Power oil transformers - Gas generation defects
KHRENNIKOV1 A.Yu., ALEKSANDROV2 N.M.
1Scientific & Research Centre of FGC of UES Rosseti
2SPE Dynamics
1 Kashirskoe highway, 22/3, 115201, Moscow
2Anisimova str., 6, 428000, Cheboksary
RUSSIA
Abstract: - The article analyzes scientific research in the field of power transformer gas generation defects
detecting by methods of Gas Dissolved Analysis (DGA) of transformer oil (chromatographic analysis) and
measurement of short-circuit impedance differences. Power transformer defects that cause gas generation and
are identified by the results of DGA should be divided conditionally into several groups: defects with
circulating currents in windings and short-circuited contours, induced by scattering flux created by wind-ups,
defects with an increase in the transient resistances of the grounding nodes of the structural elements, defects
with partial discharges of oil gaps and on the surface of solid insulation, defects with a violation of the contact
connections of the conductive circuits, defects with overheating and aging of solid insulation and transformer
oil. The difference in short-circuit impedance, measured from the sides of the higher and lower voltages,
brought to one side of the transformer, is directly dependent on the magnitude of the circulating currents
created by the scattering fields, expressed as a percentage, that is, the percentage of the number of
uncompensated turns of the windings with current.
Key-Words: - Power transformer, gas generation, Gas Dissolved Analysis (DGA), chromatographic analysis,
short-circuit impedance, scattering flux.
Received: August 8, 2022. Revised: July 21, 2023. Accepted: August 25, 2023. Published: October 2, 2023.
1 Introduction
There are various anomalous phenomena, such
as gas generation, the difference of phase currents in
windings connected to the triangle, the
inconsistency of the position of the tap-changer
(voltage regulator) for a level of nominal voltage,
partial discharges, overheating of individual parts of
the tank connector, etc. during the operation of
power oil transformers and autotransformers.
Possible causes of such anomalous phenomena
may be design and operational features, as well as
defects that have arisen during operation, under the
influence of which is the decomposition of
transformer oil and paper oil insulation with the
release of hydrocarbon gases, oxide, and carbon
dioxide.
2 Power transformer defects detecting
method
The most common and reliable method of
detecting a power transformer defect is Gas
Dissolved Analysis (DGA) of transformer
oil (chromatographic analysis), while exceeding or
increasing the concentration of individual gases in
most cases does not allow for localizing and
establishing the cause of the defect, as well as to
establish the impact of this defect on the operational
reliability [1-6].
The gas chromatograph "Crystal 5000" is
presented in the fig. 1.
Fig. 1. Gas chromatograph "Crystal 5000"
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The transformer oil of the GK type for
processing refers to naphthenic oils with positive
gas resistance. To increase stability against
oxidation, the transformer oil is subjected to
intensive processing (hydro-cleaning) of hydrogen
at high pressure and high temperature with the use
of catalysts. All chemical reactions in the process of
hydro-cleaning are in a state of equilibrium and it is
possible that they can theoretically go in the
opposite direction under conditions other than those
under which the process of hydro-cleaning took
place.
The new equilibrium is established in practice
when new transformer oil is commissioned and the
environment in which the oil exists
changes. Balance can change due to changes in
temperature, pressure, vibration, and pollution,
causing a slight gas formation. In the absence of any
external factors leading to gas generation, gas
production in the transformer for 6-8 months, and
sometimes more, months, stops. The main gas of
this gas generation is hydrogen.
The decomposition of transformer oil and solid
insulation with the release of hydrocarbon gases,
oxide, and carbon dioxide occurs under the
influence of electrical defects (electric arc in oil,
partial discharges and arc in the oil barrier isolation,
etc.) and thermal defects (heat decomposition of oil
and oil barrier insulation, solid insulation
overheating, aging of solid insulation and oil, etc.)
[7].
3 Power transformer defects
Defects that cause gas generation and are
identified by the results of DGA should be divided
conditionally into several groups, namely:
Defects of the first group - defects that cause
gas generation, are associated with structural and
operational features, namely, defects with
circulating currents in windings and short-circuited
contours, induced by scattering flux created by
wind-ups during operation and testing.
