ltc diagnostics epri

7/25/2019 Ltc Diagnostics Epri

1/10

1

Use of Gas Concentrations Ratios to Interpret LTC

Dissolved Gas Data

Fredi Jakob, Karl Jakob, Simon JonesWeidmann-ACTI

Rick Youngblood

Cinergy Corporation

I. Introduction

Fifteen years ago a common belief was that dissolved gas analysis, DGA, was not

applicable to oil circuit breakers, OCBs or load tap changers, LTCs. This assumption

was based on the idea that the gases produced during normal operation of an OCB or

LTC would be more significant than gases produced by a fault process such as contact

overheating.

The association of certain fault gases with fault types or key gas DGA interpretation

method is summarized in Table 1. These empirical correlations were developed for main

tanks of transformers but are applicable to any type of oil filled electrical apparatus. This

concept led investigators such as Youngblood1to theorize that specific fault gases are

formed when arcing under oil occurs, which is a normal process in an LTC. Overheating,

an abnormal process in an LTC, produces different key gasses. Specifically, he

suggested that acetylene and hydrogen are generated during the normal arcing process

and the hydrocarbons, methane, ethane and especially ethylene are generated when

overheating occurs in a problem LTC. These three hydrocarbons are often called hot

metal gases since they are produced when a heated conductor is in contact with mineral

oil based dielectric fluid.


2/10

2

Table 1. Key Fault Gasses.

II. Data Interpretation.

A. Concept

The interpretation of DGA data for transformers, LTCs and OCBs is empirical in

nature. The development of interpretation protocols for OCBs and LTCs parallels the

development of DGA diagnostics for the main tanks of transformers. Key gases

associated with heating problems are methane, ethane and ethylene. These gases are

listed by Halstead2in order of increasing energy required for their production. Initially

Youngblood ignored the levels of arcing gases, acetylene and hydrogen that developed

whenever an LTC operated. Subsequent work by Youngblood5indicated that arcing gases

are also diagnostically significant. For example, increased acetylene levels were often

followed by increased heating gases. The increased acetylene is due to changes in the arc

duration and/or characteristics as the contacts are eroded or covered with carbon.

The next step in the development of diagnostic protocols for LTCs was the empirical

determination of normal or threshold values. The gas retention rate in an LTC is very

dependent on breathing configuration, so this is a major factor in determining threshold

Cellulose insulation degradationCarbon OxidesArcingAcetylene

Hot Metal GassesEthylene, Ethane, Methane

Partial Discharge,Heating ArcingHydrogen

IndicationGasses


3/10

3

levels. Free breathing LTCs rapidly lose gases to the environment while sealed LTCs

retain most of the gas produced.

Threshold levels have been determined for specific models and types of LTCs by

Doble3and Baker

4. Generic levels have been set by Youngblood

5, and are useful for

specific LTC models where the threshold values have not yet been determined. Fault gas

ratios should be considered applicable for unit evaluation only when threshold values are

reached.

B. Fault Gas Concentration Ratios.

1. Normal Operation

Arcing, which generates very high temperatures, occurs with each switching

operation in an LTC. Arcing produces acetylene and hydrogen. When arcing gases are

being produced, heating gases are also produced since the oil temperature varies with

distance from the arcing contacts. At the lower oil temperatures, heating gases are

produced rather than acetylene.

If one accepts this hypothesis, one would expect that the ratios of heating to arcing

gases in a problem free unit would remain fairly constant with operational count. Since

the contact surface is being eroded and additional deposits are being formed with each

operation, these ratios will predictably increase slightly with operation count. The data

presented in a paper by Duval5supports this hypothesis. Note that all of the significant

ratios in these problem free units are well below those found in a LTC that has developed

a heating or other problem.


4/10

4

2. Heating Problems

Initially a resistive film develops on contacts, which results in an increase in contact

resistance, increased heating and an increase in heating gas concentrations. Since more

heating gases are now being produced, the ratio of heating to arcing gases increases. This

change in gas concentrations and gas concentration ratios indicates problems. As the

resistance increases, the amount of energy dissipated increases. The ratios of ethane to

methane and ethylene to ethane both increase with increasing temperature. Thus these

two heating gas ratios should also reflect increased contact resistance and heating.

