High voltage power transformer construction

High Voltage Power Transformer
Debojyoti Mukherjee
Jadavpur University
debojyotimukh@gmail.com
March 5, 2019
Debojyoti Mukherjee (JU) HV Transformer March 5, 2019 1 / 56

Overview
1 Transformer Insulation
Insulation requirements
Paper insulation
Dielectric strength, Voltage condition
2 Winding arrangements
3 Behaviour of liquid dielectrics
Properties of transformer oil
Electrode surface phenomenon
Gas evolution
Oil processing
4 Surge behaviour
5 Construction of EHV transformer
6 Short circuit behaviour

Required properties of insulators
1 High resistivity
2 High breakdown voltage (breakdown strength)
3 High dielectric constant ( r )
4 Low dissipation factor (tanδ)
5 Thermal stability
6 Chemical stability
7 Mechanical stability

Insulations in transformer
Position insulation method
Between core and LV press board
Between LV and HV Impregnated press board
Between top bottom of winding and yoke Impregnated press board
Between HV and tank Impregnated paper, cotton, oil
HV and LV coil terminals Bushings (oil ﬁlled, capacitor type)
Between conductors Enamel, varnish
Between turns Impregnated paper, cotton
Between layers Impregnated paper, cotton, pressboard
Between laminations Varnish
Between joints and connection Varnish tape
Table: Major and minor insulations in a transformer

Insulations in transformer

Paper insulation
Kraft paper
Cotton cellulose
Crˆeped paper
Highly extensible paper
Thermally upgraded paper
’Diamond dotted’ presspaper
Oil-Paper combination
In a system of insulation consisting of diﬀerent materials in series, these
share the stress in inverse proportion to their dielectric constants, so that,
for example, in the high-to-low barrier system of a transformer, the stress
in the oil will be twice that in the paper (or pressboard). The transformer
designer would like to see the dielectric constant of the paper nearer to
that of the oil so the paper and oil more nearly share the stress.

Kraft paper
Kraft paper is, by deﬁnition, made entirely from unbleached softwood
pulp manufactured by the soda pulp process; unbleached because
residual bleaching agents might hazard its electrical properties.
Kraft paper tend to be porous in order to be impregnated by oil.
Ideal breakdown strength is 8-10 kV/mm test is described in IS:
2584(1963)

Cotton Cellulose
Cotton ﬁbre may also be combined with kraft wood pulp to produce a
material which optimises the advantages of both constituents giving a
paper which has good electrical and mechanical properties as well as
maximum oil absorption capability.
Oil absobtion is very important for high to low wraps or wraps
between layers of round-wire distribution transformer high-voltage
windings where total penetration of impregnating oil may be diﬃcult
even under high vacuum.
IS: 9335-2(1998) describes the indian standards on Cellulose papers.

Crˆeped paper
It is made with an irregular close
gathering or crimp which increases its
thickness and greatly increases its
extensibility in the machine direction.
Its extensibility enables it to be shaped
to conform to irregular contours or to
form bends which may be necessary, for
example, in joining and forming tapping
leads.
A disadvantage of crˆeped paper is its
tendency to lose elasticity with time. A
better alternative in many situations is
highly extensible paper. CLUPAK
extensible presspaper is one such
material.
Figure: Leads to an on-load
tapchanger

Thermally upgraded paper
Thermally upgraded paper is treated by the addition of stabilisers
during manufacture to provide better temperature stability and
reduced thermal degradation.
The permitted temperature rise for power transformers is based on
reaching an average hot spot temperature in operation which will
ensure an acceptable life for the insulation. This is usually between
about 110o and 120oC. However, within this range of temperatures
insulation degradation is greatly increased by the presence of oxygen
and moisture, both of which are present to some extent in most
oil-ﬁlled transformers.
Thermally upgraded paper can be beneﬁcial in retarding the ageing of
paper insulation; not by permitting higher operating temperatures,
but by reducing the rate of degradation at the operating
temperatures normally reached.

