equipment. Thus, the tests methods used for insulation
maintenance and assessment are of primary importance to
the reliable operation of power system.
ELEC 4611
Power System Equipment
HIGH VOLTAGE TEST TECHNIQUES
AND INSULATION ASSESSMENT
After the insulation is designed to be able to withstand the
normal and abnormal operational conditions to which it will
be subjected, the next consideration must be the long-term
behaviour of the insulation under the expected long-term
operational and environmental conditions. The insulation
condition is the primary factor that determines the lifetime
and the operating efficacy of major HV equipment plant.
During its life in operating equipment, insulation
deterioration will occur even under normal conditions.
However, the deterioration can be accelerated greatly by a
number of factors: these include (i) excessive operating
temperature, (ii) excessive electric field strength, (iii) the
presence of moisture or other contamination, (iv) abnormal
mechanical stress or vibration, (v) environmental factors
such as UV radiation.
Because of this multiplicity of potential degrading factors
(multi-factor ageing), insulation condition monitoring is the
most important feature of asset management in relation to
the large and expensive items of high voltage power
systems. The asset management system involves
maintenance and test procedures with the aim being to
achieve the most efficient and trouble-free use of such
ELEC4611: H.V. Test Techniques and Insulation Assessment
p.1
The most generally used tests for insulation condition
assessment in high voltage electrical power equipment are
(not necessarily in order of importance):
 Dielectric Dissipation Factor (DDF) measurement.
 Dissolved gas-in-oil analysis (DGA)
• For oil impregnated insulation
• For paper degradation analysis
• For oil circuit breakers.
 Insulation Resistance (IR) measurement (and
derivative tests).
 Overvoltage Tests on equipment
• Power Frequency / HVDC
• Impulse (lightning and switching) Tests.
 Partial Discharge (PD) Tests
 Dielectric spectroscopy
• Recovery voltage measurement (RVM)
• Polarisation Depolarisation Current (PDC)
 Diagnostics specific to the insulation type (e.g.,
degree of polymerisation)
ELEC4611: H.V. Test Techniques and Insulation Assessment
p.2
Of the above diagnostics, the DDF, DGA and the IR
measurement can be done easily on-site and usually off-line
(although the actual DGA tests on oil samples taken from
the equipment normally have to be performed later in a
laboratory). The DDF measurement usually requires a
separate high voltage test source operating either at power
frequency or at some very low frequency level to reduce
reactive power requirements, but some DDF tests today can
be performed on-line using sophisticated test techniques to
determine and record phase angle difference between the
operating voltage and current for the item of equipment
under test. The DGA tests are perhaps the simplest to
perform in that only a small sample of oil is required for
chemical analysis and this sample removal can be done
while equipment is on-line. The IR and derivative tests use
a DC voltage application and must be done off-line.
Partial discharge (PD) tests have traditionally been done
off-line or in the laboratory and generally required a
separate HV source and connection to the high voltage line
via a blocking capacitor. However, modern techniques with
high frequency current transformers that can be used on
earth connections now allow on-line monitoring of PDs, but
in such cases electromagnetic interference from corona
discharge and general RF background is a major problem
that must be contended with. Overvoltage tests must be
performed as separate source tests. Normally only power
frequency OV tests can be done on-site: impulse and
switching tests are normally done only in the laboratory or
in the factory. It should be noted that HVDC tests of some
HVAC equipment items has been relatively common
ELEC4611: H.V. Test Techniques and Insulation Assessment
p.3
practice for items such as cables where the reactive power
requirements are large at rated voltage. However, while this
has been acceptable for paper insulated equipment, the use
of modern polymeric insulation with its higher propensity
to store electrical charge for long periods means that HVDC
is no longer able to be used in this way.
1. DDF (tan or Loss Angle) Testing
No insulation material is perfect: it will exhibit some finite
insulation resistance and its polarizability will cause it to
generate thermal loss at AC excitation. Thus, no
capacitance which is formed from any insulation dielectric
material can be totally loss-free. There will always be some
power loss in the dielectric due to a variety of causes,
including the ohmic loss due to leakage current (which is
usually very small), dielectric heating and partial discharge
activity in the insulation voids and material imperfections.
Dielectric heating, the electrical analogue of magnetic
hysteresis loss, results from friction in the material as
electric dipoles, molecular dipoles and atomic charges try to
follow the changing electric field applied to the insulation.
Such loss is much more dominant than ohmic loss in good
insulation at high voltage. It should be noted that the
presence of moisture will considerably increase dielectric
loss because of the high loss factor of the water molecule.
Both the leakage resistance and the dielectric loss factor are
also very temperature dependent.
ELEC4611: H.V. Test Techniques and Insulation Assessment
p.4
The losses due to PD activity are very variable and are very
voltage dependent: in good insulation they are quite low or
non-existent, but they can be significant in degraded
insulation. Measurement of DDF vs voltage will exhibit a
“tip-up” at high voltage, which can be used as an indicator
of PD activity.
As a result of all such losses, the insulation in high voltage
equipment cannot be modelled as a pure capacitance:
instead, it is modelled by the equivalent circuit shown
below with the power loss represented by a resistance R in
parallel with an ideal lossless capacitor C. The resistance R
is not a value measurable with an ohm-meter, but is a
notional value which represents the dielectric loss
magnitude and hence the quality of the insulation. It is the
electrical analogy of the core loss resistance in a
transformer equivalent circuit.
For the parallel circuit arrangement shown above, the
phasor diagram is:
The power factor is cos( 90 – ) and is thus representative
of the power loss and thus the insulation quality. However,
a better representation of the loss and hence the quality is
the angle  between the total current I and the purely
capacitive current Ic. Typically, the angle  is of the order
of milli-radians for good insulation and tens to hundreds of
m.rads for poor and very poor insulation. We can write the
various relations as follows:
Power factor = cos( 90 – )
 tan()
 
