AC Insulation Design of
Power Transformers
Fundamentals
Eng. Álvaro PORTILLO LAURINO
Transformer Consultant
Brenda 5920, Montevideo, CP11400, Uruguay
Phone: (+598) 26007982
e-mail: acport@adinet.com.uy
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Summary
In this work we will try to give an overview of
the insulation design process of high voltage
"core-type" power transformers operated in
AC networks for engineers involved in design
review tasks
All presented values and formulas are only
orientations and can vary widely
between manufacturers
Emphasize the concepts
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Introduction
• The power transformers in electric networks are subjected:
- permanently to continuously operating voltages
- sometimes to transient overvoltages caused by faults,
switching operations or lightning strikes
• To probe his ability to work for many years in service, with
permanent and transient voltage conditions, the transformers
are subjected to factory acceptance dielectric tests
• This tests trying to represent the different conditions that the
power system can impose to the transformer
• This tests are the result of more than 100 years of experience
and is generally accepted that if a transformer successfully
passes these tests they have a very high probability of work
for many decades in service without dielectric problems
• The challenge for the transformer designer is define an
insulation structure which comply with the dielectric tests
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Introduction
• The purpose of transformer insulation is to isolate parts or
electrodes at different potentials from one another but the
design of an insulation structure is not only define this
distances inside the transformer
• Previous to this is necessary to define completely:
- the geometry and number of insulation barriers
between windings and between windings and ground
- the insulation material type best suited for each part of
the transformer
- the thickness of the conductor insulation
- if is necessary or not the use of static end rings in the
windings
- the type of winding (interleaved or not)
• These definitions have a big influence in the voltage
distribution inside the transformer and in the electric fields
that appears during the dielectric tests
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Introduction
• Once the material types and geometry is completely defined
voltage distributions outside and inside the windings
according to the test voltages and to the corresponding
winding connection during test are calculated
• For AC voltages (50 to 200 Hz) the voltage distribution follows
linearly the number of turns and can be calculated very
precisely
• The calculation of impulse voltage distribution requires the
simulation of the transformer by means of an equivalent
circuit consisting of lumped R, L and C elements
• Then with this voltage distribution using simple analytical
formulae or numerical methods (like Finite Elements Method)
is possible to calculate the electric field or electrical stress in
each point inside the transformer
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1.503e+007 : >1.582e+007
1.424e+007 : 1.503e+007
1.345e+007 : 1.424e+007
1.266e+007 : 1.345e+007
1.187e+007 : 1.266e+007
1.107e+007 : 1.187e+007
1.028e+007 : 1.107e+007
9.492e+006 : 1.028e+007
8.701e+006 : 9.492e+006
7.910e+006 : 8.701e+006
7.119e+006 : 7.910e+006
6.328e+006 : 7.119e+006
5.537e+006 : 6.328e+006
4.746e+006 : 5.537e+006
3.955e+006 : 4.746e+006
3.164e+006 : 3.955e+006
2.373e+006 : 3.164e+006
1.582e+006 : 2.373e+006
7.910e+005 : 1.582e+006
<0.000e+000 : 7.910e+005
Introduction
kV → kV/mm
Density Plot: |E|, V/m
1.503e+007 : >1.582e+007
1.424e+007 : 1.503e+007
1.345e+007 : 1.424e+007
1.266e+007 : 1.345e+007
1.187e+007 : 1.266e+007
1.107e+007 : 1.187e+007
1.028e+007 : 1.107e+007
9.492e+006 : 1.028e+007
8.701e+006 : 9.492e+006
7.910e+006 : 8.701e+006
7.119e+006 : 7.910e+006
6.328e+006 : 7.119e+006
5.537e+006 : 6.328e+006
4.746e+006 : 5.537e+006
3.955e+006 : 4.746e+006
3.164e+006 : 3.955e+006
2.373e+006 : 3.164e+006
1.582e+006 : 2.373e+006
7.910e+005 : 1.582e+006
<0.000e+000 : 7.910e+005
Density Plot: |E|, V/m
ITAIPÚ
Autotransformer
470/470/157 MVA
525/241.5/13.8 kV
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Introduction
• The calculated electrical stress (kV/mm) in each point P must
be less than the admissible dielectric strength (kV/mm) of the
insulating material used in this point P for this test condition
• If not, the insulation design is modified and verified again, and
this procedure iteratively must leads to an optimised solution
• Normally the design is defined in a way that the test stress
does not exceed the PD inception values of the insulating
materials
• Finally the success of the dielectric design depends on select
high quality insulating materials with narrow dimension
tolerances and shape stability and applying adequate
stabilization, drying and impregnation processes to the
insulation materials
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Transformer Insulations
• The transformer insulations are usually classified in:
– external or major insulations
– internal or minor insulations
• External or major insulations include principally insulations
outer the windings:
– winding to winding
(gaps between windings)
– phase to phase
– windings to ground
(to core legs, to core yokes and to tank)
– winding leads
(connections between windings, connections from
windings to bushings, connections from windings to
OLTC , etc.)
