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High Voltage Testing Procedure
Electrical equipment must be capable of withstanding overvoltages during operation. Thus by suitable testing
procedure we must ensure that this is done.
High voltage testing can be broadly classified into testing of insulating materials (samples of dielectrics) and
tests on completed equipment.
The tests carried out on samples of dielectric consist generally of the measurement of permittivity, dielectric loss
per unit volume, and the dielectric strength of the material. The first two can be measured using the High
Voltage Schering Bridge.
The tests carried out on completed equipment are the measurement of capacitance, the power factor or the total
dielectric loss, the ultimate breakdown voltage and the flash-over voltage. The breakdown voltage tests on
completed equipment is only done on a few samples since it permanently damages and destroys the equipment
from further use. However since all equipment have to stand up to a certain voltage without damage under
operating conditions, all equipment are subjected to withstand tests on which the voltage applied is about twice
the normal voltage, but which is less than the breakdown voltage.
9.1 General tests carried out on High voltage equipment
9.1.1 Sustained low-frequency tests
Sustained low frequency tests are done at power frequency (50 Hz), and are the commonest of all tests. These
tests are made upon specimens of insulation materials for the determination of dielectric strength and dielectric
loss, for routine testing of supply mains, and for work tests on high voltage transformers, porcelain insulators
and other apparatus.
Since the dielectric loss is sensitive to electric stress, the tests are carried out at the highest ultimate stress
possible. For testing of porcelain insulators and in high tension cables, voltages as high as 2000 kV may be
used.
High voltage a.c. tests at 50 Hz are carried out as Routine tests on low voltage (230 or 400 V) equipment. Each
one of these devices are subjected to a high voltage of 1 kV + 2 × (working voltage). A 230 V piece of
equipment may thus be subjected to about 1.5 to 2 kV. These tests are generally carried out after manufacture
before installation.
High Voltage Testing 149
The high voltage is applied across the device
under test by means of a transformer. The
transformer need not have a high power rating.
If a very high voltage is required, the
transformer is usually build up in stages by
cascading. By means of cascading, the size of
the transformer and the insulation bushing
necessary may be reduced in size. The
transformers are usually designed to have poor
regulation so that if the device under test is
faulty and breakdown occurs, the terminal
voltage would drop due to the high current
caused. A resistance of about 1 ohm/volt is
used in series with the transformer so as to
limit the current in the event of a breakdown to about 1 A. The resistance used could be of electrolyte type
(which would be far from constant, but would be a simple device) such as a tube filled with water.
In all high voltage tests, safety precautions are taken so as to ensure that there is no access to the testing area
when the high voltage is on. There would be switches that would automatically be operated when the door to
the area is opened etc..
9.1.2 High Voltage direct current tests
These tests are done on apparatus expected to operate under direct voltage conditions, and also where, due to the
inconvenience of the use of high capacity transformers required for extra high tension alternating voltage tests
and due to transport difficulties, alternating voltage tests cannot be performed after installation.
A special feature of importance of the d.c. test is the testing of cables which are expected to operate under a.c.
conditions. If the tests are done under a.c. conditions, a high charging current would be drawn and the
transformer used would have to have a current rating. It is thus normal to subject the cable (soon after laying it,
but before energising it ) to carry out a high voltage test under d.c. conditions. The test voltage would be about 2
(working voltage ) and the voltage is maintained from 15 min to 1.5 hrs. This d.c test is not complete equivalent
to the corresponding a.c. conditions , it is the leakage resistance which would determine the voltage distribution,
while in the a.c. conditions, it is the layers of different dielectrics that determine the voltage distribution in the
cable. Although the electric field differs in the 2 cases, it is likely that the cable will stand up to the required a.c.
voltage.
The methods used to generate these high d.c. voltages have already been described.
