dga diagnostics

7
- F E A T U R E A R T I C L E Proposals for an Improvement in Transformer Diagnosis Using Dissolved Gas Analysis (DGA) Key Words: Transformer, diagnosis, gas analysis, carbon dioxide, carbon monoxide, absorption, insulating paper, Bunsen’s coefficient by HISAO KAN Optec Dai-Ichi Denko Co., Ltd., Tokyo AND TERUO MIYAMOTO Mitsubishi Electric Corporation IEC Publication S99 says that a fault involving paper insulation is probable when the COICO2 ratio is 3 or lower; we would like to propose 10. It is often found that the concentration of CO and CO2 dissolved in transformer oil is higher in summer and lower in winter, which may result in misleading conclusions if the diagnosis is based on CO and CO2 gases. The cause of the fluctuation has not been clarified so far. In this paper, we clarify the following facts: (1) the fluctuation is caused by the absorp- tion of CO and COL gases into paper insulation, (2) the absorption phenomenon is temperature-dependent with differ- ent activation energies of absorption of CO and COZ, and (3) hence, the ratio of COJCO changes depends on the tempera- ture at which the oil samples are taken. From the practical aspect of transformer maintenance and control, faults involving paper insulation need quicker action than the simple decomposition of insulating oil itself. In this paper, we also report on the improved diagnostic method of paper-insulation-related faults and their temperatures. - INTRODUCTION he diagnosis of oil-immersed transformers using the dis- solved gas analysis (DGA) technique has bleen well ac- T cepted and widely used by laboratories, transformer users, and transformer manufacturers since the introdiiction of the IEC method [l] 17 years ago and the Electrical Cooperative Research method [2] 14 years ago in Japan. Thle two typical methods have been very effective in the diagnosis of faults in transformerssuch as arc discharge, partial discharge, and over- heating. For the diagnosis of faults involving paper insulation, the methods are not as clear-cut as for discharge and overheating. For the present, paper insulation-related faults are often diag- nosed by intuition or experience when a high concentration of CO and CO2 gases is detected. IEC Publication 599 (1978) touches on the diagnosis of faults involving paper insulation based on the ratio of COJCO. In our opinion, however, the method needs further investiga- tion for the following reasons. CO2 CONCENTRATION IN INSULATING OIL OF TRANSFORMERS IN OPERATION It is well known that the CO2 concentration in insulating oil fluctuates at each oil sampling, even in the case of sound transformers. We noticed the phenomenon and conducted a follow-up survey of the CO2 concentration in oil for two years, using four transformers in operation in the field. Two of them were non-pressure-type (diaphragm-type) sealed transformers, and the other two were nitrogen-sealed-type ones. It is difficult to conduct this kind of survey if much CO2 is generated during the survey. We therefore chose four transformers that had a rather high CO2 concentration but were operated at lower loading factors. Fig. 1 shows an Arrhenius chart between the CO2 concen- tration and the oil sampling temperature. It shows a fluctuation of the concentration similar to the ones experienced in the past. The fluctuation is on a straight line. When a phenomenon is plotted on a straight line in an Arrhenius chart, the inclination of the line relates to activation energy. The fluctuation of CO2 NovembedDecember 1995 - Vol. 11, No. 6 0883-7554/95/$4.000 1995 15 Authorized licensed use limited to: Djordje Jovanovic. Downloaded on June 14,2010 at 12:10:04 UTC from IEEE Xplore. Restrictions apply.

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  • - F E A T U R E A R T I C L E

    Proposals for an Improvement in Transformer Diagnosis Using Dissolved Gas Analysis (DGA) Key Words: Transformer, diagnosis, gas analysis, carbon dioxide, carbon monoxide, absorption, insulating

    paper, Bunsens coefficient

    by HISAO KAN Optec Dai-Ichi Denko Co., Ltd., Tokyo AND TERUO MIYAMOTO Mitsubishi Electric Corporation

    IEC Publication S99 says that a fault involving paper insulation is probable when the COICO2 ratio is 3 or lower; we would like to propose 10.