 = DDF for small 
While a low DDF value is necessary for high voltage
insulation, it is not a major requirement at low (less than
600 V) voltage: PVC has a poor DDF value (about 0.1) but
[At high frequencies, a series RC equivalent is a better simulation.]
ELEC4611: H.V. Test Techniques and Insulation Assessment
p.5
ELEC4611: H.V. Test Techniques and Insulation Assessment
p.6
is widely used at 230/400V; but it is not used for HV
insulation applications.
tan is thus designated the dielectric dissipation factor
(DDF) of the insulation. Its value is a very useful indicator
of insulation efficacy, particularly when measured as a
function of voltage from below rated voltage to above rated
voltage (say 0.6–1.5 p.u.).
Note that:
and
Ic =
and thus:
tan  
IR IR

IC
I
V
Xc
= CV
1.1 DDF Measurement
IR = CVtan
and we have Power loss =
America, the method used is to measure the DDF at some
standard voltage (say 10 kV) irrespective of rated voltage
and then use this value for determining trending of DDF
over time. The advantage of this method is that it limits the
size of test transformer required. However, the voltage
variation of DDF particularly near rated voltage counts
against this method and it is usual in most other countries to
measure DDF at rated voltage. DDF is also very
temperature dependent, and this must be taken into account
when looking at trends over time. Increases of 5 or 6 times
over temperature variations of 50–60 oC are possible.
VIR = CV tan
2
The DDF (tan) (and the capacitance value C) are normally
measured by a high voltage AC (4-terminal) bridge
technique. The measurement should be done with the
insulation at rated voltage to determine its efficiency at
rated operational voltage, because DDF is voltagedependent. Also, because the variation of DDF with voltage
is an important consideration, the test should be done over a
range of voltage above and below the normal operating
voltage. Usually insulation will exhibit a “turn-up” in the
DDF vs. V curve just above operating voltage. If the
insulation is degraded the “turn-up” may occur at a lower
voltage level and this will be an indication of potential
insulation problems. In some cases, particularly in North
ELEC4611: H.V. Test Techniques and Insulation Assessment
p.7
The most commonly used bridge arrangements for DDF
measurements are the Schering Bridge and the
These tests are
Transformer Ratio Arm Bridge.
performed with the test object held at about full rated
voltage (or higher), but the variable balance impedance
components (which may have to be manually adjusted for
balance) are effectively at earth potential. [Modern bridges
are automatically balancing and do not require manual
manipulation of the dials]. Both of the above types are 4terminal bridges and thus require balance of both amplitude
and phase.
The Schering Bridge is described and analysed in detail
here. The typical circuit used with Schering bridge DDF
measurement at power frequency is:
ELEC4611: H.V. Test Techniques and Insulation Assessment
p.8
Earth
At balance:
Z1
Z2
C1
R1

Z3
Rx

H.V.
Z 2  R2
Test object
1
 jC
Z3
Z4
Note that:
Z 4  Rx 
(i) C1 and R1 are the variable impedances (they are high
precision components: R2 may sometimes also be
variable).
(ii) R2 is a standard non-inductive resistor, also of high
precision.
(iii) C is a standard high voltage precision gas capacitor
with negligible losses (typically 100 pF: the gas
dielectric makes it effectively lossless).
(iv) Rx – Cx is the test object (represented by a series
equivalent combination).
The bridge is energised at the rated voltage of the
equipment item, but there are only a few hundred volts at
most on R1, C1 and R2, which are operated to achieve
balance.
ELEC4611: H.V. Test Techniques and Insulation Assessment
Z2 Z4

Z1 Z 3
1
1
  jC1
Z1 R1
Cx
C
or
From the circuit diagram, we can write the following ( is
the angular frequency):
R2
V
Z 2 Z1

Z 4 Z3
p.9
1
jC x
and thus, for balance of the bridge, we have the following
relationship:
or:

 1
  1
 Rx  jC
R2   jC1   
 R1
  jCx

R
C
jC1 R2  2 
 j Rx C
R1 Cx
Equating real and imaginary parts, we get:
(a)
R2 C

R1 C x
(b)
 R2C1   RxC
R1
C
R2
C
i.e. Rx  1 R2
C
i.e. Cx 
ELEC4611: H.V. Test Techniques and Insulation Assessment
p.10
tan  is the required result of the DDF test measurement,
and is determined as follows (see the phasor diagram):
tan  
Rx
R C RC
  Rx Cx    2 1  1
Xx
C
R2
i.e. tan    R1C1 where  is the power frequency value,
100. Typical values of tan  are milli-radians or less at 50
Hz for good insulation.
Note that tan   at these small values hence the units are
usually milli-radians. In older documents the loss quantity
used is often the power factor which is cos( 90 – ) and is,
strictly speaking, different to tan. However, at the values
of  typical of good insulation, the two are essentially
identical. Only for very high loss angles will they differ.
When power factor is quoted, the units are either a number
or a percentage.