9.1.3 High-frequency tests
High frequency tests at frequencies varying from several kHz are important where there is a possibility of high
voltage in the lines etc., and in insulators which are expected to carry high frequency such as radio transmitting
stations. Also in the case of porcelain insulators, breakdown or flashover occurs in most cases as a result of high
frequency disturbances in the line, these being due to either switching operations or external causes. It is also
found that high frequency oscillations cause failure of insulation at a comparatively low voltage due to high
dielectric loss and heating.
High voltage tests at high frequency are made at the manufacturing works so as to obtain a design of insulator
which will satisfactorily withstand all conditions of service.
In the case of power line suspension insulators, it is possible that breakdown or flash over would occur due to
high frequency over voltages produced by faults or switching operations in the line. Sudden interruptions in the
line would give rise to resonant effects in the line which would give rise to voltage waves in the line of high
frequency. These might cause flashover of the insulators.
The behaviour of insulating materials at high frequencies are quite different to that at ordinary power frequency.
The dielectric loss per cycle is very nearly constant so that at high frequencies the dielectric loss is much higher
and the higher loss causes heating effects.The movements of charge carriers would be different.
At high frequency the polarity of electrodes might have
changed before the charge carriers have travelled from
one electrode to the other, so that they may go about
half-way and turn back (figure 9.2).
There are two kind of high frequency tests carried out.
These are
(a) Tests with apparatus which produces undamped high-frequency oscillations.
Undamped oscillations do not occur in power systems, but are useful for insulation testing purposes especially
for insulation to be in radio work.
(b) Tests with apparatus producing damped high-frequency oscillations.
When faults to earth or sudden switching of transmission lines occur, high frequency transients occur whose
frequency depends on the capacitance and inductance of the line and will be about 50 kHZ to about 200 kHZ.
These are damped out with time.
9.1.4 Surge or impulse tests
These tests are carried out in order to investigate the influence of surges in transmission lines, breakdown of
insulators and of the end turns of transformer connections to line. In impulse testing, to represent surges
generated due to lightning, the IEC Standard impulse wave of 1.2/50 s wave is generally used. By the use of
spark gaps, conditions occurring on the flash over to line are simulated. The total duration of a single lightning
strike os about 100 s, although the total duration of the lightning stroke may be a few seconds.
Overvoltages of much higher duration also arise due to line faults, switching operations etc, for which impulse
waves such as 100/5000 s duration may be used.
In surge tests it is required to apply to the circuit or apparatus under test, a high direct voltage whose value rises
from zero to maximum in a very short time and dies away again comparatively slowly . Methods of generating
such voltages have already been discussed earlier.
While impulse and high frequency tests are carried out by manufacturers , in order to ensure that their finished
products will give satisfactory performance in service , the most general tests upon insulating materials are
carried out at power frequencies .
Flash-over Tests
Porcelain insulators are designed so that spark over occurs at a lower voltage than puncture, thus safeguarding
the insulator, in service against destruction in the case of line disturbances. Flash-over tests are very importance
in this case .
Figure 9.2 Movement of charge carriers
High Voltage Testing 151
The flash-over is due to a breakdown of air at the insulator surface, and is independent of the material of the
insulator. As the flash-over under wet conditions and dry conditions differ , tests such as the one minute dry
flash-over test and the one minute wet flash-over test are performance.
(i) 50 percent dry impulse flash-over test, using an impulse generator delivering a positive 1/50 s impulse wave.
The voltage shall be increased to the 50 percent impulse flash-over voltage (the voltage at which approximately
half of the impulses applied cause flash-over of the insulator)
(ii) Dry flash-over and dry one-minute test
In this the test voltage ( given in the B.S.S.) is applied . The voltage is raised to this value in approximately 10
seconds and shall be maintained for one minute. The voltage shall then be increased gradually until flash- over
occurs .
(iii) Wet flash-over and one minute rain test
In this the insulator is sprayed throughout the test with artificial rain drawn from source of supply at a
temperature within 10 degrees of centigrade of the ambient temperature in the neighborhood of the insulator.
The resistivity of the water is to be between 9,000 and 11,000 ohm cm.
