IEEE Trans. Electr. Insul, Vol EI-13 No 5, October 1978
349
IEEE AND IEC CODES TO INTERPRET INCIPIENT FAULTS
IN TRANSFORMERS, USING GAS IN OIL ANALYSIS
R. R. Rogers
C.E.G.B. Transmission Division
Guildford, England
ABSTRACT
The detection of incipient faults in oil immersed
transformers by examination of the gases dissolved in the
oil developed from the original Buchholz relay application. The gases produced during the deterioration of
mineral oil and cellulose were examined and techniques
were established to assist in the interpretation of the
type of fault, incipient or growing, occurring within
the transformer.
The major technique now employed is the study of
ratios of pairs of certain deterioration gases. The concepts used in the development of this technique and the
modifications made to enable the technique to be established as the present method of fault interpretation,
recommended by the IEC are discussed.
INTRODUCTION
Attempts to diagnose the type of fault from the gases
evolved from the oil after the failure of mineral oil
immersed power transformers started 50 years ago [1],
and had developed by 1956 [2], into a detailed assessment of the fault from the gases collected in the
Buchholz relay. It was quickly realized that if gases
are evolved from the oil sufficient to operate a
Buchholz relay, then slowly developing faults would also
be producing decomposition gases which would be dissolved in the oil and only appear in the Buchholz at the
end of a complicated system of interchange between the
gases contained in bubbles rising to the surface and
the less soluble atmospheric gases dissolved in the oil.
It should thus be possible by analyzing the gases dissolved in the oil or in a nitrogen cushion to detect
any incipient faults which may be present in the transformer [3,4]. Originally, indications of the type of
fault occurring were developed from past history, or
from experimental work on oil filled samples where
the gases collected were differentiated as a major component of, or a percentage of the total gas accumulated.
DEVELOPMENT OF THE INTERPRETATION THEORIES
From 1968, the C.E.G.B. started regularly monitoring
by chromatographic analysis gas in oil samples from
increasing numbers of EHV transformers on a routine
basis, and by 1970 over one thousand units of 132,
275, and 400 kV voltage rating were checked at least
annually. From statistical assessment of the results,
attempts were made to set gas levels whereby 90% of
all transformers being below these levels, the
assumption could be made that transformers with gas
levels above these "Norms" were under suspicion of
containing incipient faults likely to cause future
trouble in service. However, study of the analyses
showed that all transformers in service, including
lightly loaded units known to be operating satisfactorily, evolved hydrogen and the other simple
hydrocarbon gases, albeit in very small quantities,
e.g. less than 0.1 ppm methane per month of service.
Generally the total gas content depended on the time
in service and the load conditions.
In 1970, Dornenburg [5] differentiated between faults
of thermal or electrical origin by comparing pairs of
gases with approximately equal solubilities and diffusion coefficients such as ethylene and acetylene
where an increase in the ratio of ethylene/acetylene
above unity indicated an electrical fault, and also
used the ratio of methane/hydrogen to suggest an indication of a thermal fault if the ratio was greater than
0.1 or a corona discharge if the ratio was less than
0.1. This method was recognized as promising, in that
it eliminated the effect of oil volume and it simplified the choice of unit since one is measuring one gas
produced per unit of another. In particular, the plot
of successive samples forms a basis which can be used
to deduce whether the fault is constant in nature.
0018-9367/78/1000-0349$00.75
(Q 1978
IEEE
350
IEEE Trans. Electr. Insul, Vol EI-13 No 5, October 1978
THE FIRST RATIOS
-a
L-
.=
6L°1
a
Hydrogen
H2
Cs
CL
/
I
CODE
1
The above considerations led to the choice of 4 ratios
for fault diagnosis, based on the order given in
Fig. 1, i.e. methane/hydrogen, ethane/methane,
ethylene/ethane and acetylene/ethylene [7]. The significant change point of each ratio was assumed arbitrarily to be unity and Table 1 illustrates the first
elementary diagnoses used. In the tabulation, a
value above 1 is indicated by 1 and a value below 1 by
zero.