    It is often found that the concentration of CO and CO2 dissolved in transformer oil is higher in summer and lower in winter, which may result in misleading conclusions if the diagnosis is based on CO and CO2 gases. The cause of the fluctuation has not been clarified so far. In this paper, we clarify the following facts: (1) the fluctuation is caused by the absorp- tion of CO and COL gases into paper insulation, (2) the absorption phenomenon is temperature-dependent with differ- ent activation energies of absorption of CO and COZ, and (3) hence, the ratio of COJCO changes depends on the tempera- ture at which the oil samples are taken.

    From the practical aspect of transformer maintenance and control, faults involving paper insulation need quicker action than the simple decomposition of insulating oil itself. In this paper, we also report on the improved diagnostic method of paper-insulation-related faults and their temperatures. -

    INTRODUCTION he diagnosis of oil-immersed transformers using the dis- solved gas analysis (DGA) technique has bleen well ac- T cepted and widely used by laboratories, transformer users,

    and transformer manufacturers since the introdiiction of the IEC method [l] 17 years ago and the Electrical Cooperative Research method [2] 14 years ago in Japan. Thle two typical methods have been very effective in the diagnosis of faults in transformers such as arc discharge, partial discharge, and over- heating.

    For the diagnosis of faults involving paper insulation, the methods are not as clear-cut as for discharge and overheating. For the present, paper insulation-related faults are often diag- nosed by intuition or experience when a high concentration of CO and CO2 gases is detected.

    IEC Publication 599 (1978) touches on the diagnosis of faults involving paper insulation based on the ratio of COJCO. In our opinion, however, the method needs further investiga- tion for the following reasons.

    CO2 CONCENTRATION IN INSULATING OIL OF TRANSFORMERS IN OPERATION

    It is well known that the CO2 concentration in insulating oil fluctuates at each oil sampling, even in the case of sound transformers. We noticed the phenomenon and conducted a follow-up survey of the CO2 concentration in oil for two years, using four transformers in operation in the field. Two of them were non-pressure-type (diaphragm-type) sealed transformers, and the other two were nitrogen-sealed-type ones. It is difficult to conduct this kind of survey if much CO2 is generated during the survey. We therefore chose four transformers that had a rather high CO2 concentration but were operated at lower loading factors.

    Fig. 1 shows an Arrhenius chart between the CO2 concen- tration and the oil sampling temperature. It shows a fluctuation of the concentration similar to the ones experienced in the past. The fluctuation is on a straight line. When a phenomenon is plotted on a straight line in an Arrhenius chart, the inclination of the line relates to activation energy. The fluctuation of CO2

    NovembedDecember 1995 - Vol. 11, No. 6 0883-7554/95/$4.000 1995 15

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  • 100000

    I I I I

    K - E 1000 c

    K 0

    1 3.0 3.4 3.8

    Reciprocal absolute temperature (1 000/K)

    Fig. 1 Arrhenius plots between CO2 and sampling temperature

    concentration in insulating oil is expressed by the following formula:

    (1) where M is the CO2 concentration in oil (ppm), MO is a constant (ppm), E is the activation energy (cal/mol), R is 1.987 dmoldeg, and T is the absolute temperature at whch the oil sample is taken.

    CO1 gas in insulating oil has been generated by the decom- position of cellulosic insulation. It is therefore assumed that CO2 gas must have a good affinity with cellulosic insulation and that the gas may be absorbed into paper insulation very well. We assumed that the fluctuation is caused by the absorption phenomenon, and we conducted an experiment to confirm the assumption.

    M = MO exp (-E/RT)

    TEST ON ABSORPTION OF CO;! AND CO INTO INSULAT~W PAPER

    Absorption of Dissolved Gases into Paper Insulation Fig. 2 shows the container used for the test. It is a stainless

    steel container equipped with bellows to take up a change in oil volume due to temperature. It was filled with 1,200 ml of oil degassed under vacuum, and then various lunds of gases were dissolved in the oil.

    The laboratory test was run with an oi1:paper ratio of 3:l. The four transformers plotted in Fig. 1 had oi1:paper ratios of about 3 : 1 and 5: 1. In examining other data, it seemed that the laboratory test results may be applicable with reasonable accu- racy to actual transformers having oi1:paper ratios up to 9:l.