Windings with an asymmetrical and uneven
distribution of linear load relative to the middle of
windings and other windings (input coils from the
end of windings, adjustment coils, etc.),
asymmetrical location of windings relative to the
magnetic window systems, asymmetrical location of
shunts relative to the ends of windings, etc. refer to
the design features of such transformers.
Uneven load of split windings (more than 20%)
of transformers with split windings short-circuit of
parallel conductors, multi-height windings,
deformation of windings, turn-to-turn short-circuit,
i.e. modes and defects leading to the uneven and
asymmetrical distribution of loads by the height of
the windings are the operational features of
transformers.
Electromagnetic scattering is an incomplete
electromagnetic connection in a transformer caused
by the presence of magnetic flux that is not common
to both windings, i.e. closed outside the magnetic
system, and called the scattering flux. The degree of
incomplete electromagnetic connection has a great
impact on many technical parameters, including
short circuit parameters.
It is known that the magnetic field (stream) of
scattering, in a real transformer can be represented
in the form of three fields: a longitudinal field,
created by a full number of winding turns with
current; a cross-field caused by the final height-
width ratio of windings and the second cross field
caused by the uneven distribution of linear load by
the height of windings.
Defects that cause gas generation are associated
with the presence of magnetic scattering flux in the
closed circuits of the magnetic scattering space of
windings.
The induced voltage from the scattering fields is
applied precisely to the insulation gaps of the joints
in the presence of butt connectors in the circuits;
therefore, a section is formed with a high
inhomogeneous electric field, causing discharge
phenomena in the oil gap of the joint.
The prolonged exposure (months and years) of
this insignificant electric field to the transformer oil
in the joint area leads to the gradual accumulation of
gases in the oil.
This process can be called partial discharges at
the joints of closed loops.
Here, a typical example can be for distribution
transformers a violation of the insulation of the tie
rods, and yoke pressing beams, when as a result
short circuits are formed with butt gaps in the
openings of the channels of the yoke beams with
studs.
Discharge phenomena can occur between the
parallels themselves, which are separate electrodes,
between which in the insulation gap (butt connector)
an electromagnetic force (EMF) is applied in the
case of parallel branches. The relatively small value
of this EMF during prolonged (for months)
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exposure leads to the decomposition of oil and gas
formation.
The scattering field of the transformer induces
EMF in the wires and circuits located in the field
zone, under the influence of which currents flow.
These currents are closed inside the wires and
between the parallel branches of the windings and
short-circuited circuits, including in the magnetic
system, and, unlike the load currents, do not go
beyond the windings and circuits, and can occur
during testing. In turn, the circulating currents in the
windings create additional fluxes, which, combined
with the main scattering fluxes, increase the
magnetic fluxes in the yokes of the magnetic system
and the tank, causing them to heat up. And the
circulating currents in the butt connectors of short-
circuited circuits, including at the joints of the plates
of the magnetic system, create discharge
phenomena. The maximum magnetic flux in the
core of a single-phase transformer is shown in fig. 2.
Fig. 2. Maximum magnetic flux in the core of a
single-phase transformer.
The results of DGA of oil, and the results of
transformer inspection prone to gas generation,
confirm the presence of oil decomposition products
(soot) at the joints of the plates of the magnetic
system and the joints of the short-circuited contours
of structural elements caused by electrical
discharges and thermal effects on these areas [7].
Thus, the defects of the first group that cause gas
generation are accompanied by the release of gases
associated with defects of an electrical and thermal
nature, and the source of gas generation is the
magnetic system and structural elements, as well as
windings with short circuits of parallel conductors
and turn-to-turn short-circuits.
Defects of the second group - defects that cause
gas generation, are associated with an increase in
the transient resistances of the grounding nodes of
the structural elements, the presence of structural
elements under floating potential, and increased
potentials in the structural elements and taps of the
high-voltage bushings of transformers.