The ratios were chosen so that a series of four
zeros indicated satisfactory operation of the trans-
former.
Statistical study of nearly ten thousand C.E.G.B.
analyses suggested that certain types of fault
conditions could be differentiated within more detailed ranges and combinations of gas ratios. This was
confirmed by internal examination of a number of suspect transformers together with post mortems on faulty
units as well as by design studies of hot spots likely
to be found in transformers under operational conditions. The refined code of ranges of gas ratios is
shown in Table 2 and a fault diagnosis table based on
the code is given in Table 3 [8]. The use of the Code
facilitates the programming of a computer to provide a
fault diagnosis directly from a chromatograph detector
record.
Methane
CH4
gas
Ethane
C2H6
Ethylene
C2 H4
Acetylene
C2 H2
INTERNATIONAL STUDY OF THE RATIOS TECHNIQUE
The various interpretation schemes employed in Europe
were summarized in a CIGRE Study
a trilinear scheme was described
Paper in 1975 [9], and
by Duval in 1974 [14].
In order to establish the identification of actual
Fig. 1:
oil
as
Comparative rates of evoZution of gases from
a function of deccmrposition energy.
A C.E.R.L. report produced in 1970 by Halstead but
not published unti' 1973 [6], made a theoretical thermodynamic assessment of the formation of the simple decomposition gaseous hydrocarbons in mineral oil which
suggested that on the basis of equilibrium pressures at
various temperatures, the proportion of each gaseous
hydrocarbon in comparison with each of the other hydrocarbon gases varied with the temperature of the point
of decomposition. This led to the assumption that the
rate of evolution of any particular gaseous hydrocarbon
varied with temperature, and that at a particular
temperature there would be a maximum rate of evolution
of that gas and that each gas would attain its maximum rate at a different temperature.
Study of the
Halstead thermodynamic equilibria suggested that with
increasing temperature, the maxima would be in turn
methane, ethane, ethylene and acetylene, and Fig. 1
shows a non-qualitative representation of this hypothesis. Hydrogen evolution is shown first, but this is
represented initially by a large amount of hydrogen
produced by ionic bombardment under discharge conditions (i.e. "cold"), followed by a continual increase
of hydrogen representing the increasing rudimentary
fractionating of the long chain paraffin molecule
under increasing temperature conditions.
faults, the CIGRE WG 15-01 assessed one hundred sets
of analyses from transformers with known faults, contributed from the records of members of the Working Group.
The results of considerable laboratory work on small
specimens were provided by several members in order to
assess the probable temperatures at which the ratios
indicated significant change. In the light of these
results and further theoretical assessments, the
significant changeover values of the ratios for both
electrical and thermal faults were then modified.
Because the ratio ethane/methane only indicated a
limited temperature range of decomposition, but did not
assist in further identifying the fault, it was
deleted. It was considered that the use of only three
ratios would simplify interpretation. To assist understanding of the technique, the tables were re-organized
to produce a more rational progression of faults from
minor incipient faults up to the major known faults
identified in the above survey. The modified tables
were included in IEC Document 10A (Secretariat) 53 and
detailed at the 1977 Doble Conference [10] .
It should be emphasized that the tables were intended
demonstrate the range of phenomena expected to be
likely to occur in a transformer in service, from normal
ageing, through incipient electrical or thermal phenomena
likely to reduce transformer life expectancy by negligible amounts (but nevertheless important for understanding of the reliability of a transformer under
service conditions), to phenomena likely to cause
Buchholz relay operation within a short time and thus
requiring emergency action. In order to assist this
identification, the tables were further modified at a
recent IEC-10 meeting. The new table as shown in
Table 4 is included in IEC Document 10A (Central Office)
37.
to
Rogers:
Interpretation
Incipient
of
TABLE 1
C C4C2H6
C2H4 C2i 2
H2
CH
C H
o
0
1
1
0
0
1
0
0
0
2
351
Faults
ORIGINAL FAULT DIAGNOSIS
-
Percentage
2H
Diagnosis
0
0
If CH 4
0
1
1
0
0
0
0
0
1
1
0
0
0
0
0
0
1
0
0
0
1
1
1
1
4
24
26
Sfled
CH4
discharge otherwise - normal deterioration
- below 150°C (?)