Diamond dotted presspaper
The diamond dotted presspaper represents
a more acceptable method of achieving
high mechanical strength without the
associated difficulty of impregnation.
The resin dots create a large bonding
surface while ensuring that the paper can
be effectively dried and oil impregnation
efficiently carried out.
When the winding is heated the adhesive
dots melt and cure so creating permanent
bonding sites which will be unaffected by
subsequent heating cycles without affecting
its mechanical strength.

Pressboard
At its most simple, pressboard represents nothing more than thick
insulation paper made by laying up a number of layers of paper at the
wet stage of manufacture.
Pressboard is available in thicknesses up to 8 mm and is generally
used at thicknesses of around 2-3 mm.
Types of pressboard-
Calendered pressboard
Mouldable pressboard
Precompressed pressboard (Preferred in Power Transformer)
Laminated pressboard
IS: 1576(1992) is the standard for insulating pressboard.

tan δ = IR
IC
=
V
R
V
ωC
= 1
ωCR ∝ 1
f
Hence, at low frequency, the tan delta
number is higher, and the measurement
becomes easier.
Ploss = VI cos φ = V 2ωC cos(90 − φ) =
V 2ωC sin(δ) = V 2ωC tan δ
i.e dielectric loss is proportional to tanδ
The tan delta controller unit takes
measurement of tan delta values. A loss
angle analyzer is connected with tan
delta measuring unit to compare the
tan delta values at normal voltage and
higher voltages and analyze the results.

Overview
Paper insulation
Gas evolution
Oil processing
4 Surge behaviour

Winding arrangement

Types of windings
Disc winding
Helical winding
Double helical winding
Multi-layer helical winding
Cylindrical winding
Cross-over winding

Disk Winding
Used in high capacity
transformers
A disc coil can be made
from a single strand or
several strands of insulated
conductors wound in a series
of parallel discs of horizontal
orientation.
Individual discs are
separated (axially) from each
other by pressboard spacers.
Spacers are used for
supporting the coils as well
as to provide path for oil to
ﬂow.

Helical Winding
Double helical are used in LV side of high
power transformers where the number of
turns is less. Single helical winding with
normal height may not ﬁll the window
height completely. the conductors are
divided in two parallel circuits and are
placed in two vertical layers shifted in the
axial direction.
Multi-layer helical are used mostly in HV
sides of transformers where the number of
turns is high. As if all the turns are
arranged axially then height may exceed the
window, in that case the total turns are
divided and arranged concentrically.

Cyllindrical Winding
These are used for transformers
with high current and low
voltage rating.
The individual cylindrical layers
are placed axially one over the
other. Each cylinder may
contain more than one layer of
conductors in parallel.
Oil ducts may be used between
cylinders to enhance cooling.

Cross-over Winding
Each coil consists of a number
of layers with a number of turns
per layer. The complete winding
consists of number of such coils
connected in series.

Overview
Paper insulation
Gas evolution
Oil processing
4 Surge behaviour

Viscosity & Pour point
The ease with which this convection flow(thermo-siphoning effect) can be
induced clearly is very dependent on the viscosity of the fluid and it is
therefore important for a transformer oil to have a low viscosity.
Low viscosity will assist in the penetration of oil into narrow ducts and
assist in the circulation through windings to prevent local over-heating
which would result from poorer flow rates in the less accessible areas.
Initial impregnation is also greatly accelerated by the use of oil which is
thin enough to penetrate into multi-layers of paper insulation found in
areas of high stress in extra high-voltage transformers.
The pour point of a fluid is the lowest temperature at which the fluid is
capable of any observable flow.

Volatility & Flash point
Normally transformers are expected to have a life of at least 30 years. It is
desirable not to have to constantly think of making good evaporation
losses during this lifetime, nor is it acceptable that the composition of the
oil should change due to loss of its more volatile elements. Low volatility is
therefore a desirable feature.
It will be recognised that fire and explosion are to some extent potential
risks whenever petroleum oils are used in electrical equipment. It is
therefore necessary that the temperature of the oil in service should be
very much lower than the flash point.
Certain types of electrical fault can also give rise to comparatively volatile
lower molecular weight hydrocarbons or to inflammable gases due to
break-down of the heavier constituent molecules of the transformer oil.