    The laboratory tests were carried out under conditions as similar to those of actual transformers as possible. Because modern transformers are made with pressboard comprised of highly refined natural cellulose with no additives, the type of paper would not seem to affect the laboratory test results very

    Fig. 2 Container used forthe test

    much. The oil used in the laboratory studies was shown by DGA to be similar to the oils in the transformers. There may be an influence of moisture and oxygen, but this study did not include the effects of these variables.

    It was recognized that the temperature in the laboratory test was more uniform than in the transformers. However, since the conductor insulation, which is usually hotter than oil, is only about 10% of the total paper insulation, there is a good possibility that the results of the laboratory test model the bulk of the paper insulation found in the transformer. However, it is recognized that it may be necessary to limit the application of t h s method to forced oil-cooled transformers in which the temperature dstribution is more uniform.

    The target values for the concentrations of the gases dis- solved in oil were chosen as follows: CO2: 10,000 ppm, CO: 1,000 ppm, hydrocarbon gases: 200 ppm. The figures were chosen because sound transformers with a diaphragm-type oil preservatlon system generally have a ratio of COJCO in the vicinity of 10.

    Hydrocarbon gases such as methane, ethane, and propane are the subject gases of routine DGA for transformer diagnosis.

    16 IEEE Electrical Insulation Magazine

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  • They were also injected into the container to find out if they were absorbed into paper insulation.

    DGA was performed by means of a method called strip- ping. The method requires only 1.5 milliliters of oil, while conventional DGA needs several tens of milliliters of oil per analysis. Carrier gas (helium) is supplied to a gas chroma- tograph at a constant flow rate. An oil sump is provided halfway to the supply piping, where gases dissolved in the sample oil are extracted by bubbling the helium gas through the oil. The extracted gases are sent to the gas chromatograph with the carrier gas and are analyzed. The gas chromatograph should be equipped with a high sensitivity gas detector because the amount of the gases extracted by bubbling is very small.

    DGA was carried out immediately after the gases were injected into the container, and the result was used as the initial concentration of the gases for the test. Table I shows the result. The gases were assumed to be dissolved as soon as they were injected, because the oil had been degassed under vacuum.

    Fig. 3 shows the fluctuation of the gas concentration. The oil temperature was changed in the order of 30+ 80+60+40+90+70+ 100C. The concentration of CO2 and CO showed appreciable fluctuation depending on the tempera- ture, while other hydrocarbon gases showed almost no sign of fluctuation.

    The concentration of CO2 and CO is lower at lower oil temperatures and higher at higher oil temperatures. The trend agrees with the field experience that the concentration is higher in summer and lower in winter. We therefore assumed that the fluctuation was caused by the absorption phenomenon. The concentration of CO2 was reduced to about 1/5 of the initial concentration when the container was kept at 30C. CO showed similar fluctuation, but to a much lesser degree. Hy- drocarbon gases such as methane, ethane, and propane showed almost no sign of absorption into paper insulation. It is very fortunate that the diagnoses made in the past based on the hydrocarbon gases are not affected by the findings of our test.

    Fig. 3 gives the following finding: The CO2 concentration resumes its initial value at 80,90, and 100C. It is temperature- dependent at temperatures below 80C. The finding indicates that paper insulation no longer absorbs CO2 at 80C and above.

    Fig. 4 shows the result of Fig. 3 plotted in ankrhenius chart. The activation energy can be obtained from the inclination of the straight line portion below 80C. The calculation results

    Soluble gas Content in oil (pprn)

    1 Gabon dioxide (CO2) I 10900 I 1 Cabon monoxide (CO) I 845 I I Methane(CH4) I 21 0 I

    12000 1 I I ]1200

    10000 1000

    h h

    8000 800 & v Q

    0 a r E

    6000 600 5 0 C

    8 4000 400 2

    _ - - c

    - c N 0

    2000 200

    I I I 10 20 40 60 80

    Heating time (Day)

    Fig. 3 Fluctuation of gas concentration by temperature

    100000

    100 2.4 2.8 3.2 3.6

    Reciprocal absolute temperature (1 000/K)

    Fig. 4 Arrhenius plots of results of Fig. 3

    are 5.2 kcal/mol for CO2 and 1.3 kcaVmol for CO. The temperature dependence curve of CO2 is plotted in Fig. 1 together with the curves for transformers in operation in the field. The inclination of those curves shows good agreement. From that fact we conclude that the fluctuation of CO2 dis- solved in the oil of transformers in operation is caused by the absorption of CO2 into paper insulation.