- 150 - 2000C (?)
- 200 - 3000C (?)
General conductor overheating
Circulating currents and/or overheated
joints
Flashover without power follow-through
Tapchanger selector breaking current
Arc with power follow-through - or
persistent sparking
C2H4
2
Less than 1.0
a4
Not less than 3.0
2H
2H6
2
4
0
5
0
0
0
1
1
0
0
0
0
0
0
0
1/2
1/2
0
0
1
1
0
0
0
5
(
(>
(
Not less than 3.0
-
2 6
1/2
2
0
9.7
5
1)
0
1
2
(< 1)
1)
0
1
(< 1)
0
3)
2
(< 0.5)
(¢ 0.5,< 3)
0
1,< 3)
(
1
1
( 3)
2
FAULT DIAGNOSIS TABLE BASED ON CODE GIVEN IN TABLE 2
2H4 C2H2
0
0
0
0
0
1
1
2
0
9.0
2.1
1.1
1, < 3)
( 3)
(¢
Between 0.5 and 3.0
CH
7.8
11.1
0.1)
0.1,<
(
Less than 0.5
2
2 4
ai
9.0
REFINED CODE
Between 1.0 and 3.0
TABLE 3
34.2
11.8
Code
CH
2 6
H
H
-
Less than 1.0
Not less than 1.0
4
C
2.0
Slight overheating
Slight overheating
Slight overheating
Not greater than 0.1
Between 0.1 and 1.0
Between 1.0 and 3.0
Not less than 3.0
C H
C
Partial
0.1
or
H2
Range
H_H2
of
Sampled
TABLE 2
Gas Ratio
TABLE
CH
2 4
0
0
0
0
0
0
0
0
1
1/2
2
1/2
ianosis
_
_
_
_
_
_
_
Normal Deterioration
Partial Discharge
Slight Overheating
Overheating
Overheating
General Conductor
-
-
_
_
_
_
_
_
_
_
below 150 C (?)
150 - 200°C (?)
200- 300°C (?)
Overheating
Winding Circulating
_
Currents
Core and Tank Circulating Currents, overheated joints
Flashover without Power Follow Through
Arc with Power Follow Through
Continuous Sparking to Floating Potential
Partial Discharge with Tracking (note CO)
IEEE Trans, Electr.
35 2
TABLE 4
-
Insul, Vol EI-13 No 5, October 1978
CODE FOR EXAMINING ANALYSIS OF GAS DISSOLVED IN MINERAL OIL
Ratios of Characteristic
Gases
Code of range of ratios
CRH
CHi
.4
22
C2H4
H2
CH
2 4
C 2H6
0
1
1
2
1
0
2
2
0
0
1
2
<0.1
0.1- 1
1-3
>3
Case
0
1
2
3
4
Characteristic Fault
Typical Examples
0
0
0
Normal ageing
0
but not
1
0
Discharges in gas filled
cavities resulting from
incomplete impregnation, or
super saturation or cavitation
or high humidity
1
1
0
As above, but leading to tracking
or perforation of solid
No fault
Partial discharges of
low energy density
significant
Partial discharges of
high energy density
insulation
Discharges of low
energy (see Note 1)
l-12
0
1-e2
Discharges of high
1
0
2
Discharges with power follow
through. Arcing-breakdown of
oil between windings or coils
or between coils to earth.