In the case of the testing of insulating materials , it is not the voltage which produces spark-over breakdown
which is important , but rather the voltage for puncture of a given thickness ( ie. dielectric strength ) . The
measurements made on insulating materials are usually , therefore , those of dielectric strength and of dielectric
loss and power factor , the latter been intimately connected with the dielectric strength of the material.
It is found that the dielectric strength of a given material depends , apart from chemical and physical properties
of the material itself, upon many factors including,
(a) thickness of the sample tested
(b) shape of the sample
© previous electrical and thermal treatment of the sample
(d) shape , size , material and arrangement of the electrodes
(e) nature of the contact which the electrodes make with the sample
(f) waveform and frequency of the applied voltage (if alternating )
(g) rate of application of the testing voltage and the time during which it is maintained at a constant value .
(h) temperature and humidity when the test is carried out
(i) moisture content of the sample.
9.2 Testing of solid dielectric materials
9.2.1 Nature of dielectric breakdown
Dielectric losses occur in insulating materials, when an electrostatic field is applied to them. These losses result
in the formation of heat within the material. Most insulating materials are bad thermal conductors,so that, even
though the heat so produced is small, it is not rapidly carried away by the material. Now, the conductivity of
such materials increases considerably with increase of temperature, and the dielectric losses, therefore, rise and
produce more heat, the temperature thus building up from the small initial temperature rise. If the rate of
increase of heat dissipated, with rise of temperature, is greater than the rate of increase of dielectric loss with
temperature rise, a stable condition (thermal balance) will be reached. If, however, the latter rate of increase is
greater than the former, the insulation will breakdown owing to the excessive heat production, which burns the
material.
High Voltage Engineering - J R Lucas, 2001 152
Now, the dielectric losses per cubic centimetre in a given material and at a given temperature, are directly
proportional to the frequency of the electric field and to the square of the field strength. Hence the decrease in
breakdown voltage with increasing time of application and increasing temperature and also the dependence of
this voltage upon the shape, size, and material of the electrodes and upon the form the electric field.
The measurement of dielectric loss in insulating materials are very important, as they give a fair indication as to
comparative dielectric strengths of such materials. In the case of cable, dielectric loss measurements are now
generally recognized as the most reliable guide to the quality and condition of the cable.
9.2.2 Determination of dielectric strength of solid dielectrics
A sheet or disc of the material of not less than 10cm in diameter, is taken
and recessed on both sides so as to accommodate the spherical electrodes
(2.5 cm in diameter) with a wall or partition of the material between them
0.5mm thick. The electrical stress is applied to the specimen by means of
the two spheres fitting into the recesses without leaving any clearance,
especially at the centre. The applied voltage is of approximately sine
waveform at 50Hz. This voltage is commenced at about 1/3 the full value
and increased rapidly to the full testing voltage.
Sometimes insulators after manufacture are found to contain flaws in the form of voids or air spots.These spots
(due to non-homogeneity) have a lower breakdown strength than the material itself, and if present would
gradually deteriorate and cause ultimate breakdown after a number of years.
High degree ionisations caused in these spots would give rise to high energy electrons which would bombard the
rest of the material, causing physical decomposition. In plastic type of materials,there might be carbonizations,
polymerisations, chemical decomposition etc.,which would gradually diffuse into the material the by-products,
causing chemical destruction.
The useful life of a component using such material will depend on the weak spots and the applied voltage. If the
applied voltage is small, the life of the component is longer. From design considerations the voltage to be
applied if a particular life span is required can be calculated.
The schering bridge type of measurement gives an average type of measurement,where the p.f. and the power
loss indicates the value over the whole of the length.Thus small flaws if present would not cause much of a
variation in the overall p.f. Thus in the schering bridge type of measurement such flaws would not be brought
out.
The loss factor of a material does not vary much for
low voltages, but as the voltage is increased at a
certain value it starts increasing at a faster rate. This
is the long time safe working voltage, since beyond
this, the specimen would keep on deteriorating.