Chemical Stability
Oils consisting of high molecular weight hydrocarbon molecules can suﬀer
degradation due to decomposition of these molecules into lighter more
volatile fractions. This process is also accelerated by temperature.
This should not occur at normal operting temerature but inevitable under
fault conditions.
Acid Content
Another undesirable component in the transformer oil is acid. It causes
corrosion of metal surfaces of the transformer unit and destroys the solid
insulation.

Parameters Standard values
Electrical Properties
Dielectric stregth
New unfiltered oil, Min 30kV(rms)
After filtration, Min If above values not attained oil shall
be filtered, 60kV(rms)
Specific resistance
At 90oC, Min 35 × 1012Ω − cm
At 27oC, Min 1500 × 1012Ω − cm
Dielectric dissipation
factor(tan δ) at 90oC,
Ma
0.002

Parameters Standard values
Chemical Properties
Water content, Max 50ppm
Total acidity, Max 0.03 mg KOH/G
Total sludge, Max(ageing) 0.05% of weight
Flash point Pensky-
Marton(closed), Min
140oC
Pour point, Max −6oC
Kinematic viscosity at 27oC 27 cSt
Density at 29.5oC, Max 0.89 g/cm2
Table: Major properties from IS: 335(1993)

Bridging in contaminated oil
Contaminants such as metal filings or cellulosic residue can be formed
in the oil, especially for transformers with aged paper insulation.
Non-uniform fields are present within the transformer. These
contaminants tend to move towards high field regions due to
dielectrophoresis (DEP) forces
Once in the high field regions, these particles may acquire a charge
when in contact with different parts of the transformer and contribute
to a leakage current. They also could form a bridge over a period of
time. The bridge may potentially act as a conducting path between
two different potentials within the transformer structure, leading to
partial discharges or insulation failure.
Contaminated particles were always formed bridges between
electrodes under DC electric field (Converter transformer). AC field
does not induce any bridging. Combination of AC and DC enhances
the bridging dynamics.

Bubble formation
Bubbles will reduce the electric strength of transformer oil, and even
result in the breakdown of the insulation.
The discharge types of the oil-impregnated pressboard is diﬀerent
with bubbles, and under DC, the main discharge type is ﬂashover
along the oil-impregnated pressboard, while under AC, the main
discharge type is breakdown through the oil-impregnated pressboard.
It is easier to generate the bubbles in ageing paper than in new paper
at a high temperature.The bubbles will be harmful for transformer if
the moisture content in the pressboard is above 2%.
The average breakdown voltage with bubbles is 15.1kV. The
breakdown voltages of oil-impregnated pressboard have a 48.9%
decrease on average.

Dissolved Gas Analysis (DGA)
Whenever a transformer undergoes abnormal thermal and electrical
stresses, certain gases are produced due to the decomposition of the
transformer oil.
When the fault is major, the production of decomposed gases are
signiﬁcant and they get collected in a Buchholz relay. But when
abnormal thermal and electrical stresses are not signiﬁcantly high the
gasses due to decomposition of transformer insulating oil will get
enough time to dissolve in the oil.
Most commonly used method of determining the content of these
gases in oil, is using a Vacuum Gas Extraction Apparatus and Gas
Chronographs. Using this apparatus, gasses are extracted from oil by
stirring it under vacuum. These extracted gasses are then introduced
in gas Chronographs for measurement of each component.

Common gas found during testing
Fault Key Gas Results
Corona
Discharge
Hydrogen Low energy discharges create methene and hydrogen and smaller quantities
of ethylene and ethane.
Arcing Acetylene Large amounts of hydrogen or acetylene or minor quantities of ethylene and
metene can be produced.
Overheated
Cellulose
Carbon
Monoxide
If cellulose is overheated, then it will prodece carbon monoxide.
Overheated
Oil
Methene
and
Ethylene
Overheating oil will produce methene and ethylene(300oF) or methene and
hydrogen(1112oF). Traces of acetylene might be created if the unit has
electrical contacts or if the problem is severe.