0.01
0.1
0.2
0.5
tan 
0.01000
0.10033
0.20271
0.5463
cos  90   
0.009999
0.09983
0.19867
0.4794
%
0.01
0.5
2.0
14.0
1.2 Relevance of DDF
(a) They can give a measure of impurities (particularly
moisture) in the insulation.
(b) DDF provides some information about partial
discharge activity in insulation.
(c) CV2tan gives the dielectric heat loss and this will
determine, to a considerable degree, the operating
temperature of insulation at high voltage.
(d) Plots of tan vs. voltage below and above rated
voltage will give useful information on the insulation
condition.
As the Schering Bridge uses a series equivalent circuit for
Rx – Cx, the value of Cx may vary from that for a parallel
circuit, but this only occurs for high levels of dissipation
factors (DDF).
One of the disadvantages of using DDF testing (or any
testing that requires application of rated voltage) comes
when the test object has a high capacitance, such as an HV
cable. Then the off-line source has to have large reactive
power capacity to energise the item to rated voltage. Two
methods are used to get around this problem: (a) Use of
very low frequency (about 0.1 Hz) AC excitation (b) Use of
an external inductance to give resonant operation with the
cable capacitance at or near power frequency.
tan or DDF (and the capacitance) value C are
important characteristics of high voltage insulation
because of the following features:
Because the reactive power requirement of a capacitance C
is CV2, a simple method of reduction of the required
transformer rating is to decrease the frequency. Thus if 0.1
Hz is used instead of 50 Hz the reactive power requirement
is decreased by a factor of 500 and only a very small
ELEC4611: H.V. Test Techniques and Insulation Assessment
ELEC4611: H.V. Test Techniques and Insulation Assessment
p.11
p.12
transformer is needed for the test. This, of course, relies on
the 0.1 Hz setting up an AC electric field distribution in the
insulation rather than a DC field and while there has been
some reservation about this, it is generally accepted that
such low frequencies do give an AC distribution and the
tests is thus valid.
The other method uses a tuneable inductor to give a circuit
resonant frequency somewhere around the 40-60 Hz range.
When the inductor is tuned to resonance the inductor and
capacitor voltages are able to be held at rated values but
cancel each other and the supply transformer is then
required only to supply the power loss in the inductor
winding resistance, and this is able to be made very low and
the transformer very small. High temperature
superconducting windings have been suggested.
The following figure shows the circuit diagram of a seriesresonant system. Note that the internal leakage inductance
of the transformer (not shown) also contributes to the
resonance process. Thus, resonance operation is determined
by the combination of the total inductance Le (tuning
inductor and transformer inductance) and the load
capacitance C. At resonance,   1 LeC and the load
voltage V2 which appears on the capacitance load is given
by:
V2 
  j  C V
1



