dissolved gas analysis-1

Shanthala Power Research Corporation

Shanthala Power Research Corporation, Hubli.

Shantha Power Research Corporation

Dissolved Gas Analysis by Gas Chromatography

A Modern Diagnostic tool for detection of faults in power Transformers in operation


Contamination by gases

Gases present can be divided into two categories

1. Those which are dissolved in the oil from the atmosphere.

2. Those which are generated insitu.

1. Atmospheric gases – oxygen, nitrogen and carbon dioxide.Dissolution depends on nature of gas, composition of gas, temperature and pressure.


2. The gases generated inside the transformers are hydrogen and hydrocarbon gases.

The causes are:

a. Thermal decomposition.

b. Electrical stress.

c. Electrolysis.

d. Vapourisatoin.

e. Chemical reaction


MECHANISM OF GAS FORMATION:

Oil molecules CH4 C2H2, C2H2, C2H6, H2 (G) + C

(solid)--(1)

C (SOLID) + 2H2 (G) CH4(G) (2)

2CH4 (G) C2H4 (G) + 2H2 (G)

(3)

C2H4 (G) C2H2 (G) + H2 (G)

(4)

C2H4 (G) + H2 (G) C2H6 (G) (5)

C2H2 (G) + 2H2 (G) C2H6 (G) (6)


DISSOLVED GAS ANALYSISSampling of oil. IEC 567-1977 IS 9434-1992

Extraction of dissolved gases from oil by introduction of oil into a pre evacuated known volume. The evolved gases are compressed to atmospheric pressure and the total volume is measured.

Separation of gas from the extracted mixture is effected in a.G.C.The integrated area of each peak is determined. The gas represented by each peak is identified by comparison of eluted times with those obtained for standard gas mixture used for calibration.

Quantification.The volume concentration of each gas in the oil sample is calculated in terms of parts per million, taking into account the volume of oil used for degassing.

Interpretation of data. concentration limit on the basis of service tr. history key gas method combustible gas method Rogers ratio or IEC ratio method.

IEC 599-1979 is 10593-1992


Standards applicable

• Sampling of oil - IEC 567-1971

IS 9434-1992

• Extraction of gases - ASTM-831-41

• Analysis of gases - IS 9434-1992

By gas Chromograph

• Interpretation of - IS 10593-1992

Data


GAS ANALYSIS BY GAS CHROMATOGRAPHY

Principle : Chromatography is a physical process of separation. Components to be separated are distributed between two phases, one of which is a stationary phase of large surface area and the other a fluid that percolates through a mobile phase.

The stationary phase is an adsorbent like silica gel, Alumina, molecular sieves; Polymer beeds etc.The process of adsorption will have different retention time characteristics under given experimental conditions. Therefore identification of components is possible. Standard retention times obtained with calibration gases.


Gas chromatography system

Infinite Source -------------- Regulator -------- Injection-----

of Gas Pressure Port

Capillary

Column ----------- Detectors ---------- Amplifier -----

Adsorbent

Recording System / Data Station


CALIBRATION OF GAS CHROMOTOGRAPHThe gas chromatograph is required to be calibrated with standard gases of known composition. The retention time and areas of peaks of the known quality of gas mixture are obtained to compare with that of sample gas.

Typical composition of standards gas

Methane

ETANE 100 ppm, in balance Nitrogen

Ethylene

Acetylene

Hydrogen 100 %

Carbon monoxide 5000 ppm, in balance Hydrogen

Carbon dioxide


LABELLING OF OIL SAMPLES

Oil sample preferably in duplicate should be properly labeled before despatch to the laboratory and the following information is to be sent along with each sample.

1. Plant location(i.e. Division, Sub-division MUSS etc.,)

2. Identification of equipment(i.e. Name plate details of Transformers.)

3. Object of sampling(i.e. whether sampling is consequent to the tripping of the transformer through protective relays or a normal routine periodic testing.)