    It is generally said that the activation energy of absorption is in the vicinity of 1 kcal/mol. The 5.2 kcaVmol obtained for CO2 is much larger than expected. The large fluctuation of the CO2 concentration seems to be attributable to the high activa- tion energy.

    NovemberDecember 1995 - Vol. 11, No. 6 17

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  • 10

    0 0 " 1 s

    0.1 0 10 20 30 40

    Aging time (rnin)

    Fig. 5 Ageing time dependence of CodCO

    100

    10

    8

    1

    0 1 0 5 1.5 2 5 3 5

    Reciprocal absolute temperature (1 000/K)

    Fig 6 Temperature dependence of COdCO

    Quantify of CO2 and CO The phenomenon that paper insulation loses its absorbing

    capability at 80C and above makes it possible to convert a measured concentration to the value at 80C. The value at 80C is equal to the total amount of the gases that exist in a transformer. It can be calculated by the following formulae, (2) and (3):

    for CO2 M(C02) = M1 exp [2620 (l/I-O.O0283)] (2)

    for CO M(CO) = M2 exp [650 (lJT-O.O0283)] (3)

    where M(C02) and M(C0) are the concentration of COzand CO converted to 80C, M1 and M 2 are the same at the temperame T (absolute, IC) at whch sample oil was taken, and 0.00283 (K ') is the inverse of 80C expressed in K [1/(273+80)]. 2650 and 650 are the activation energy figues lvided by the gas constant of 1.987

    cal/moldeg. The ddference in the activation energy of CO1 and CO results in temperature-dependence characteristics of the COdCO ratio. The following are examples of the application of the ratio derived from equations (2) and (3).

    EXAMPLES OF APPLICATIQN OF

    Temperature-Dependence of C02ICO Ratio IEC Publication 599 (1978) says that a fault involving paper

    insulation is probable when the CO2 /CO ratio is 3 or lower. It is based on the fact that the generation of CO increases faster than CO2 as the decomposition temperature of cellulose in- creases. We made a study of what temperature range of cellulose decomposition the figure of 3 corresponds to and of the possibility of estimating the heating temperature from COJCO. The following is the result of our study

    The temperature of faults that take place inside a transformer can reach a value higher than the flash point of the transformer oil (140C). Because it is dangerous to heat oil-immersed paper beyond the flash point, the test was conducted in the following manner to obtain COJCO figures. The absence of oil in the test may have affected the test result, but there was no other safe way to heat paper beyond the flash point of oil.

    A gas chromatograph equipped with a thermal decomposi- tion oven was used, which can heat a sample up to a predeter- mined temperature instantaneously. It can decompose thermally 6 mg of insulating paper in a helium atmosphere at a desired temperature. The decomposition gas was lrectly sent to the gas chromatograph and analyzed. The decomposition temperatures were so much higher than 80C that there was no absorption of CO2 and CO into the paper insulation.

    The COdCO ratio should be stable when the decomposition temperature is kept constant in order to obtain correct tem- perature-dependence characteristics of COJCO. Fig. 5 shows the time-dependent change of COdCO when insulating paper is heated at constant temperatures of 280C and 550C. It shows that the time-dependence characteristics of COdCO are stable when the decomposition temperature is constant and that COdCO is smaller as the temperature goes higher. It indicates that the temperature-dependence charactetistics of CodCO can be obtained by the test.

    Based on the result of the preliminary test, we heated insulating paper at 150"-650"C. Fig. 6 shows the result, in which Arrhenius plots consist of two straight lines intersecting at around 400C. The existence of the inflection point may mean that the thermal decomposition mechanism of insulating paper is different in the ranges above and below the point, for reasons yet to be clarified. The result shown in Fig. 6 makes it possible to estimate the thermal decomposition temperature of insulating paper in the following manner.