Currents arise in them under the influence of
inductive EMF, scattering fields in short-circuited
circuits located in the zone of an electromagnetic
field.
Circulating currents cause heating and discharge
phenomena at the joints of contact connections of
grounding elements. In addition, discharge
phenomena occur between structural elements under
floating potential and grounded structural elements.
Defects with violation of the contact resistance
of the grounding elements and the presence of
elements under a floating potential are accompanied
by gas generation associated with defects of an
electrical nature. In this case, the defects of the first
group enhance the discharge phenomena in the
grounding elements.
Defects of the third group - defects that cause
gas generation, are associated with partial
discharges of oil gaps and on the surface of solid
insulation under the influence of applied voltage,
test voltage, lightning, and switching overvoltages,
as well as gaps with increased electric field
strengths.
Defects of the third group are accompanied by
the gas generation associated with defects of an
electrical nature. In this case, the defects of the first
group enhance the discharge phenomena.
Defects of the fourth group - defects that cause
gas generation are associated with a violation of the
contact connections of the conductive circuits.
The most common defects with violation of the
contact connections of conductive circuits are bolted
and soldered joints, i.e. contact connections of
bushings, on-load tap-changer, without-load tap-
changer, and taps of the high-voltage bushings of
transformers.
Defects in conductive circuits are accompanied
by the gas generation associated with defects of a
thermal nature. In this case, defects of the first,
second, and third groups do not affect the defects of
the fourth group.
Defects of the fifth group - defects that cause
gas generation are associated with overheating and
aging of solid insulation and transformer oil.
It is known that thermocatalytic degradation of
solid insulation is usually accompanied by
degradation (water yield) with an increase in the
concentration of carbon dioxide (CO2) and oxide
(CO), as well as their increased ratio and increased
moisture content of transformer oil [1, 2, 14, 16].
Defects of aging of solid insulation are
accompanied by a decrease in the degree of
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polymerization, an increase in the moisture content
of the oil, and an increase in carbon monoxide and
dioxide. Moreover, defects of the first and third
groups enhance the aging processes of insulation
and transformer oil [8-14].
The analysis of transformer defect groups and
their interconnections show that the defects leading
to gas generation, in addition to the defect of the
conductive circuits, are associated with the
condition state of the windings, i.e. with a change
and increase in scattering fields.
The short-circuit impedance is one of the
parameters influenced by the scattering fluxes. It is
also known that with the same geometrical
dimensions and the same arrangements of the coils,
their inductances and inductive resistances are
proportional to the squares of the number of turns,
that is, there is a relation between inductive
impedances of higher and lower voltage:
, (1)
where – short-circuit impedance, measured
from the side of higher voltage, Ohm; -
short-circuit impedance measured from the low
voltage side, Ohm; KT - the ratio of transformation,
the degree of regulation.
Short-circuit impedance, measured from the side
of the higher and lower voltages brought to one of
the sides of the transformer, are not equal to each
other in a real transformer in the presence of any
defects. The measurement scheme of the short-
circuit impedance Zk of the transformer in HV-LV
mode is presented in the following fig. 3
Fig. 3 Measurement scheme of the short-circuit
impedance Zk of a transformer in HV-LV mode
(HV-LV windings (phase A measurement).
Thus, the difference in short-circuit impedance
measured from the sides of the higher and lower
voltages arises when the geometric dimensions
(deformation, different heights, shorting of parallels,
etc.) differ and the arrangement of the coils or the
presence of other defects [15].
The difference of short-circuit impedance
reduced to the side of the higher voltage is
determined from the expression:
(2)
The transformer must be taken out for repair if
the value of deviates from the base by 3% or
from the calculated by the passport by 5% in the
HV-LV mode according to [12, 13, 16, 18].
However, from [15] it is known that the
difference in short-circuit impedance, measured
from the HV side and the LV side, more than 2.0%
indicates a defect in the windings, and less than
2.0% is associated with defects in the contact
connections of conductive circuits and ground
circuits structural elements, as well as partial
discharges under the influence of operating voltage
and lightning switching overvoltages [16-22].