Selector breaking current
energy
Continuous sparking in oil
between bad connections of
different potential or to
floating potential. Breakdown
of oil between solid materials
5
Thermal fault of
temperature <1500C
(see Note 2)
0
0
1
Insulated Conductor overheating
6
Thermal fault of low
temgerature range
0
2
0
)Localised overheating of the
)core due to concentrations of
)flux. Increasing hot spot
)temperatures; varying from
Thermal fault of
medium temperature
0
2
1
0
2
2
)shorting links in core, over)heating of copper due to eddy
)currents, bad contacts/joints
)(pyrolitic carbon formation)
)up to core and tank
)circulating
150 C to 300 C
(see Note 3)
7
range
8
3000C.7000C
Thermal fault of high
temperature range
>7000C (see Note 4)
)small hot spots in core,
)currents
Note 1:
For the purpose of this tabZe there wiZZ be a tendency for the
ratio C2H2/C2H4 to rise from 0.1 to above 3 and for the ratio
C2H4/C2H6 to rise from 1-3 to above 3 as the spark develops
in intensity. The characteristic code of the fauZt at an
incipient stage wiZZ then be 1.0.1
Note 2:
In this case the gases come mainZy from the
Note 3:
This fauZt condition is normaZZy indicated by increasing gas
concentrations. Ratio CH4/H2 is normaZZy about 1, the actuaZ
vaZue above or below unity is dependent on many factors such as
design of oiZ preservation system, actuaZ Zevel of temperature
and oiZ quality.
decomposition of the
soZid insuZation, this explains the value of the ratio C2H4/C2H6.
Rogers:
Interpretation of InciDient Faults
353
Note 4: An increasing vaZue of the amount of C2H2 may indicate the
hot point is higher than 1000°C.
GeneraZ remarks (1) Significant vaZues quoted for ratios should be
regarded
typicaZ onZy.
(2) Transformers fitted with in-tank on-Zoad tapchangers may indicate fauZts of type 202/102
depending on seepage or transmission of arc
decomposition
products in the diverter switch
tank into the transformer tank oiZ.
(3) Combinations of the ratios not incZuded above
may occur in practice. Consideration is being
given to the interpretation of such combinations.
CELLULOSE DETERIORATION
The effect of cellulose deterioration in assisting in
the diagnosis of the phenomena should not be ignored.
The relationship between carbon monoxide and carbon
dioxide under normal or abnormal conditions depends on
the amount of oxygen available to assist the thermal
decomposition of cellulose [8,11,12].
Partial discharges of high energy density (Table 4
case 2) are generally accompanied by tracking of
solid insulation and the production of measurable quantities of CO and CO2. Similarly for insulated conductor overheating (case 5), the increase of the
unsaturated hydrocarbons such as ethylene compared
with the saturated ethane should also be accompanied
by increased CO and CO2.
PROCEDURE
Any indication of abnormal deterioration should
be followed by a procedure such as detailed in Ref. 8
or IEC-IOA (Central Office) 37. Similar flow charts
could be programmed into a computer which could
examine previous records, request further samples and
recommend necessary action tothe maintenance engineer.
The IEC document emphasizes that any resulting action
must be undertaken only after proper engineering
judgment.
as
efficient of variation, any adjustment can be neglected.
The indication of changing values of ratios allied to
increased rate of gas production is of true significance, regardless of the type of oil or transformer.
The data in Table 4 is based on present day
knowledge
and it is likely that with further international
experience, there may be modifications to the significant
ratios and new interpretations of other ratio combinations.
CONCLUSIONS
The application of the ratios
to the interpretation of incipient faults technique
in power transformers
by dissolved gas analysis, has proved beneficial in
that forced outage and repair costs have been consider-
ably reduced.
The application of the interpretation
recommended in this paper should enable procedure
the smaller
utility, without the need for extended statistical
and laboratory investigation, to
the technique
to reduce the overall costs for apply
maintaining power
transformers in service.
Further application of this technique as a monitor
the factory proving tests of
transformers, is being developed employingpower
very much
greater detection sensitivities than used in the field.
This shows great promise in that a number
of
likely to cause future trouble in service havefaults
been
detected, although in each case the transformer
had
satisfactorily passed the routine tests.
during
DISCUSSION
As most of the original statistical work was done on
high aromatic content, napthenic base oil and free
breathing conservator transformers, reservations were
made on the application of the ratios technique to
sealed transformers filled with inhibited paraffinic
base oil.