Furan analysis
Transformer core and winding have mainly paper insulation. The base
of the paper is cellulose. The cellulose structure is a long chain of
molecules. As the paper becomes aged, these long chains are broken
into a number of shorter parts.
In a transformer, the aging eﬀect of paper insulation is accelerated
due to the oxidation that occurs in oil. When insulating paper
becomes mechanically weak, it can not withstand the mechanical
stresses applied during an electrical short circuit leading to electrical
breakdown.
When oil is soaked into paper, it is damaged by heat and some unique
oil soluble compounds are realized and dissolved in the oil along with
CO2 and CO. These compounds belong to the Furfuraldehyde group.
These are sometimes called Furfural in short. Among all Furfurals
compounds 2-Furfural is the most predominant. These Furfural family
compound can only be released from destructive heating of cellulose
or paper.

Transformer oil processing

Filtration- particle removal
Dehydration- water removal
Degassiﬁcation- removal of gas
Puriﬁcation- all of the above
Regeneration- acidity, colour
Desludging- sludge dissolution
Anti-oxident- DBPC blend-back

Maintenance of oil falls into two catagories:
Preventative maintenance: includes regular care for desicant
breathers nitrogen blanketing systems and monitering of proper
additives level.
Resorative maintenance: represents an attempt to return the
contaminated oil and insulation to its original or as-new quality.

Oil filters
Main purpose is to remove solids from transformer oil.
Surface type filter cartridges with ratings of 0.5-15
microns are most practical for transformer oil
Pleated paper designs give very large filtration area to
smaller space

Fuller’s Earth method
The term Fuller’s earth is used for any clay which has adequate
decolorizing and purifying capabilities.
’Attapulgus clay’ deposits found in Georgia and Florida posseses
superior decolorizing and absorption powers.
Activated Fuller’s earth is highly porous and its active surface area
reaches more than 100m2/gm

Fuller’s Earth method
The oil is regenerated by passing through a Fullers Earth filter
medium for molecular filtration. Contaminants and oil degradation
products are trapped in the granules of the sorbent.
After sorbent saturation, the CMM-R unit automoatically switches to
sorbent reactivation mode to remove contaminants from the sorbent
into a special collector and charcoal filter.
Figure: GlobeCore Oil Regeneration System

Thermo-Vacuum method
Most economical method in the removal of moisture and gases in
EHV transformers.
Using vacuum, moisture can be boiled out of oil at room temperature.
This vacuum process prevents overheating and oxidation of oil.

Figure: Life expectency of transformer oil insulation

Overview
Paper insulation
Gas evolution
Oil processing
4 Surge behaviour

Surge behaviour
The transients to which transformers are mainly subjected are:
Impact of high-voltage and high-frequency waves arising from various
causes, including switching in.
System switching transients with slower wavefronts than the above.
Switching inrush currents.
Short-circuit currents.

Impact of high-voltage and high-frequency waves
Transformer windings may be subject to the sudden impact of
high-frequency waves arising from switching operations, atmospheric
lightning discharges, load rejections, insulator ﬂashovers and
short-circuits, and, in fact, from almost any change in the
electrostatic and electromagnetic conditions of the circuits involved.
Inter-turn insulation failure occurs at coil ends and where there is any
appreciable change in winding geometry. The failures which have
occurred on the line-end coils have been due chieﬂy to the
concentration of voltage arising on those coils as a result of the
relative values and distribution of the inductance and capacitance
between the turns of the coils.