The relationship between the causes of gas
generation that occurred during operation due to a
structural defect (asymmetric and uneven
distribution of the linear load relative to the middle
of the windings and windings of different heights) is
shown in the example of the 4 MVA/35 kV
transformer.
4 Example 1.
4 MVA/35 kV transformer is operated in the
system of Kolymagaz and it has been in operation
for five years, the load was not more than 50%, and
there was a strong gas generation (triggering of gas
protection to the signal). Composition of gases:
hydrogen and a small amount of hydrocarbon gases.
During disassembly, on the surfaces of the active
part, a gray coating was detected with a thickness of
up to 0.5 mm. This deposit (sludge) is probably the
product of oil oxidation with a large amount of
hydrogen released. No other visible defects, except
for different heights and asymmetric distribution of
the turns of the windings along their height and
increased electric field strengths between the layers
of the windings, were found.
Control turns in the form of two coils were
wound on top of the regulating windings for
detection of presence of asymmetry of the scattering
field. The turns were located symmetrically relative
to the middle of the windings and connected in
parallel. Currents were measured in parallel
connected branches of the control turns during the
short-circuit experiment. Current in parallel
connected turns can occur only in the presence of
asymmetry of the transverse scattering field.
The measurement results of short-circuit
impedance and circulating current in the control
turns are given in table 1.
From the values given in table 1, it follows:
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1. The technical characteristics of the
transformer correspond to the volume of the
standards for testing electrical equipment and the set
of operational documentation of the manufacturer.
2. The presence of current in parallel connected
branches of the control winding indicates the
difference in the EMF in the branches induced by
asymmetric transverse scattering fields, i.e. created
by the uneven and asymmetric distribution of the
load current along the height of the windings.
3. The difference between the short-circuit
impedance measured from the sides of the higher
and lower voltages given to one of the sides of the
transformer is directly dependent on the magnitude
of the circulating current, and expressed as a
percentage, there is a percentage of the number of
uncompensated turns of current windings.
Table 1 - The results of measurements of short-
circuit impedance and circulating currents in the
control turns of 4 MVA/35 kV transformer.
Position
OLTC
Short-
circuit
mode
Short-
circuit
impeda
nce
,
Ohm
Circulating
current in
control
turns, A
Short-
circuit
resistance
difference
(),%
Quantity
of
uncompe
nsated
turns with
electric
current,
Aw
1
2
3
4
5
6
1
HV-LV
30,54
5,2
5,87
3250
LV-HV
2,307
4,8
5
HV-LV
LV-HV
23,91
2,19
3,6
3,15
5,62
3100
9
HV-LV
18,15
2,5
2,3
1200
LV-HV
2,11
2,25
In order to express the circulating current, some
explanation is needed.
The phase power of the transformer is written as
follows:
(3)
Rated phase voltage of HV winding:
Unom.1ph= (ω/√2)w1ВmSs, (4)
where ω -the circular frequency of the network; w1 -
the number of turns of the HV winding; Bm - the
amplitude of the magnetic induction in the core in
idle mode, as a rule for power transformers Bm =
1.65 T, a constant value;
Ss -the active section of the core. Here, the
circular frequency of the network is ω = 2πf, where
f - the network frequency.
The active section of a round core:
(5),
where D - is the diameter of the circumscribed
circle of the core of the magnetic circuit,
ks -the fill factor of the packages of the magnetic
circuit steel, approximately ks = 0.965,
kgeom - geometric coefficient filling the circular
cross-section of the core with packages of electrical
steel, approximately kgeom = 0.925.
Then
, (6)
Вm = const.
Circulating current can be expressed as follows:
(7)
where
Bm - the amplitude of the magnetic induction in
the core in idle mode, as a rule for power
transformers Bm = 1.65 T, the value is constant.
In this formula, the dimension [] is
obtained due to the adoption of the condition Bm =
const for most power transformers.