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Phase to Phase Insulation
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Winding to Ground Insulation
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HV Winding Lead Insulation
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Connections from Windings to OLTC
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Transformer Insulations
• Internal or minor insulations include principally insulations
inside the windings:
– conductor to conductor
– turn to turn
– section to section (axially along windings)
– layer to layer
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Turn to Turn Insulation
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Section to Section Insulation
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Section to Section Insulation
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Transformer Insulations
• Other essential elements in order to achieve a good dielectric
design:
–
–
–
–
Angle caps
Angle rings
Static end rings
Internal surge arresters
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Angle Caps
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Angle Rings
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Static End Rings
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Internal Surge Arresters
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Internal Surge Arresters
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Power System Overvoltages
The transformers during operation are subject continuously to
operating voltages and occasionally to overvoltages
The overvoltages occurring in the power systems can be
divided into:
lightning overvoltages
aperiodic voltage waves with duration of one to tens of
microseconds
– switching overvoltages
damped oscillatory voltage waves with duration up to
thousands of microseconds
– temporary overvoltages
voltage waves at or close to the power frequency lasting
for few minutes
–
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Dielectric Tests
Tests intended to verify the insulation withstand to operational
voltage and to transient overvoltages
• The Applied Voltage Test at industrial frequency (50 or 60 Hz)
With the applied voltage test the withstand strength of the
external insulations (winding to winding and windings to
earth) to service and temporary overvoltages is verified
In this test there is not turn-to-turn voltage
• The Induced Voltage Test (short and long duration) at
industrial frequency (between 100 to 200 Hz)
With the induced voltage test we verified principally the
internal insulations (turn to turn, section to section) and also
external insulations (phase to phase, winding to winding and
windings to earth) to service and temporary overvoltages
The long duration induced voltage test with PD measurement
is intended to verify that the transformer will be free of
harmful partial discharges under normal operating
conditions
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Dielectric Tests
• The Switching Impulse Test is intended to verify the capability
of insulation to withstand slow rise time (greater than 100 µs)
transient voltages typically associated with switching
operations in service
With this test the internal and external insulations are verified
to switching transients
The fundamental test wave frequency is in the order of 2.5 kHz
The voltage impulse shall have a time to peak of at least 100 μs,
a time above 90 % of the specified amplitude of at least 200 μs,
and a time to zero of a minimum of 1000 μs.
This impulse wave shape is purposely different
from the standard waveshape of 250/2500 μs
recommended in IEC 60060-1, since IEC
60060-1 is intended for equipment without a
saturable magnetic circuit.
The time to peak is chosen to be long enough
to give an essentially linear voltage
distribution along the windings
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Dielectric Tests
• The Lightning Impulse Test (full wave and chopped wave) is
intended to verify the insulation withstand to fast rise time
(around 1 µs) transients overvoltages occurring in the power
system as a result of lightning strikes
With this test the internal and external insulations are verified
to lightning transients
The fundamental test wave frequency is in the order of 250 kHz
The chopped wave test voltage impulse has a higher peak value
and contains higher frequency components than the full wave
impulse
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Dielectric Tests - Connections
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Voltage Distribution in Windings
The distribution of voltage to ground along the coils (Fig.a), for the different
tests, is illustrated in the following figures:
Fig.b shows the distribution of voltage in the applied voltage test (not turnto-turn voltage)
Fig.c shows the distribution of voltage in the induced voltage test (voltage
inductively distributed, proportional to the number of turns, through all
windings)
Fig.d shows the distribution of voltage in the atmospheric impulse test
(oscillating voltages that produces non-uniform stresses in winding under
test).
