFORMATION

1. DATE OF COMMISSIONING

2. DATE OF LAST OIL FILLING

3. DATE OF LAST OIL FILTRATION/ DETANKING / TOPPING UP / TRANSFORMER REPAIRS

4. DATE OF SAMPLING

5. OIL VOLUME IN SYSTEM

6. ANY ABNORMALITY IN WORKING

C] OPERATIONAL

1. LOAD ON DUTY : MAX & MIN

2. OIL TEPERATURE : MAX & NORMAL

D] PREVIOUS TEST RESULTS

1. TEST REPORT NO. & DATE IF TESTED AT ERDA

2. TEST RESULTS INCLUDING DATE OF TESTING

IF TESTED IN OTHER LAB

FREQUENCY OF SAMPLING

NEW TRANSFORMERS

FIRST DGA TEST : BEFORE HEAT RUN TEST ON TRANSFORMER

SECOND DGA TEST : AFTER HEAT RUN TEST ON TRANSFORMER (2&24 HRS)

THIRD DGA TEST : BEFORE ENERGIZING TRANSFORMER (B.M.)

FOURTH DGA TEST : WITHIN THREE MONTHS OF SERVICE

IN-SERVICE TRANSFORMERS ANNUALLY AS A ROUTINE CKECKING BEFORE (LATEST DATA) & AFTER FILTRATION

(FRESH B.M.)/TOPPING UP

TRANSFORMERS AFTER OVERHAULING,

REPAIRS, MAINTANANCE BEFORE ENERGISING TRANSFORMER WITHIN THREE MONTHS OF SERVICE

Detection Sensitivity Limit

Sr.No.

Gas DetectionSensitivity

(ppm)1. Hydrogen 52. Oxygen 503. Nitrogen 504. Hydrocarbons 15. Carbon Oxide 25

Interpretation of Results

• Ensure that gas concentrations are high enough to warrant further investigation.

• Gas being present even in normal operating condition without any fault being present.

• Gases might have formed on the occasion of previous faults or during repairs by brazing, welding etc., and not completely removed.

• Since gases are produced in normal ageing also, the service duration of the oil has to be taken into account.

PERMISSIBLE LIMITS OF DISSOLVED GASES IN OIL

OF A HEALTHY TRANSFORMER

GAS <4YRS IN PPM) 4-10 YRS (PPM) >10 YRS (PPM)

H2 100-150 200-300 200-300

CH4 50-70 100-150 200-300

C2H2 20-30 30-50 100-150

C2H4 100-150 150-200 200-400

C2H6 30-50 100-150 800-1000

CO 200-300 400-500 600-700

CO2 3000-3500 4000-5000 9000-12000

Diagnosis of Health of A Transformer / Fault

1. Benchmark Results

2. Permissible Limit

3. IS Ratio Method

4. Muller’s Ratio Method

5. Key Gas Method

6. TCG Content

7. Rate of Rise of Gas

Code of range of ratios

C2H2C2H4

CH4H2

C2H4C2H6

< 0.1 0 1 00.1 – 1 1 0 01 – 3 1 2 1>3 2 2 2

BEFORE & AFTER HEAT RUN TEST

EXISTING METHOD OF INTERPRETATION

NOT SUITABLE FOR NEW TRANSFORMER INITIALLY SMALL QUANTITY OF GASES

DISSOLVED IN OIL DURATION OF TEST IS VERY SHORT VERY SMALL RISE IN GAS CONCENTRATION

NEW GUIDELINE

NEW GUIDELINE IS SUGGESTED BY CIGRE WORKING GROUP FOR THE INTERPRETATION OF DGA RESULTS BEFORE & AFTER HEAT RUN TESTS.