    Estimation of Thermal Decomposition Temperature of lnsulafing Paper

    The two straight lines in Fig. 6 are expressed by the following formulae:

    low temperature side CodCO = 3.21 x exp(4380BT) (4)

    18 IEEE Electrical Insulation Magazine

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  • high temperature side CodCO = 6.16 x 10;' exp (6590BT)

    It seems to us that the criterion of 3 is too low from an insulation deterioration point of view A larger figure is desirable for the early detection of faults. We would like to propose 10, which corresponds to 1 1O"C, considering the accuracy of DGA. The life of insulating paper at 110C is calculated as 10 years according to equation (6)'

    (5) where R is 1.987 c4moldeg and T is the thermal clecomposition temperature expressed in K.

    The equations (4) and (5) give the codCO values when insulating paper is thermally decomposed in a helium atmos-

    Gas content (ppm)

    Gas content in gas space"

    phere. Some additional consideration is necessary when they are applied to transformers in operation in the field.

    CO2 and CO gases are generated slowly or quickly when a fault involving paper insulation takes place in a transformer. When a gas increase has been detected, DGAis carried out twice at an interval of two to four weeks. Then the total amount of CO2 and CO is calculated by the equations (2) and (3) based on the two analyses. The difference between the two analyses is taken to calculate the final value of COJCO, thus eliminating the effect of CO2 and CO gases that have been generated by normal ageing before the fault took place. The method can be applied to transformers with a diaphragm-type conservator. The correction described in the section below, "COJCO Value for Transformers with Gas Space above Oil," is necessary when the method is applied to nitrogen-sealed-type transformers.

    CO2/CO VALUE FOR TRANSFORMERS WITH A DIAPHRAGM-TYPE CONSERVATOR

    It may be possible to estimate the operating temperature of transformers by extrapolating Fig. 6 to lower temperatures. To confirm the adequacy of Fig. 6, the data of transformers whose operation records are clearly available are plotted in Fig. 6 together with the laboratory data of Fig. 6. The transformers in nuclear power stations were chosen because they are always loaded at full load and their operation record is readily avail- able. Fig. 6 indicates that the data obtained from the transform- ers and the laboratory data show good agreement.

    As mentioned in a previous section, IEC Publiccation 599 says that a fault involving paper insulation is probable when the COJCO ratio is 3 or lower. The phenomenon of absorption of CO2 and CO into paper insulation is not taken into considera- tion in the case of the IEC, of course. The figure "3" corre- sponds to 210C according to the equations (2) and (3). Let us check what will happen if paper insulation is exposed to 210C.

    The equation (6) has been reported based on an accelerated ageing test carried out at 140C and higher [3]:

    (6) where z is the life (hr.), fi is the degree of polymeriization (DP) of insulating paper to be used as a criterion for the determination of the life, and T is the heating temperature (K). In Japan, transformers are designed to withstand a short-circuit electromagnetic force of 1200 kg/cm2 [4], and it is said that the DP value of insulating paper should be 450 and higher to withstand the force [S;]. The life of a transformer is only 10 hours whenT is 483K (210C) and pis 450.

    Insulating paper is not highly temperature-resistant; but it has the characteristic of being good insulation even after it has been considerably aged. Consequently, a transformer can be in serious condition from a mechanical strength point of view even when it is operating satisfactorily but with a sign of overheating.

    log T = 7250/'4.00334 912.50

    Sampling temp. (OC)

    26 50

    CO2 CO CO2 CO

    68 17 120 17

    CO2/CO VALUE FOR TRANSFORMERS WITH GAS SPACE ABOVE OIL

    The method mentioned above is applicable only to trans- formers with a daphragm-type conservator, in which all the gases generated remain in the oil and insulation. Additional consideration is necessary when the method is applied to transformers with a gas space above the oil. Part of such gases as COZ, CO, CH4, and H2 migrate into the gas space to reach equilibrium between the concentrations of gases in the oil and in the gas space. In the case of nitrogen-sealed transformers with gas release and replenishment equipment, nitrogen gas in the gas space is released to the atmosphere and then replen- ished, depending on the oil temperature fluctuation. However, the release and replenishment are infrequent enough to justify the assumption that equilibrium is maintained in this type of transformer, also. The solubility of gases in insulating oil is called a Bunsen coefficient.

    The Bunsen coefficient is temperature dependent. Fig. 7 shows the temperature-dependence characteristics of Bunsen coefficients of CO2 and CO. CO2 and CO have different coefficients at the same temperature, which means that it is inaccurate to diagnose the transformer condition based only on gases dissolved in oil.