International experience now indicates that the
ratios selected apply equally to inhibited paraffinic
base oil. With nitrogen sealed transformers the
changed equilibrium conditions across the oil/nitrogen
or air surface may be adjusted if necessary by the
relative diffusion and solubilities but can generally
be considered of limited significance. For diaphragm
sealed units the ratios could be adjusted in accordance
with the diffusion coefficients [13]. Nevertheless,
as the significant values of ratios quoted in Table 4
should be regarded as typical only, with a large co-
AC KNOWLEDGMENT
The author wishes to thank the Directors of the
Transmission Development and Construction Division of
the Central Electricity Generating Board for permission
to publish this paper, and acknowledges the work of
colleagues in IEC lOA WG.02.
IEEE Trans. Electr. Insul, Vol EI-13 No 5, October 1978
354
REFERENCES
[1]
M. Buchholz "The Buchholz protection system and
its application in practice" E.T.Z. 49 (1928) 34
pp 1257-1262.
[2]
V. H. Howe, L. Massey, A.C.M. Wilson. "Identity
and significance of gases collected in Buchholz
protectors." M. V. Gazette May 1956.
[3]
P. S. Pugh, H. H. Wagner. "Detection of incipient
faults by gas analysis." TAIEE 80 (1961) pp 189195.
[4]
L. C. Aicher, J. P. Vora. "Gas analysis - a
transformer diagnostic tool." Allis Chalmers
Electrical Review 28 (1963) 1, pp 22-24.
[5]
B. Fallou, F. Viale, I. Davies, R. R. Rogers,
E. Dornenburg. "Application of Physico-Chemical
Methods of analysis to the study of deterioration
in the insulation of electrical apparatus."
CIGRE 1970 Report 15-07.
[6]
W. D. Halstead. "A thermodynamic assessment of
the formation of gaseous hydrocarbons in faulty
transformers." J. Inst. Petroleum 59
Sept. 1973) 569 pp 239-241.
[7]
B. Barraclough, E. Bayley, I. Davies, K.
Robinson, R. R. Rogers and C. Shanks. "CEGB
Experience of the analysis of dissolved gas in
transformer oil for the detection of incipient
faults." IEE Conference on Diagnostic Testing
of High Voltage Power Apparatus in Service 6-8
March, 1973.
[8]
R. R. Rogers. "UK Experience in the Interpretation
of Incipient Faults in Power Transformers by
Dissolved Gas-in-Oil Chromatographic Analysis."
Doble Client Conference 1975 Paper 42 AIC 75.
[9] CIGRE 15-01.
"Detection of and Research for the
Characteristics of an Incipient Fault from
Analysis of Dissolved Gases in the Oil of an
Insulation." Electra 42 (October 1975) pp 31-52.
[10]
R. R. Rogers. As Ref. 8. A Progress Report.
Doble Client Conference 1977 Paper 44 AIC 77.
[11]
M. Thibault, J. Raboud. "Application of dissolved gas analyses to the maintenance of
transformers". Rev. Gen. Elec. 84 (Feb 1975) 2,
pp 81-90.
[12]
R. Muller, H. Schliesing, K. Soldner. "Assessment
of working condition of transformers by gas analysis." Elektrizitatswirtschaft 76 (1977) 11,
pp 345-349.
[13]
R. Andersson, U. Roderick, V. Jaakkola,
N. Ostmann, "The transfer of fault gases in
transformers and its effect upon the interpretation of gas analysis data." CIGRE 1976
Report 12-02.
[14] M. Duval. "Fault gases in oil-filled breathing
EHV power transformers. The interpretation of gas
analysis data". IEEE PES Conference paper No.
C74 476-8 (1974).
Manuscript was received 11 November 1977, in finaZ
form 1 May 1978.
This paper was presented at the IEEE Power Enginerring
Society Winter meeting, January 1978 in Nsw York, N.Y.