TABLE -4 GAS CONCENTRATION IN OIL B.H.R.T.MIN. DETECTION SENSITIVITY OF EQUIPMENT(Sd) MAX.PERMISSIBLE VALUE OF GASES (X1)

S.NO. GAS Sd PPM (V/V) (X1) PPM (V/V)

1. CO2 10.0 200.0

2. CO 5.0 80.0

3. H2 2.0 15.0

4. CH4 0.1 2.5

5. C2H6 0.1 2.0

6. C2H4 0.1 2.0

7. C2H2 0.1 1.0

Maximum Analytical Spread (Sa)

Sa (x) = 0.1 (x)1 + 2 Sd (x)

x = ANY GAS

(x)1 = ORIGINAL QUANTITY OF GAS X

Sd (x) CAN BE TAKEN FROM TABLE - 4

DIFFERENCE IN CONC. OF ANY GAS BHRT & AHRT IS LESS THAN Sa OF GAS - TR. IS HEALTY.

No. Criteria Range 1 Range 2

1. (C2H2)2 – (C2H2)1 <Sa (C2H2) <Sa(C2H2)

2. 1/t {[H2)2] – (H2)1 +

{[CH4)2 – (CH4)1] +

{[C2H4)2 – (C2H4)1] +

{[C2H6)2 – (C2H6)1]} +

<2 ppm (v/v)/hr. <5 ppm (v / v) hr

3. 1/t [(CO)2 – (CO)1] < 5 ppm (v/v) / hr. < 25 ppm (v / v) hr

4. 1/t [(CO2)2 – (CO2)1] < 20 ppm (v/v) / hr.

< 100 ppm (v / v) hr

5. (C2H4)2 – (C2H4)1

(C2H6)2 – (C2H6)1

< 1 ppm (v/v) / hr. < 3 ppm (v/v) / hr.

• Here t is duration of heat run test in hours, (X)2 and (X)1 are

concentrations of gas X, after and before the test.

• 5 th criteria may only be used if increase in gas concentration of C2H4

& C2H6 BHRT & AHRT are > 10 Sa

INTERPRETATION

• All in Range 1 – No Thermal fault

• One or more are > Range 1 < Range 2 – Fault caused by Load current or Leakage flux * Close monitoring required for oil during service.

• One or more are > Range 2 –*more analysis during heat run test.

FAULT DIAG:THERMAL FAULT OF HIGH TEMP(700oC)FAULT OBSERVED:HOT SPOT IN ONE OF THE

CONNECTING LEADS HAVING JOINT

3.64 MVA, 22.5/1.73 kVDATE OF COMM. 5-2-88RATIOS : 0 2 2H2 1049CH4 6986C2H2 1476C2H4 39998C2H6 6462CO 40773CO2 94998

No Sign of any abnormality from out side.

Fault diagnosis : Thermal Fault

Observation : Presence of hot sport in one of

the connecting leads having joint

F.DIAG:THERMAL FAULT OF MED.TEMP(3000C-7000C)FAULT OBSERVED:BOLT ON STAMPING OF

TRANSFORMER WINDING GOT MELTED

3.22 MVA, 22.5/1.73 kVDATE OF COMM. 28-5-82RATIOS : 0 2 1H2 45CH4 296C2H2 4C2H4 2228C2H6 1292CO 68CO2 1458

No Sign of any abnormality from out side.

Fault diagnosis : Thermal Fault

Observation : Top as well as bottom metallic bolts used to hold stamping of the

transformer winding got melted.

FAULT DIAG : ELECTRICAL DISCHARGESF.OBSERVED:ELECTRICAL SPARKING ON H.T.LEADS

18.4 MVA, 33/0.173 kVDATE OF COMM.OCT `76RATIOS : 1 0 2H2 2113CH4 319C2H2 408C2H4 469C2H6 82CO 1632CO2 4190

Fault diagnosis : Electrical discharges

Observation : Electrical discharges on high

voltage leads inside the main tank

was observed. The wooden planks

used to isolate HV leads clearly

show sign of electrical discharges.

F.DIAG:ELECTRICAL DISCHARGES OF LOW ENERGYF.OBSERVED:WINDING FAILURE

33 kVDATE OF COMM. 1984RATIOS : 2 0 2H2 240CH4 26C2H2 202C2H4 51C2H6 3CO 199CO2 64

Fault diagnosis : Presence of severe electrical

discharges

Observation : Total winding was damaged

significantly

Thank you