Table 2. Gas Formation as a Function of Operation Count. (Duval

5

)Operations: 500 3600 49000 Gas produced/operation

Gas/500 Gas/3600 Gas/49000

Hydrogen 6870 12125 14320 13.74 3.37 0.29

Methane 1028 5386 10740 2.05 1.50 0.22

Acetylene 5500 35420 53670 11.00 9.84 1.10

Ethylene 900 6400 35839 1.80 1.78 0.73

Ethane 79 400 3944 0.16 0.11 0.08

*Note: Some gas is always lost with time. Therefore, the gas concentration per

operation is expected to decrease with operation count. Duval did not provide breathing

configurations for this data.

Heating to Arcing Ratios

HydrogenAcetylene

Ethylene

+

Acetylene

Ethylene

HydrogenAcetylene

EthaneEthyleneMethane

+

++

Temperature Dependant Ratios

ethane

Ethane

Ethane

Ethylene


5/10

5

C. Application of Gas Concentration Ratios.

As is the case with main tank DGA, ratios are not valid unless at least one of the

fault gases exceeds its threshold value. Threshold values used by Weidmann-ACTI are

model specific, whenever this data is available. If model specific data is not available,

we use the monthly watch levels developed by Youngblood5as our threshold values.

Normal values we use for the ratios represent the 90th

percentile of fault gas

concentrations from a very large number of DGA results (~2500 units). These 90th

percentile values which are listed in Table 3are generic and do not take into account the

difference in gassing rates of specific units. Ratios are, as expected, model specific.

Table 4lists the ratios calculated for a Westinghouse LTC, which according to Bakers5

threshold levels requires attention.

Table 3. Generic 90th

Percentile Fault Gas Ratios.

Note the difference between these model specific ratios listed in Table 4, and the

generic values which are listed in Table 3.

R1 R2 R3 R4 R5

Acetylene

Ethylene

HydrogenAcetylene

Ethylene

+

HydrogenAcetylene

MethaneEthaneEthylene

+

++

Methane

Ethane

Ethane

Ethylene

0.3378 0.5000 0.9157

0.2067

4.83


6/10

6

Table 4. Unit Specific Ratio Comparison.WESTINGHOUSE LTC UTT

Ethylene Ethane Methane Acetylene Hydrogen CO CO2

LT3 Gas

Level

3,000 250 1,000 5,000 5,000 3,000 10,000

Ratio 1 Ratio 2 Ratio 3 Ratio 4 Ratio 5

0.60 0.30 0.43 0.25 12

McGRAW EDISON LTC 550-BL

LT3

2,000 400 400 400 500 1,000 3,000

Ratio 1 Ratio 2 Ratio 3 Ratio 4 Ratio 5

5.00 2.22 3.11 1.00 5

* LT3 is Bakers designation of a unit requiring inspection.

III. Case Studies

Two case studies for Cinergy data are presented below. This data, from Cinergys

initial work, was interpreted without consideration of fault gas concentration ratios. Case

history II is of particular interest since serious damage occurred in the six month interval

between scheduled tests. The data indicates that critical gas ratios such as the ratio of

ethylene to acetylene doubled from March 92 to Feb 93. This points out the necessity

of trending both ratios and gas concentrations. We believe that the extent of the coking

would have been less severe if the unit had been inspected in Feb 93 based on the

doubling of the significant ratios, rather than to have waited for six additional months.


7/10

7

IV. Future Work

The empirical analysis of DGA data for LTCs is well developed. Gas

concentration levels and gas concentration ratios can differentiate between normal and

problem units. We believe that both the concentration and ratio values will work best if

they are model specific. The compilation of these values requires user feedback on

problem units. Trending of both the gas concentrations and ratios is always the best

method to identify incipient problems.

Another concept under investigation is normalization of fault gas data. We

believe that during normal switching operations, the ratio of ethylene to acetylene is

fixed. This ratio should remain constant for different numbers of operations.

Furthermore, since these two gases have approximately the same solubility in mineral oil

and approximately the same escape rates from the oil, the ratios should remain fairly

constant even for free breathing units. For example if the concentration of both ethylene

and acetylene are 100 ppm then the ratio is one. If a heating problem is superimposed on

the normal arcing process and the gas levels are 175 ppm for ethylene and 150 ppm for

acetylene there would be an additional 25 ppm of ethylene due to the heating problem.