Re  j  Le   1
C



ELEC4611: H.V. Test Techniques and Insulation Assessment

V1
 ReC
p.13
The phasor diagram of the system is also shown and, at
resonance, it is possible that V2  V1 . The quality factor
Q  1 CR   Le R gives the voltage multiplication factor.
Thus, depending on the values of Le and C, the load voltage
can be made many times the applied voltage magnitude V1.
There will be some small series resistance Re that will cause
the only power loss needed to be supplied by the source.
2. Dissolved Gas Analysis (DGA)
The majority of large power transformers, many older
cables and many switchgear components are insulated by
oil and by oil-impregnated paper. Whenever any faults
occur that may cause deterioration of the oil-based
insulation, the generation of gases by discharges in the oil is
almost always a concurrent event. These gases are then
dissolved in the oil and an analysis of the gas constituents
and their quantities (and particularly their relative
ELEC4611: H.V. Test Techniques and Insulation Assessment
p.14
quantities) can provide very useful information about the
fault. In many cases it is possible to identify the type of
fault from the relative quantities (ratios) of the gas
components.
The gases which are generated in such oil-based insulation
include:
Carbon monoxide CO
Carbon dioxide
CO2
Hydrogen
H2
Methane
CH4
Ethane
C 2H 6
Ethylene
C2H4
Acetylene
C2H 2
The above is not the full list of gases produced as many
higher hydrocarbons are also generated. However the above
are the ones that are best able to be used to determine any
deterioration effects of the oil and to identify any particular
fault types that may be causing gas generation. Also, O2 and
N2 are also often monitored to give an indication of possible
oxidation. The analysis is done by standard gas
chromatograph (GC) methods on samples taken from the
oil in the equipment item. Most utilities have GC
equipment routinely available.
In general, CO and CO2 are generated by hot spots such as
may arise from hot metal surfaces such as the core or a
magnetic shield or a winding hot spot in the transformer.
Hydrogen, methane and, to a lesser extent, ethane may also
be generated by such effects. Ethylene and acetylene are
ELEC4611: H.V. Test Techniques and Insulation Assessment
p.15
not generated by hot spots. Partial discharges will generate
hydrogen, methane, ethane and ethylene. High power
arcing will generate hydrogen, ethylene and acetylene.
Acetylene is only generated by power arcing and is thus a
general indicator for arcing when present. However arcing
will occur in on load tap changers and so the presence of
acetylene must be looked at carefully to determine whether
it arises from OLTC operation.
2.1 DGA Analysis Methods
The key gas method of
association as follows:
Hydrogen:
Ethylene:
Acetylene:
CO and CO2:
analysis identifies fault types by
Partial discharge
Overheating in oil
Arcing in oil
Solid insulation deterioration
(usually paper)
In addition there are the various ratio methods which take
the ratios of the quantities of specific pairs of gases and
then use these ratios to predict the type of fault. The ratio
methods currently in general use include:
 The Rogers ratio method.
 The IEC (International Electrotechnical Committee)
ratio method.
 The Dornenberg ratio method.
 The Duval triangle method (this analyses the data
using groups of three gases).
ELEC4611: H.V. Test Techniques and Insulation Assessment
p.16
In addition to the above there are a number of other
methods (e.g. IEEE and Laborelec) and also many in-house
methods of analysis used by various utilities, based on their
accumulated experience. In recent years fuzzy logic
analysis has become popular for the assessment of
dissolved gas data.
DGA is a valuable diagnostic technique and is probably one
of the most widely practiced techniques in current use. It is
simple to do, does not require disconnection of equipment
from supply and most utilities have the chemical analysis
tools available. The problems with the technique are that
there may be a delay in getting results and as it is often used
as a routine maintenance technique, the data is generally
not analysed as thoroughly as is warranted. The other
problem is that DGA gives only a measure of the integrated
effects of the fault. It does not give any information as to
whether the fault has been in existence for a short or long
time and thus it is not easy to identify the magnitude of the
fault. It is also not able to give continuous on-line
monitoring. There are some continuous on-line DGA
monitors (HYDRAN monitors) available but they are
restricted in the range of gases that can be identified and not
so wide-ranging in identifying fault types as the full DGA
laboratory tests.
In addition to normal DGA, which essentially identifies
mainly problems with oil, there are more sophisticated test
techniques of a similar type which are used to monitor more
accurately the degradation of the paper insulation. These
tests look at gases generated by the chemical decomposition
ELEC4611: H.V. Test Techniques and Insulation Assessment
p.17
of the cellulosic structure of the paper as it ages. When the
paper is degenerated by age or by some other deteriorative
effect the chains of glucose molecules that make up the
cellulose polymer structure break down into smaller chain
lengths and the bonds between the molecules, when broken,
give moisture and also gas generation. The decrease in the
chain lengths (de-polymerization) also reduces the tensile
strength of the cellulose material and this then means loss
of any useful insulation function. The reduction in
insulation efficacy can be measured either by the gases
given off (not the moisture, as this masked by other effects)
or by measurement of the degree of polymerization (DP) of
the cellulose.
Good (new) paper insulation has a DP of about 1300-1400
(the DP is essentially the number of molecules in a typical
polymer chain) while paper with no insulation value has a
DP of about 200-300. The gases generated in the
decomposition of paper are much more complicated in
structure than those generated by oil decomposition. Thus
these gases are more difficult to extract and analyse than
the relatively simple hydrocarbons analysed by DGA
techniques. The gases produced are the so-called furans or
furfuraldehyde group of gases. Their measurement requires
the use of HPLC or high performance liquid
chromatography, a more sophisticated technique than the
GC and one which is not generally available in-house to