    The temperature dependence of Bunsen coefficients is ex- pressed by the following formulae, according to our experi- ment:

    (7)

    (8)

    The Bunsen coefficient of CO2 increases as the oil tempera- ture goes up, while that of CO decreases. The larger the Bunsen coefficient, the more soluble the gas is in oil. CO2 is more soluble in oil than CO.

    k (C02) = 0.0864 exp (1340BT)

    k (CO) = 1.2 exp (-164O/RT)

    Gas content in oil

    Gas content in paper*

    Total gas content

    559 13 839 16

    2151 18 1684 19

    2778 48 2643 52

    * Converted to equivalent gas concentration in oil

    November/December 1995 - Vol. 11, No. 6 19

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  • 10

    C 2 s 0.1 m

    I , , I I

    2.5 3.0 3.5 4.0 Reciprocal absolute temperature (1000/K)

    Fig. 7 Temperature dependence of Bunsen coefficient

    When insulating oil with gases dissolved in it is in contact with gas space, concentration equilibrium takes place between the oil and the gas. Equation ( 9 ) applies:

    CO = Ca (l+V/kv) (9)

    where CO is initial gas concentration in oil, Ca is gas concentration at equilibrium, k is a Bunsen coefficient, Vis gas space volume, and v is oil volume. The volume of gas space in a transformer is usually about 10% of that of oil. Equation (10) applies:

    VIV = 0.1 (10) CO in equation (9) gives the sum of the gases that exist in

    the oil and in the gas space. It should further be added with the amount of the gas absorbed in the paper insulation to obtain the total amount of the gas in a transformer.

    As an example, the above-mentioned method was applied to a transformer in operation in the field rated at 490 MVA, 275 kc! Two oil samples were taken at an interval of eight months. The oil temperatures at which the samples were taken were 26C and 50C. The calculation result is shown in Table 11. The gas content in oil column shows the values obtained by DGA. The table shows that the condition of the transformer did not change materially even though the gas concentration detected by conventional DGA was quite different (e.g., Col : 559 ppm versus 839 ppm). In the case of CO, the gas in the oil is only about one-third of the total amount that exists in the transformer. In the case of CO2, it is only 1/3 to 1/5 of the total. It should be noted that conventional DGA can detect only the tip of the iceberg.

    CASES WHERE @Qn/CO METHOD QOES NOT APPLY The above-mentioned method cannot be applied to the

    following cases:

    1) Transformers with an Open-Twe Conservator Besides the diaphragm-sealed or nitrogen-sealed transform-

    ers treated in the previous sections, transformers with an open-type conservator are widely used. Insulating paper in this kind of transformer deteriorates in the presence of oxygen, and hence, the method cannot be applied. It is hoped that data can be collected for this kind of transformer to establish a new method for DGA.

    2) Six Months after Refilling of Oil Insulating oil in a transformer is degassed and refilled when

    the transformer is opened for inspection, repair, or whatever reason. There is no dissolved gas right after the refilling, but the gases absorbed in the paper insulation gradually diffuse into the oil to reach equilibrium. Our experience shows that it takes a few months to reach it, during which time the COJCO figure is unstable. It is recommended that the method not be applied for six months after the degassing of the oil.

    3) Six Months after a Transformer Is Put in Operation A transformer is heated a couple of times for drying during

    its manufacture, during which time CO2 gas is generated in the paper insulation or is absorbed from the atmosphere into the paper. The atmosphere contains 400 ppm of COZ, so that the gas absorbed from the atmosphere into the paper is not negli- gible. The absorbed CO2 gradually diffuses into the oil after the transformer is put into operation. It is due to the diffusion that some CO2 is detected soon after a transformer is put into operation, and hence, it is not advisable to apply the method for six months after the start of operation.

    4) Restriaions on Oil-to-Paper Ratio and Type of Cooling of Transformers

    As described earlier, the laboratory test results may be applied with reasonable accuracy to transformers having the oil-to-paper ratio of up to 9:l. Because non-uniform tempera- m e mstribution within a transformer may affect the accuracy of the method, it would be necessary to restrict the application of the method only to forced-oil cooled transformers.

    coNcLuslo~s Our study has revealed many important facts as summarized

    below: 1) Our laboratory test and field survey on transformers in

    operation revealed that CO2 and CO gases are absorbed into paper insulation very well.