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Voltage Distribution in Windings
• In the atmospheric impulse test the voltage distribution
depends of the capacitances and inductances (self and
mutual) of the windings
The initial voltage distribution inside the windings is
capacitive and at the end of the transient this voltage
distribution is inductive
During the transient the voltage in each point of the winding
is oscillatory with frequencies equal to the natural
frequencies of the transformer and with a damping
depending of the transformer losses
• In the case of switching impulse test the voltage distribution
is almost linear, similar to that experience during an induced
voltage withstand test, and when specifying switching impulse
test is not performed short-duration induced voltage test
(Table 1 of IEC 60076-3:2013)
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External Insulation Design
In a paper-oil insulation system, stressed with AC voltage:
• the maximum admissible field stress of pressboard is higher
than 20 kVrms/mm
• for an oil gap of around 5 mm values less than 12 kVrms/mm are
admissible
• This difference is augmented by the fact that the permittivities
of the two materials differ by a factor 2, resulting in field
values twice as high in oil that in the adjacent board
• Furthermore, the relative strength of oil for an increasing gap
width decreases:
Therefore, in a paper-oil insulation system the solid material is
used only to subdivide oil gaps and to insulate electrodes. The
design of such systems concentrates in general on the electric
strength of the oil gaps and of solid-liquid interfaces
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External Insulation Design – Weidmann Curves
These curves express the maximum admissible design value as
a value of uniform electric field of low probability of partial
discharge inception for 1 min AC test voltage (less than 1%)
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External Insulation Design
• Partial discharges should be excluded even during dielectric
tests of insulations structures
• This design concept is extremely important
• Localization of partial discharges during transformer testing is
unsafe and should be avoided to the greatest possible extent
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External Insulation Design
• Another type of breakdown that can occur in insulation
structures consisting of solids and fluids is creep breakdown
• This occurs along a solid surface that is in contact with a
liquid or gas
• These potential breakdown surfaces are nearly unavoidable in
insulation design.
• In the end insulation area the pressboard must be used in
such a manner that creep stress are practically precluded
• To achieve this the pressboard-oil boundary
surfaces must run, as far as possible, parallel
to the equipotential surfaces.
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External Insulation Design
For the solid-liquid interfaces the maximum admissible creep
tangential stress EC-AC along clean pressboard surfaces in
degassed oil, in terms of the creep distance dC (in mm) along
the surface, can be calculated using the following formula:
EC-AC in kVrms/mm, 1% probability of PD inception
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External Insulation Design
• Several parameters influence the breakdown behaviour of
transformer oil and in consequence to this the oil-design
curves
• The most evident parameter is the duration of voltage
application on an insulation configuration
• Breakdown test has shown that oil-paper insulation exhibits
an exponential decrease of strength when the duration of the
voltage application is increased
• This volt-time breakdown characteristic can be represented
with a equation of the type:
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External Insulation Design - DIL
• Design curves must reflect this dependence; therefore these
curves are defined for a reference duration of 1 minute, AC,
power frequency
• A multiplication factor is introduced to adapt the design
curves for different time duration, e.g. lightning impulse (BIL),
switching impulse (SIL), 1 hour induced voltage, etc.
• This factor is called Design Insulation Level (DIL) and it
increases (reduces) the respective design curve value if the
voltage application time is shorter (longer) than 1 minute:
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External Insulation Design - DIL
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External Insulation Design
•
•
•
•
The evaluation of extenal paper-oil insulation systems consist of
calculation of stress and the subsequent comparison of stress
values with admissible design values
The calculation of stress is divide into three parts:
Calculation of Voltage distributions within windings according to
the specific test voltages and to the corresponding windings
connections during the tests
These voltages are converted to the equivalent voltage at 1
minute power frequency voltage
The maximum of these equivalents in each insulation clearance will
define the insulation design in this insulation clearance
Finally, usually using the Finite Elements Method (FEM) or
analytical formulas for simple geometries, the electric field within
the insulation clearances is determined for the maximum
equivalent voltage.
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External Insulation Design
Winding to Winding Insulation
Angle
End Rings
collars
HV
HV Winding
LV Winding
Radial
Spacers
Key
spacers
LV
Sticks
Pressboard barriers
Pressboard barriers
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External Insulation Design
From the point of view of the electric field calculation this
configuration is very simple. Disregarding curvature effects, the
electric field is uniform along all the height of the windings and
can be calculated using the elementary formulas of a plane
capacitor:
Where U is the applied voltage between the windings, dOil is the
total oil width, dPsb is the total pressboard width and (ε = 2.2)
and (ε = 4.4) are the permittivities of oil and pressboard
respectively.