many utilities.
ELEC4611: H.V. Test Techniques and Insulation Assessment
p.18
[It should be noted that the "paper" referred to above is an
inclusive term which also includes pressboard and wood
etc. which are cellulosic in their chemical structure].
(iii) a so-called absorption current component (Ia). It is Ia
which is dielectric- dependent and thus useful for
insulation condition analysis.
The various transient current components are shown below:
3. Insulation Resistance (IR)
Insulation resistance is a very simple test to apply and can
be done very quickly and easily, but must be done off-line.
The IR must be measured using DC voltage (usually
between 500 V DC to 10 kV DC, depending on the rated
voltage of the equipment insulation being tested). However
although it is a simple test, the insulation resistance value in
itself is not a particularly useful parameter except when the
insulation is extremely poor and near to failure. However
the time variation of the insulation resistance during the
measurement is a much more useful property because the
IR-t characteristic variation depends on the dielectric
polarisation properties of the insulation and thus on its
dielectric (insulation) condition. A number of useful
parameters for insulation assessment can thus be derived
from the IR vs. time variation.
The transient current that flows in a dielectric material
when it is subjected to a DC voltage step is made up of a
number of current components, all of which are time
varying. The total current (I) is composed of three
components:
The Ohmic leakage current (Ie) is that (constant)
component which is left after a long period (one minute or
more) of DC voltage application. It is determined by
moisture content, and by any contamination, as well as by
the intrinsic material insulation resistivity and geometry.
Because of the effect of the contaminants in or on the
insulation, leakage current is important for insulation
condition monitoring.
(i) a conduction (Ohmic leakage) current (Ie),
(ii) a displacement (true capacitive) current part (Ic), and
The absorption current (Ia) is affected by the nature and
condition of the insulation dielectric material and is thus an
important component for insulation condition monitoring. It
ELEC4611: H.V. Test Techniques and Insulation Assessment
ELEC4611: H.V. Test Techniques and Insulation Assessment
p.19
p.20
will be representative of the intrinsic condition and
degradation of the dielectric.
The capacitive current (Ic) is determined only by the
insulation capacitance. It is not so important for condition
monitoring, although moisture can affect it.
From the above, it can be seen that the leakage current and
the absorption current are the two quantities which are most
indicative of insulation quality and their comparative values
and trends over time can give useful information about the
condition of the dielectric.
The insulation resistance (IR)-derived parameters which are
used for assessment are:
(a) Short-time Test
Here, the insulation resistance is measured just once,
after 60 seconds of DC voltage application. This is the
simplest but least useful form of IR test. It is however
extensively used on a routine basis.
(b) Resistance-time variation
The time variation of insulation is recorded for a period
up to 10 minutes. For good insulation, resistance
increases with time. This is not a specific test in itself,
but the data obtained is used to derive the following
quantities:
(c) Polarisation Index (PI)
ELEC4611: H.V. Test Techniques and Insulation Assessment
p.21
This quantity is the ratio of insulation resistance
measured at 10 minutes to the resistance measured at 1
minute. A value of 1-2 is characteristic of poor
insulation; a value greater than 3 is representative of
good insulation.
(d) Step Voltage Test
This test uses different DC voltage levels applied in
steps: e.g. 500 V followed by 2500 V over 60 seconds.
This test is used extensively on low voltage motor
windings.
(e) Dielectric Absorption Ratio
This is a variation of the Polarisation Index parameter.
Usually the absorption ratio is obtained from the IR
value at 60 seconds divided by that at 30 seconds. A
value less than 1.3 is characteristic of poor insulation,
and a value greater than 1.5 is characteristic of good
insulation.
Insulation resistance is measured simply by using a
“MEGGER” type tester. The tester is normally batteryoperated and should be capable of application of any
voltage in the range up to 10,000 volts DC, with an
accuracy of about 1-2%. In the basic form, the tester
provides a reading of insulation resistance only and the
values must be recorded over time and then plotted to
determine the required IR-derived parameters. Modern
instruments however are microprocessor-controlled to
apply the chosen voltage (including the step variations if
ELEC4611: H.V. Test Techniques and Insulation Assessment
p.22
necessary), record the data over time and then calculate the
parameters with the test result then printed out.
In some tests it may be necessary to differentiate between
surface and bulk (or volume) insulation resistance in the
measurement and this will require the use of guard
electrodes to direct the electric field application to
appropriate configurations.
Typical insulation resistances of high voltage equipment are
in the hundreds of Mega-ohm to some Giga-ohm levels and
at these levels the accuracy of measured resistances are
only about 10-15% when performed in the field. Care must
be taken to ensure that no surface leakage current paths are
involved in the measurement as these will obscure the test
result. The instrument accuracy should be checked
regularly against standard high value resistors. The
advantage of using parameters which involve calculation of
a ratio of resistances is that any measurement errors are
minimised (assuming the errors are systematic).
DC test voltage requirements for IR testing for different
ratings of equipment.
AC Voltage rating
230 V
400 V
2,200–5,000 V
5000–15000 V
more than 15000 V
DC Test Voltage level
500 V
500–1000 V
2500–5000 V
5000–15000 V
10000–15000 V
ELEC4611: H.V. Test Techniques and Insulation Assessment
Typical IR values (at the lower end) for different voltage
ratings: also showing the effects of temperature on IR.
Voltage
(kV)
6.6
11
33
66
IR (@200 C)
(M)
400
800
1000
1200
IR (@600 C)
(M)
25
50
65
75
4. Overvoltage Tests
Overvoltage tests are essentially pass or fail tests and are
usually performed at the final stage of manufacture in the