    2) The gases are absorbed more at lower temperatures. They are hardly absorbed at 8 0C and above.

    3) Equation (1) has been obtained for the temperature-de- pendence of the COL concentration in oil below 80C.

    20 IEEE Electrical Insulation Magazine

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  • 4) From conventional DGA and the temperature at which oil samples are taken, we can now calculate the total amount of the gases that exist in a transformer by equations (2) and (3 ) .

    5) The temperature dependence of the CO2/CO ratio can be expressed by equations (4) and (5).

    6) As a criterion for the diagnosis of faults involving paper insulation, a CodCO ratio of 10 is proposed, .which corre- sponds to the ageing temperature of 110C for insulating paper. This proposal takes the life of the insulating paper into consid- eration.

    7) For nitrogen-sealed-type transformers, it is proposed that the total amount of gases, including the gases in the gas space, be calculated. It is expected that the accuracy of the diagnosis will be improved by the use of the new method.

    From the above findings, we would like to propose the following as an improvement in transformer diagnosis using DGA:

    1) When a high concentration of CO2 and CO is detected in the insulating oil of a transformer, two DGAs shlould be made at an interval of two to four weeks.

    2) The DGA results should be converted to the total amount of CO2 and CO that exists in the entire transformer, using equations (2) and (3 ) . The difference between the two analyses is used in order for the diagnosis to exclude the gases that have been generated by normal ageing before a fault took place.

    3 ) The figure of 10 is proposed as a criterion for judging fault condition, which corresponds to 110C.

    ~ A O KAN graduated from Tokyo Institute of Technology in 1954 with a B.S. degree in electrical engineering. He has worked for the Mitsubishi Electric Corporation as a transformer design and a

    development engineer from 1954 to 1989, and for the Optec Dai-Ich Denko Co., Ltd. as chief engineer since 1989. He served as chairman for the transformer committee of JEE (1970-80) and as a Japanese delegate for CIGRE SC 12 (Xrans- formers) (1979-87). Hemaybereachedat: Optec Dai-Ichi Denko Co., Ltd., 3-1-1 Marunouch, Chyoda-ku, Tokyo 100, Japan.

    TERUO Ahmmo graduated from Tokyo Metro- politan University in 1968 with an M.S. degree in chemistry and received a doctorate of engineering degree from Osaka University in 1976. Since 1968 he has worked for the Mitsubishi Electric Corporation, primarily on the research and de- velopment of insulating materials for generators. Since 1974 he has worked on the application of

    transformer materials and on life and abnormahty diagnosis tech- nologies for transformers. He may be reached at: Mitsubishi Electric Corporation, Ako Works, 651 Tenwa, Ako City, Hyogo Prefecture 678-02, Japan.

    REFERENCES 1. IEC Pub. 599, Interpretation of the Analysis of Gases in Transformers and Other Oil-Filled Electrical Equipment in Service, 1978. 2. Mamtenance and Control of Oil-Immersed Electrical Equipment by Dissolved Gas Analysis, Society of Electrical Cooperative Research 36, No. 1, 1980, (in Japanese). 3. R. Tamura, H. Anem, T. Ishii, and T. Kawamura, Diagnosis on Agng Deterioration of Insulating Paper in Transformers by Gas Analysis, JIEE Transaction A, 101(1), 30,1981, (in Japanese). 4, Mechanical Strength of Transformer Windmgs under Short Circuit, JIEE Transformer Committee Report Part 1, No. 89, 1969, (in Japanese). 5. Crimal Value of the Average Degree of Polymerizatlon for Electrical Insulating Paper Used in Transformers, JEM (Japanese Electrical

    % Manufacturers Association Standard) 1463, (in Japanese).

    Erratum In the feature article, Insulating and Semiconductive Jackets for Medium and High Voltage Underground Power

    Cable Applications, by Gordon Graham and Steve Szaniszlo in the September/October 1995 issue of E1 Magazine, an error was printed in Table 111, Protective Properties of Typical Black Insulating Jacket Compounds. In the Physical Property column under Moisture Vapor Trans., the correct figure for PVC is 2 10.0.

    NovembedDecember 1995 - Vol. 11, No. 6 21

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