- the total circulating current in short-
circuited circuits, including in the magnetic system,
A; D - the diameter of the core of the magnetic
system, m;
w1 - the number of turns of the HV winding;
- difference of short-circuit impedance
measured from the sides of the higher and lower
voltages, reduced to one of the sides of the
transformer,%
Snom. - rated power of the transformer, kVA.
4. The minimum value of the difference in short-
circuit impedance at the minimum tap of the on-load
tap-changer, and the maximum value of the
difference of short-circuit impedance at the
maximum tap of the on-load tap-changer indicate an
asymmetric and uneven arrangement of the turns of
the control winding relative to the lower and higher
voltage windings.
5. Uncompensated turns of windings with current
(3250 Aw at max position, 3100 Aw at nom.
position, 1200 Aw at min position) are compensated
by circulating currents in short-circuits, including in
the magnetic system, causing electrical discharges at
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Volume 18, 2023
the joints of the plates and their heating due to the
asymmetric arrangement of the regulating turns of
the winding, which is the cause of gas generation
with the release of hydrogen H2 and a small amount
of hydrocarbon gases.
Conclusions
1. A method for identifying and localizing a
defect by measuring short-circuit impedance on the
sides of the higher and lower voltages allows to
identification of defects that affect the gas
generation that occurred during design,
manufacture, operation, and repair, as well as
determine the effect of the defect on operational
reliability.
2. The difference in short-circuit impedance,
measured from the sides of the higher and lower
voltages, brought to one side of the transformer, is
directly dependent on the magnitude of the
circulating currents created by the scattering fields,
expressed as a percentage, that is, the percentage of
the number of uncompensated turns (circulating
current) of the windings with current.
3. Uncompensated turns of windings with a
current of more than 2.0% and and uneven load of
the LV1 and LV2 windings of transformers with
split windings of more than 20% lead to local
heating of structural elements, including the yoke of
the magnetic system and the occurrence of
discharges in the contact joints of short-circuited
circuits, including at the joints of the plates of the
magnetic system.
4. In the presence of gas generation and the
absence of a defect in the windings (∆Zk <2.0%),
defects in the transformer are associated with a
violation of the contact connections of the
conductive circuits and ground circuits, the
occurrence of repeated grounding with the
formation of short-circuited circuits and the
presence of structural elements under floating
potential, as well as partial discharges under the
influence of applied voltage, lightning and
switching overvoltages.
5. In accordance with the recommendations of
IEC-6099 and RD 153.340-46.302-00 for
transformers with split windings and transformers
for which > ±2.0%, and whose technical
characteristics satisfy the volume of the standards
for testing electrical equipment and passport values
of the manufacturer, at an increased concentration
for individual gases, higher boundary concentrations
of gases dissolved in oil should be established, both
for individual gases and for the volume of gas
content.
6. Some future research directions are the
modeling of processes in power transformers when
defects associated with gas formation occur.
7. The use of artificial intelligence and neural
networks will make it possible, based on this
method and groups of defects that cause gas
formation and identified by the results of DGA, to
create an expert diagnostic system with elements of
artificial intelligence for diagnosing a defect or
damage and localizing the location of the defect.
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Sources of Funding for Research Presented in a
Scientific Article or Scientific Article Itself
The authors are greatly acknowledged for
supporting this study to Mr. Avtaev P.N. and his
research of method for identifying a defect in a
power transformer. Cooperation of Universities and
Innovation Development, Doctoral School project
“Complex diagnostic modeling of technical
parameters of power transformer-reactor electrical
equipment condition”, 2009, has made publishing of
this article possible
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DOI: 10.37394/232016.2023.18.13
Khrennikov A. Yu., Aleksandrov N. M.
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Volume 18, 2023
Sources of Funding for Research Presented in a
Scientific Article or Scientific Article Itself
No funding was received for conducting this study.
Conflict of Interest
The authors have no conflicts of interest to declare
that are relevant to the content of this article.
Creative Commons Attribution License 4.0
(Attribution 4.0 International, CC BY 4.0)
This article is published under the terms of the
Creative Commons Attribution License 4.0
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