Impact of high-voltage and high-frequency waves
External protection is provided by means of coordinating gaps or
surge diverters coupled with the use of insulation coordination and
winding design has developed to a stage at which more eﬀective
measures are available than reinforcement of end turns.
This development followed on from a fuller understanding of the
response of windings to high-frequency transients and a recognition of
the part played by capacitances at these high frequencies.
Lightning impulse L=inductance
Cs=Series(turn-to-turn) capacitance
Cg =Shunt(turn-to-earth) capacitance
rL=Winding resistance
rs=Loss component of series capacitance
rg =Loss component of shunt capacitance

Lightning impulse
At the instant of incidence of the impulse the capacitance elements alone
react to the front of the wave establishing an initial distribution of
potential which is usually non-uniform. At the end of the phenomenon,
during the tail of the wave, the resistance elements govern the response
establishing a ﬁnal distribution which is usually uniform.

Switching in-rush
If the transformer is energized at a voltage zero then the flux demand
during the first half voltage cycle can be as high as twice the normal
maximum flux. This causes an excessive unidirectional current to flow,
referred to as the magnetizing inrush current.

Switching in-rush
The initial value of the current taken on no-load by the transformer at
the instant of switching in is principally determined by the point of
the voltage wave at which switching in occurs, but it is also partly
dependent on the magnitude and polarity of the residual magnetism
which may be left in the core after previously switching out.
It is also worthy of note that transformers with butt-type yokes do not
retain residual magnetism so much as if the yokes and cores are
interleaved. In-rush current may therefore be less with butt yoke
transformers, though the disadvantages of this form of construction
for ordinary power transformers far outweigh any advantage which
might be gained in respect of a minimised current in-rush.

Overview
Paper insulation
Gas evolution
Oil processing
4 Surge behaviour

Construction of EHV transformer
All the Transformers of primary voltage class from 66kV and above
are conventionally termed as Extra High Voltage (EHV)
These transformers are generally used for stepping down voltage from
Transmission lines.
EHV transformers generally have forced cooling and on-load tap
changer.
IS: 2026 is the indian standard for EHV power transformers.

Forced Cooling
The types of transformer based on cooling mechanisms are:
ONAN
ONAF
OFAF
OFWF
OTI and WTI are used to detect the temperature rise and automatically
oil pumps and cooling fans are operated.

On-load Tap Changer(OLTC)
Tappings are provided in HV winding (as current in HV is less so
sparking will also be less). If load voltage changes due to change in
current the tap is changed automatically during on-load conditions.
This is done to improve voltage regulation.
Two diverter resistors are used which limit the sparking current during
the tap changing process. The rotating contactor, connects two
terminals at a time.
OLTC is kept on a separate oil tank isolating it thermally from the
windings; as due to sparking during tap changing the oil degrades
faster in this OLTC tank.
IEEE Std C57.131-2012 is referred for OLTC.

Overview
Paper insulation
Gas evolution
Oil processing
4 Surge behaviour

Short circuit considerations
Short circuits or faults can and do occur on electric power and
distribution systems. When a fault occurs on the load side of a
transformer, the fault current will pass through the transformer. As
components on these systems, transformers need to be able to
withstand these fault currents.
The thermal stress is caused by the high current causing heating in
the transformer. Both the RMS symmetrical current magnitude and
duration of the fault contribute to the heating of the transformer.
One case in particular requires special attention transformers directly
connected to a generator. When a generator is supplying a load and
the load is suddenly disconnected, the output voltage of the generator
rises signiﬁcantly for a short time until the excitation system
decreases the voltage. In such cases, it is recommended that IEEE
C57.116 IEEE Guide for Transformers Directly Connected to
Generators be reviewed to determine the short circuit withstand
requirement for the transformer.

Short circuit withstand capacity
IS: 2026/5 (2011) describes a power transformer’s ability to withstand
short circuits

Standards on Power transformers
IS No. Title
2026 Power transformers:
(Part 2):2010 Temperature-rise (ﬁrst revision)
(Part 3):2009 Insulation levels, dielectric tests and external clearances
(Part 4):1977 Terminal markings, tappings and connections
(Part 5):2011 Ability to withstand short circuit (ﬁrst revision)

The End

High voltage power transformer construction

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