factory for power frequency and impulse tests and again
after installation in the case of the power frequency
overvoltage test. These tests may also be performed on
suspect equipment or on refurbished equipment. The power
frequency test can be done on-site, but the impulse and
switching tests are normally done in the test laboratory or
factory only.
Overvoltage tests can cause damage to the insulation if the
tests and test voltage levels are not properly controlled. The
table below shows the range of standard test voltages for
power frequency and impulse voltage tests for transformers.
Note that the power frequency overvoltage test requirement
is about twice system voltage for one minute: the criterion
of success is simply that no insulation breakdown failure
p.23
ELEC4611: H.V. Test Techniques and Insulation Assessment
p.24
occurs. Normally only one such test is performed. Voltage
is monitored during the test to aid in identifying any
insulation breakdown, as the breakdown current is limited
to prevent significant damage.
The impulse test level required for specific equipment
varies greatly with rated voltage of the equipment. A
number of impulses tests (perhaps about 5 may be required)
and different polarities may also be required. The impulse
voltage waveform is recorded and will indicate if any
failure occurs during the test. The equipment buyer may
also specify other insulation tests that must be done before
the item of equipment is accepted from the manufacturer.
For example an induced overvoltage test at a higher
frequency may be performed on transformers. Similarly a
chopped wave test may be needed in some cases.
4.1 Power Frequency Overvoltage Test
of reactive power requirements. For HV power cable
overvoltage tests, HV resonant test sets using tunable
inductors to achieve resonant conditions with the cable
capacitance are used. At resonance, the HV transformer
only needs to supply the pure Ohmic losses, which are
generally quite small as they arise only from leakage
currents in the cable. Paper oil insulated power cables were
usually tested with HVDC which required much lower
rating of supply equipment. However XLPE cables cannot
be tested with HVDC because of potential problems with
remnant charge build up after test with DC.
It can be seen from the table that a typical power frequency
test voltage is about 2 per unit or 2U0. There are some
reservations about imposing such a high test voltage on new
equipment but the accepted standard tests require these
levels.
Typical test voltage levels for overvoltage tests on power
transformers (AS/NZS 60076.3:2008)
These are relatively simple to achieve if the necessary
power supplies are available. For on-site tests this will
require use of a mobile HV transformer, which may not be
easily available for the full transmission voltage range of a
transmission utility's operation. Generally only items with
low capacitance can be tested on-site because of the
problems of supplying large quantities of reactive power
with low capacity mobile test transformers. Thus, only
items such as transformers, instrument transformers, circuit
breakers and similar equipment items can be tested at full
voltage on site. Cables cannot be tested in this way because
ELEC4611: H.V. Test Techniques and Insulation Assessment
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ELEC4611: H.V. Test Techniques and Insulation Assessment
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4.2 Impulse testing
There are two types of transient overvoltages that must be
tested for in high voltage equipment. These are:
 lightning overvoltages
 switching overvoltages
Power system equipment items must be tested for their
ability to withstand the effects of such transient voltages,
the magnitudes of which are typical to their voltage rating,
without suffering dielectric breakdown.
4.2.1 Lightning impulse testing
Lightning impulses can be up to 1000 kV or more in
amplitude and the associated current may be up to 100 kA
in each stroke, although 10-20 kA is typical. If the actual
strike is to an overhead line, a travelling wave results which
then moves along the transmission line (unless it causes
breakdown of the air insulation between lines). This HV
propagating surge may test the electrical insulation of any
equipment connected to the line and thus the equipment
must be tested for its withstand ability to such transients.
Even if the strike is not to a line, but occurs close to it,
induced overvoltages may be coupled into the line
inductively or capacitively.
The test waveform used for equipment to test against
lightning impulse voltage is the generally agreed shape of
1.2/50 s. The voltage rises to a peak in 1.2 s and decays
ELEC4611: H.V. Test Techniques and Insulation Assessment
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ELEC4611: H.V. Test Techniques and Insulation Assessment
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to half peak in 50s. There are some tolerances allowed for
the rise time and decay time when testing (typically about
+/- 20%). In some cases chopped wave tests are performed
where the voltage is driven to zero very quickly. This test
can be used to provide tests of, for example, inter-turn
insulation where the capacitive voltage coupling is
enhanced by the fast chopping. If failure occurs under
impulse testing, the disruptive discharge will produce
something akin to a chopped wave shape.
The figures below shows typical lightning impulse voltage
waveshapes for (a) full wave, (b) chopped wave on
decaying side and (c) chopped wave on the rise. Typical
standard test voltage amplitudes are listed in the previous
tables.
(ii) Chopped wave (tail)
Lightning impulse test waveforms
(i) Standard 1.2/50
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(iii) Chopped wave (front)
p.29
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4.2.2 Switching impulse testing
Switching impulses occur as a result of operation of circuit
breakers, switches etc. in the power system. Their shape is
very dependent on the system parameters and there are thus
very substantial variations in magnitudes and in shape of
switching impulses. However their risetime is generally
much slower than that of lightning impulses and their
duration is also generally much longer.
The amplitude of typical switching impulses is about 2-3
p.u. and is thus a little lower than lightning impulse
amplitudes. However the longer duration may stress the
insulation equally as much as lightning effects or even more
