-
- 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
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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.
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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.
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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)
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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
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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.
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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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