One could thus normalize our results using a ratio of:

acetylene

arcingnormalduringproducedethyleneethylenetotal

arcingtodueacetylene

heatingtodueethylene )____(_

)__(

)__( =


8/10

8

Date Comments C2H2 CH4 C2H6 C2H4 H2

8/31/92 Removed from Service 8527 3279 1135 9606 9083

12/17/93 Post Repair 501 387 16 375 2883

5/1/94 Normal 541 534 9 313 3800

8/17/95 placed on 6 month watch 648 590 52 836 3995

LTC Case Study I

Westinghouse UTT-A 138KV x 69KV x 13.8KV Sealed

Date Comments Ratio 1 Ratio 2 Ratio 3 Ratio 4 Ratio 5

8/31/1992 Removed from Service 1.13 0.55 0.8 0.35 0.15

12/17/1993 Post Repair 0.75 0.11 0.23 - -5/1/1994 Normal 0.58 0.07 0.2 - -

8/17/1995 Placed on 6 Month Watch 1.29 0.18 0.32 0.06 0.09

C2H4 / C2H6Ratio5

C2H6 / CH4Ratio4

CH4 + C2H4 + C2H6 / C2H2 + H2Ratio3

C2H4 / C2H2 + H2Ratio2

C2H4 / C2H2Ratio 1

31-Aug-92Based on the DGA result this unit was immediately removed from service. The fault

was determined to be due to contact misalignment.

17-Dec-93

Typical levels of gasses for a sealed, unit, annual monitoring was indicated.

1-May-94

Again gas levels are typical for a sealed unit, annual monitoring continued.

17-August-95

Again the Gas levels are indicative on normal operation, however the fifty percent increase

in the level of Acetylene indicated increased surveillance.


9/10

9

12-Mar-92

This unit indicated the early stages of mechanical difficulties. While the

Acetylene and Hydrogen levels are elevated, the level of Ethylene is less than 100

ppm. Indicating a continuance of annual monitoring

LTC Case Study II

Federal Pacific TC-25 69KV x 12KV 20MVA Desiccant Breather

Date Comments C2H2 CH4 C2H6 C2H4 H2

3/12/1992 Annual DGA Test Cycle 589 60 2 89 144

2/1/1993 6 Month Test Cycle 1625 342 70 534 3099

8/12/1993 Thermal Runaway 1633 53434 55535 253024 2217

Date Comments Ratio 1 Ratio 2 Ratio 3 Ratio 4 Ratio 5

3/12/92 Annual DGA Test Cycle 0.15 0.12 0.21 - 0.15

2/1/93 6 Month Test Cycle 0.32 0.11 0.2 0.21 0.33

8/12/93 Thermal Runaway 155 66 94 1.03 155

C2H4 / C2H6Ratio5

C2H6 / CH4Ratio4

CH4 + C2H4 + C2H6 / C2H2 + H2Ratio3

C2H4 / C2H2 + H2Ratio2

C2H4 / C2H2Ratio 1

1-Feb-93

At this time the unit was placed on a 6 month monitoring, due to the elevated,

Acetylene, Hydrogen, and Ethylene levels. At 534 ppm Ethylene immediate

removal from service was not indicated.

12-Aug-96

Too late, by August, the unit was in thermal runaway. As indicated by the extremely

high level of Ethylene, at 253,024 ppm. Repairs included a Tap Shaft board, Slip

Rings, and a New Reversing Switch assembly.


10/10

10

References:

1. Youngblood, R., Jacob, F., Haupert, T.J. Application of DGA to Detection of HotSpots in Load Tap-Changers, Minutes of the Sixtieth Annual International

Conference of Doble Clients, 1993, Sec. 6-4.1.

2. Halstead, W. D., A Thermodynamic Assessment of the Formation of Gaseous

Hydrocarbons in Faulty Transformers,Journal of the Institute of Petroleum, Vol. 59,

Sept. 1959, pp. 239-241.

3. Doble Client Transformer Committee Subcommittee Report on Transformer Load

Tap Changer Dissolved Gas Analysis September 24, 2001.

4. Baker, Charles. Personal correspondence. 2002.

5. Youngblood, R., Baker, C., Jacob, F., Perjanik, N. Application of Dissolved GasAnalysis to Load Tap Changers.

6. Duval, Michel. A Review of Faults Detectable by gas-in-Oil Analysis inTransformers. IEEE Electrical Insulation Magazine May/June 2002, Vol. 18 no. 3,

pp. 8-17.

ltc diagnostics epri

Documents