in some cases.
A standard switching impulse waveform has a risetime of
about 250 s and a decay to half peak of about 2500 s.
This is designated as the 250/2500 s switching waveform.
There are tolerances of about 20% on the rise and fall time
values in testing. Because of the variability in shape for
different systems, there are other standard waveshapes
which can be used if required by the test situation: these
include the 100/2500 s and the 500/2500 s. The
amplitudes of switching impulses are generally a little
lower than lightning impulses, with the amplitude
depending on the application.
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The figure above shows a typical waveshape of a switching
transient test voltage.
[Note that switching transients are not applied as chopped
waves.]
4.2.3 Impulse voltages for LV systems and
Communications systems
With the increasing susceptibility of modern electronics to
impulse voltages and with the increasing use of power
electronics and such hard switching elements as IGBTs,
there is now a need to impulse test low voltage equipment.
There is also particular need to test information technology
equipment for impulse voltages and in the case of IT
equipment there are very many different impulse voltage
waveshapes that are used for these tests.
ELEC4611: H.V. Test Techniques and Insulation Assessment
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interconnecting NSW and Queensland and one in a cable
interconnecting parts of NSW and SA.
4.3 High Voltage DC testing
The use of high voltage DC testing for AC equipment with
DC voltage test levels about the same as the peak AC
voltage has been used for many years in testing, particularly
when testing equipment with high reactive power
requirements such as cables. On-site testing of power cables
using HVDC was practised for many years because of the
benefits of small transformer/supply power requirements.
The use of HVDC to test HVAC equipment has some
advantages and some disadvantages. One of the main
disadvantages occurs with modern polymeric insulation.
Great care must be exercised when using high voltage DC
for the testing of such insulation. The very high insulation
resistance means that the effective time constant of
discharge of any accumulated charge and any recovered
charge is very high for such material. Thus, when normal
voltage is re-applied, the charge retention may cause
overvoltages and damage the insulation. Modern XLPE
cable insulation is prone to this problem and HVDC testing
is not performed on such cables. Paper insulated cables,
however, are not affected in this way and HVDC has been
used for many years to test such cables.
Polymeric insulation has always been considered too much
of a risk to use for HVDC cables and although paperinsulated AC cables have been almost totally supplanted by
XLPE, XLPE has not been used for DC applications for the
above reasons. Recently however, a polymer has been
produced that can be used for HVDC cables and has been
used in two applications in Australia: one in a DC cable
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Advantages of HVDC Testing
 Can be used on equipment with high capacitance such
as cables
 Less damaging to insulation than HVAC
 The duration of voltage application is thus not as
critical as with HVAC
 Insulation resistance tests can be performed
concurrently
 The size of the test equipment supply is much reduced
from HVAC
Disadvantages of HVDC testing
 Electric field distribution in insulation is different to
that with AC stress
 Residual charge is a problem: there may be space
charge problems
 Needs a longer time to perform tests than for AC HV
tests
 Some insulation defects are not detectable with HVDC
 Electric field distribution is temperature (load)
dependent via resistivity
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5. Partial Discharge Tests
Partial discharges in insulation can cause substantial
degradation of the insulation because of the very high
energies and associated heat of ionized particles which are
produced in the partial discharge ionization process. These
ions and electrons can then change the chemical structure
and composition of the insulation, thereby degrading it over
time. The primary result of the chemical change is
carbonisation of the insulation material. The damage due to
PDs is primarily with organic insulation: mica-based
insulation such as is found in large HV motor and generator
winding insulation is able to withstand very high levels of
partial discharge activity and heat without any significant
deleterious effect on insulation properties.
Because of this direct damage to insulation and because
PDs can be monitored directly, PD testing and monitoring
is perhaps the best insulation assessment technique
available. Modern developments in computer-based data
acquisition systems have allowed the development of PD
testing to a stage where it can give a very sensitive measure
of the insulation integrity. To this end there are very
substantial programs of PD research which are aimed at:
 Developing continuous on-line PD monitors
 Development of signal processing techniques to
remove interference
 Using PD data to determine fault type and location
 Using PD data to estimate the remnant life of
insulation
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These are very ambitious programs and may not be fully
realisable, but PD monitoring is the area where most
development is occurring at present.
PD monitoring covers the widest spectrum of voltages and
equipment. It is used for transformers, cables, switchgear,
bushings, insulators, busbars systems, SF6 gas insulated
systems, instrument transformers, motors and generators.
The PDs can be monitored electrically by resistive,
inductive or capacitive sensors or by UHF aerial-coupling
units at frequencies up to some GHz. PDs can also be
monitored by using piezoelectric detectors sensitive to the
ultrasonic acoustic pressure waves generated by the PDs.
The major problems with the electrical detection of PDs are
interference from corona discharge for example, and the
difficulties in some cases of gaining access to the
appropriate location for the sensor in or on the equipment
item.
A discharging sample may be simply represented by the
equivalent circuit in the Figure below. This is known as the
abc model. The applied alternating voltage V is increased
until the voltage across the cavity (Cv) produces a
discharge, with a subsequent pulse across the sample of
Vx .