4. Date of Sampling .


7. Volume of oil in equipment

8. Temperature oil sample when drawn

9. Date of commissioning of the transformer

10. Date of last filtration of oil

11. Tap changer (if any) whether it is integral part of the transformer or isolated part

12. Details of repairs (if any) carried out since the commissioning of the transformer.


PERMISSIBLE LIMITS

Permissible concentrations of dissolved gases in the oil of a healthy transformer (Transformer Union AG)

GasLess than four year in service

4 – 10 years > 10yrs

Hydrogen 10 / 150 ppm 200 / 300 ppm 200 / 300 ppm

Methane 50 / 70 ppm 100 / 150 ppm 200 / 300 ppm

Acetylene 20 / 30 ppm 30 / 50 ppm 100 / 150 ppm

Ethylene 100 / 150 ppm 150 / 200 ppm 200 / 400 ppm

Carbon Monoxide

200 / 300 ppm 400 / 500 ppm 600 / 700 ppm

Carbon Dioxide 3000 / 3500 ppm 4000 / 5000 ppm 9000 / 1200 ppm

Limitations of permissible limitsHow Permissible Can Be Formed


Interpretation of Transformer faults by Various Methods

Key Gas Method

Fault Key Gases

Partial Discharge Methane and Hydrogen

Arcing Acetylene

Thermic/Overheating Ethylene

Degradation of Cellulose Insulation Carbon Monoxide


V Ratio MethodsRogers Ratio: This is Four Ratio method and helps in defining the faults precisely.

CH4

H2

C2H6

CH4

C2H4

C2H6

C2H2

C2H4

Diagnosis

0 0 0 0 If CH4 / H2 = 0 or 0.1 partial discharge,

otherwise normal deterioration.1 0 0 0 Slight overheating below 150ºC

1 1 0 0 Slight overheating below 150ºC - 200ºC

0 1 0 0 Slight overheating below 200ºC - 300ºC

0 1 1 0 General conductor overheating.

1 0 1 0 Circulating currents and / or overheating joints.

0 0 0 1 Flash over without power follow through.

0 1 0 1 Tap Changer selector breaking current.

0 0 1 1 Arc with power follow through or persistent sparking.


IEC 599 RATIO METHOD

C2H2 / C2H4, CH4 / H2, C2H4 / C2H6

Code of Range of Ratio C2H2 / C2H4 CH4 / H2 C2H4 / C2H6

Range of Gases < 0.1 0 1 0 0.1 - 1.0 1 0 0 1 – 2 2 2 1 > 3 2 2 2 No Fault 0 0 0 RD. of High 0 1 0 Intensity Discharge of 1-2 0 1-2 Low Energy Discharge of 1 0 2 High Energy


Thermal Fault Of low Temp.

Up to 100ºC 0 0 1 150ºC - 300ºC 0 2 0

300ºC - 700ºC 0 2 1

> 700ºC 0 2 2

Code of Range of Ratio C2H2 / C2H4 CH4 / H2 C2H4 / C2H6


Case-1320kVA Transformer.Date of Testing 11-02-03 11-05-03 12-07-03

Filtration ----------- 30-04-03 ----------

Methane in ppm 177 93.62 1453

Ethane in ppm 211 15.44 2978

Ethylene in ppm 39 33.40 393

Acetylene in ppm 75 ND 13900

Hydrogen in ppm 96 ND 736

Carbon Di Oxide in ppm 490 1011.23 74

TGC % by Volume 10 09 10


Typical case of contact points completely burnt of Transformer OLTC.


Closer look of contact Points.


Case-2

• Transformer Details:• 2 MVA, 3.3kV/433V• Year of Installation :1995, Filtration :2003Date of Testing 15-10-2003 22-12-2003Methane in ppm 86.56 126.27Ethane in ppm 17.35 30.97Ethylene in ppm 224.33 360.28Acetylene in ppm 6.49 3.16Hydrogen in ppm 3.72 2.71Carbon Di Oxide in ppm 1141.90 1342.06TGC % by Volume 10.54 8.02DGA indicates Methane and Ethylene gas conc. are more than the limit.It is suspected that there is a Thermic Fault due to overheating of joints or bolts or

core.The utility has confirmed the fault as overheating of joints.