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External Insulation Design
To simplify we suppose that all cylinders have the same width and all
the oil ducts have the same width (this is not usual in practical cases):
In this case, for AC test (1 min, 50 Hz), with degassed oil and
insulated electrodes, applying equation for oil and equation for
pressboard the design conditions will be:
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External Insulation Design
Like an example, consider the design of a 245 kVrms main gap
The voltages to be applied in the tests are:
Um 245/ SI 850/ LI 1050/ LIC 1155/ AC 460 kV
According to DIL factor approach the main gap will be designed for:
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External Insulation Design
Both Winding to Winding Insulations (LV-HV gaps) are
equivalents from the point of view of the dielectric design
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External Insulation Design
Phase to phase insulation
Winding to tank insulation
Disregarding curvature effects these
insulations are designed in the same way of
winding to winding insulation
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Winding to core return
legs insulation
Winding to core return
legs insulation
44
External Insulation Design
Non-uniform electric fields
• End winding insulation
• Winding to core yokes insulations
• Winding leads insulations
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External Insulation Design
• For non-uniform electrical fields a very conservative approach
would be to limit local maximum stresses to values given by
the oil design curves for the full length gap
In this case the larger part of the gap is not stressed at the
limit of its dielectric strength
This is not satisfactory as it leads to excessive dimensions and
high costs
• On the other hand it would be risky to compare the average
electric field stress with the design curves
In highly non-uniform fields average values can be low
compared with the maximum value in the gap
These highly stressed gap parts intervals might be
overstressed
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External Insulation Design
• The method for determining the electric strength in the case
of non-uniform fields was developed and verified by
experiment [Weidmann]
• Suppose a oil gap d with highly non-uniform electric field
profile
Beginning in the high field region, average stresses are
calculated for gap intervals which are sucessively increasing
from up to the total length:
These average stresses are compared with the oil design
curves values for a gap interval z
The dielectric strength
must be higher than the average
stress
for all the gap intervals z from z = 0 to z = d
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External Insulation Design
Design curves for non-uniform electric fields
This method is widely used to define maximum permissible
voltages in insulating structures with highly non-uniformed
electrical fields and breakdown tests showed that these
voltages are equivalent to a breakdown probability of 2%.
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External Insulation Design - Example
Case 1
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Case 2
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Case3
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External Insulation Design
Example
q( r ) Minimum
Oil Gap 1
Oil Gap 2
Oil Gap 3
Case 1
1.19 @ r =125 mm
−−−−−
−−−−−
Case 2
1.18 @ r =110.5 mm 2.30 @ r =177.5 mm 3.39 @ r =245 mm
Case 3
1.62 @ r =54 mm
1.64 @ r =73.5 mm 1.64 @ r =245 mm
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Internal Insulation Design
• For the design of internal insulations the same rules explained
in external insulations can be applied with the exception of
DIL factor
• DIL must not be applied for internal design of the windings
insulation
• These insulations shall be designed for each type of stress
(service, AC tests, impulse tests, etc.)
• The internal insulation design is strongly dependent of the
type of winding and of the measures taken by the designer to
improve the lightning impulse voltage distribution by means
of winding series capacitance increase (interleaved disk
windings and intershield disk windings)
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Internal Insulation Design
Turn to Turn Insulation - Kraft Paper - Admissible Field Stress
n = 0.22 for impulse tests
n = 0.33 for AC tests
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Internal Insulation Design
Section to Section Insulation
A oil duct is generally provided between sections of the disctype and helical-type windings to ensure the required
dissipation of heat
Due to the creepage distance formed by the pressboard spacers
placed between the sections, the admissible voltage stress in
these ducts is much lower than that tolerable for an oil gap of
equal thickness
Some manufacturers use curves defining the admissible section
to section strength in function of the thickness of conductor
insulation, parametric in the distance between the sections.
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Internal Insulation Design
Section to Section Insulation
A more direct approach is to use directly an equation for the
admissible creepage stress in uniform fields for AC (50 Hz, 1
min) and lightning impulse voltages:
Other values that are verified are the oil electric field at the
internal and external edges of the conductors of each section
The limits for these stresses are around 11 kV/mm for AC (50
Hz, 1 min) and 29 kV/mm for lightning impulse voltages.
All these values and formulas are only orientations and can vary
widely between manufacturers
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Thank you very much
for your attention
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