CC 
(1)
qv   Cv  c a   Vv
C

C
c
a 

Cc
where Vv 
 Vx
Cc  Cv
Vx = instantaneous applied voltage
and
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The sudden change Vv will produce a voltage pulse at the
sample terminals:
Cc
(2)
Vx 
 Vv
Cc  Ca
and the apparent change in charge across the sample on
breakdown of Cv is:

CC 
qx  Vx  Ca  c v   Vx .Ca  Vx .Cx (3)
Cc  Cv 

qv Cv  Cc
partial discharge dissipated in void


Cc
apparent partial discharge at terminal
qx
Cc
C
or: qx 
if Cv  Cc
qv  c qv
Cv  Cc
Cv
i.e. the apparent (measured) PD is much smaller than
the actual PD discharged in an elementary cavity for the
simple case considered.
[Note: Ca also includes any external capacitance that is in
parallel with test object, Cx].
If Ca  Cc (this is usually the case but not always, e.g. for a
point-plane system) then, before breakdown of the void,
equation (1) becomes:
qv   Cv  Cc  .Vv
(4)
and if Cv  Cc [Cv assumed to be much thinner and same
cross section as Cc] equation (3) becomes:
qx   Ca  Cc  .Vx
Cc
.Vv
  Ca  Cc 
Cc  Ca
 Cc .Vv
(5)
[from eq.2]
(6)
as measurable at terminals. Note that Cc and Vv are rarely
known individually. Now from (4) and (6):
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Figure: (a) Representation of a void in a solid, (b) the
abc model, and (c) possible voltage waveforms.
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The partial breakdown of the insulation causes a charge
reduction and thus creates a momentary voltage collapse
between the two main terminals of the test object. This gives
rise to an electrical current pulse of very short duration and
small amplitude. It can be detected in the external circuit
connected to the test object. Note that the small voltage
collapse is superimposed on the large 50 Hz supply voltage.
Basically, the function of a discharge detector is to decouple
the high-frequency PD signal from the supply and to amplify
this signal. A conventional circuit to detect the PD current
pulse is shown in the Figure below.
Figure: Basic circuit for PD measurements.
Cx represents the capacitance of the test specimen. Ck is the
coupling (blocking) capacitor which, of course, must be
discharge-free and appropriately rated for use at highvoltage. The measuring impedance Z is connected in series
with Ck. Following a PD, a redistribution of the remaining
charge between Cx and Ck will give rise to a corresponding
current pulse flowing through Ck and the measuring
impedance Z. The signal picked up by Z is fed through a
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long cable to an amplifier which is at some metres away
from the HV area. In some systems, the amplifier is
incorporated into Z. Measurement sensitivity is dependent on
the relative values between Cx and Ck as this will determine
the proportion of restoring charge flowing through Z.
Preferably, Ck should be as large as possible to maximize the
external charge displacement.
6. Dielectric spectroscopy
Solid and liquid dielectric insulants have a response to an
applied DC electric field that is determined by their
intrinsic dielectric properties, including their dipole
structure, electronic and ionic polarisation, interfacial
polarization and many other facets, including chemical
composition. The actual dielectric response of the dielectric
material is thus a sensitive determinant of the insulation
state and hence of its condition. Any impurities or change
in structure and chemistry for example should theoretically
be able to be determined by determining the dielectric
response to an applied voltage of a HV insulation material.
We have already discussed one version of this general
method in that the capacitance and DDF of insulation
measured with a Schering bridge is one part of the
dielectric response. However there are other facets of the
more general dielectric response that can be used to focus
on particular aspects of aged materials. This includes in
particular moisture in insulation and its effect on insulation
level and ageing.
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When an electric field E is applied to a polar dielectric,
polarisation of the material increases the electric
displacement flux D by an amount P, the polarization flux
and thus raises the relative permittivity to a value greater
than 1:
D  P   o E   r o E
We are particularly concerned with the time variation of the
response as this will allow determination of dielectric
response in both time and frequency domains:
frequency range that is of interest to us in insulation
assessment for use at power frequencies. The
transformation requires the dielectric to be linear however
and in some cases, such as XLPE with high water tree
components, the dielectric is not linear.
It should be noted that the dielectric dissipation factor
(DDF) and capacitance measurements at power frequency,
or whatever the test frequency used is, gives just one point
on the dielectric response frequency-domain curve.
D t   P t    o E t 
It is thus the polarization P(t) that is of interest. It is of
interest both as the response to an applied electric field (the
polarisation response) and its response when that electric
field is removed (de-polarisation or relaxation response).
Both of these can give useful information about the
insulation condition.
The response as above can be obtained in the time domain
by application of a DC voltage step for a defined period of
time, followed by a short-circuiting for a defined period and
then followed by an open circuit relaxation period.
Typically the applied DC voltage level is about 10% of the
rated AC voltage. This then does not cause any damage to
the dielectric and in the case of very good dielectrics does
not cause any problems with residual charge on the material
after the test.
It is possible to obtain the response in the frequency domain
by performing a Fourier Transformation of the time domain
response to a step. This then gives the dielectric response
over a large frequency range. However it is only the lower
7. Other Diagnostic Tests
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The present situation in the electrical industry is aimed at
extending the life of large items of capital equipment and
thus asset management and condition monitoring are now
very important aspects of operation. There is a very
substantial program of development of new monitoring
techniques. These include, for example,








Frequency response analysis (FRA) of windings.
Very low frequency (VLF) testing of cables.
On-line DDF monitoring systems.
On-line PD monitoring of cables.
Fibre-optic acoustic sensors for PDs.
Degree of polymerisation testing of composite insulators.
On-line leakage current monitors for insulators.
Oscillating voltage tests of cable joints.
p.42
 UHF (up to a few GHz) PD monitoring systems for
transformers.
 Infrared imaging techniques.
 Corona discharge visualisation techniques.
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