Case-3

• Transformer Details:• 20 MVA, 66kV/6.9kV• Year of Installation :1985, Filtration :2003Date of Testing 03-12-2004 28-01-2005Methane in ppm 169.82 293.06Ethane in ppm 170.05 238.58Ethylene in ppm 502.02 737.99Acetylene in ppm 26.54 20.26Hydrogen in ppm 1.96 68.82Carbon Di Oxide in ppm 1758.663215.98TGC % by Volume 8.02 12.30DGA indicates Methane, Ethane and Ethylene gas conc. are more than the

limit.It is suspected that there is a Thermic Fault due to overheating.


Typical of a failure of 65 MVA Transformer –Thermic Fault Bucholtz actuation reported Bucholtz gas brought for analysis

Methane in ppm 244Ethane in ppm 166Ethylene in ppm 1107Acetylene in ppm 129Hydrogen in ppm 1578Carbon Di Oxide in ppm 3208 AS Bucholtz gas contains high concentration of hydrogen gases & since ethylene concentration is very high it is suspected to be a severe overheating fault The utility has confirmed the presence of overheating joint


Case-420 MVA Transformer

Date of Testing 12-05-03 14-06-03

Methane in ppm 91 162

Ethane in ppm 81 230

Ethylene in ppm 42 94

Acetylene in ppm 158 831

Hydrogen in ppm ND ND

Carbon Di Oxide in ppm 245 174 Findings:

On visual examination, it was found that Transformer had arced from its primary winding to ground. Further it was also found that the insulating spacers were completely burned on the primary winding.


Typical case of a Arcing failure of a 20 MVA Transformer.


DGA FOR NEW TRANSFORMERS

• Useful quality control test for new transformers

• Gas analysis before & after factory tests (heat run test, H.V. Test, temperature raise test) can reveal the internal condition

• For New Transformers Gas Concentration Are Very Low – 2 PPM

• while interpreting new equipments after factory tests, the pressure of gases due to following should not be forgotten

• Old fault corrected but oil not degassed

• Evolution of hydrogen due to catalytic dehydrogenation of oil

• Inadequate drying before impregnating may give rise to hydrogen or oxygen from electrolysis of water molecules.

• Repairs by way of brazing and / or welding may give rise to various gases


THE TYPICAL FAULTS THAT MAY OCCUR DURING ROUTINE TESTS ARE:

• Arcing due to clearance to the tank and to adjoining winding

• Overheating of the joints in OLTC and brazed points

• Power follow through with continuous arcing

• Inter turn windings failure

• Shield ring failure

• Core bolt fault


GREATEST ADVANTAGE OF DGA TECHNIQUE

• Avoidance of unplanned outage as transformer defects are detected at incipient stages itself so that timely remedial measures can be undertaken to prevent damage or total loss of equipment

• Status of health check for transformer periodically

• Is a quality test for new transformer / repaired transformer before dispatch, installation & commissioning

• Cleaning transformer without internal faults, when they have tripped due to other reasons


Greatest advantage of DGA technique

• Avoidance of unplanned outage as transformer defects are detected at incipient stages itself so that timely remedial measures can be undertaken to prevent damage or total loss of equipment

• Status of health check for transformer periodically

• Is a quality test for new transformer / repaired transformer before dispatch, installation & commissioning

• Cleaning transformer without internal faults, when they have tripped due to other reasons

• Several gases where transformers have been saved from total destruction, the confidence in DGA technique is so high that the transformers are sent to repairs by no other evidence other than that of DGA


The Transformer failed due to Periodic Maintenance was not carried out.

dissolved gas analysis-1

Documents

sample gas

power transformers

composition of gas

gas chromatographyprinciple

nature of gas

gas chromographinterpretation

analysis of gases

known quality of gas