If the apparatus need be used only for a short period,
the applied voltage can be higher than this safe
value.
In a long length of cable, the greater part of the cable
would be in good conditions but with a few weak
spots here and there.
Due to the presence of the interwinding capacitance and the capacitances to earth of the transformer windings,
the upper elements of the transformer windings tend to be more heavily stressed than the lower portions.
Due to the velocity of propagation of the impulse voltage would not be evenly distributed in the winding. Due to
sharp rise of the voltage of the surge. there is a large difference of voltage caused in the winding as the wave
front travels up the winding. Thus there would be an overvoltage across adjacent windings.
Depending on the termination, there will be reflections at the far end of the winding. If the termination is a short
circuit, at the lowest point the voltage wave whose amplitude is same as the original wave but of opposite
polarity is reflected. For a line which is open circuited, the reflected wave would be of the same magnitude and
of the same sign.
Arising out of the reflections at the far end , there would be some coils heavily stressed. The position of the
heavily stressed coils depending on the velocity of propagation.
If flashover occurs at the gap (lightning arrestor) the voltage of the impulse suddenly drops to zero when
flashover occurs. This can be represented by a full wave, and a negative wave starting from the time flashover
occurs. The chopped wave, though it reduces the voltage of the surge to zero, will have a severe effect of the
winding due to sharp drop in the voltage. Thus it is always necessary to subject the transformer during tests to
chopped wave conditions. Generally the method is to apply full-waves and see whether damage has occurred
and then to apply the chopped waves and to see whether damage has occurred and then to apply the chopped
waves and to see whether damage has occurred.
9.5 Tests on Insulators
The tests on insulators can be divided into three groups. These are the type tests, sample tests and the routing
tests.
9.5.1 Type tests
These tests are done to determine whether the particular design is suitable for the purpose.
(a) Withstand Test: The insulator should be mounted so as to simulate practical conditions. A 1/50 s wave
of the specified voltage (corrected for humidity, air density etc.,) is applied. Flashover or puncture should not
occur. [If puncture occurs, the insulator is permanently damaged]. The test is repeated five times for each
polarity.
(b) Flash-over test: A 1/50 s wave is applied. The voltage is gradually increased to the 50% impulse flashover
voltage. The test is done for both polarities. There should be no puncture of insulation during these tests.
© Dry One-minute test: The insulator, clean and dry, shall be mounted as specified and the prescribed
voltage (corrected for ambient conditions) should be gradually brought up (at power frequency) and maintained
for one minute. Thee shall not be puncture or flash-over during the test.
Dry flash-over test: The voltage shall then be increased gradually until flash-over occurs. This is repeated
ten times. There shall be no damage to the insulator.
Figure 9.13 - variations of voltage and voltage gradient
vx
(neutral) 0 1 x/l 1 x/l
V ξ x
High Voltage Testing 159
(d) One-minute Rain test: The insulator is sprayed throughout the test with artificial rain drawn from a source
of supply at a temperature within 10o
C of the ambient temperature of the neighbourhood of the insulator. The
rain is sprayed at an angle of 45 o
on the insulator at the prescribed rate of 3 mm/minute. The resistivity of the
water should be 100 ohm-m ± 10%. The prescribed voltage is maintained for one minute.
Wet flash-over test: The voltage shall then be increased gradually until flash-over occurs. This is repeated
ten times. There shall be no damage to the insulator.
(e) Visible discharge test: This states that after the room has been darkened and the specified test voltage
applied, after five minutes, there should be no visible signs of corona.
9.5.2 Sample Tests
The sample is tested fully, up to and including the point of breakdown. This is done only on a few samples of
the insulator.
(a) Temperature cycle test: The complete test shall consist of five transfers (hot-cold-hot-..), each transfer
not exceeding 30 s.
(b) Mechanical loading test: The insulator shall be mechanically loaded up to the point of failure. When
failure occurs, the load should not be less than 2000 lbf.
© Electro-mechanical test: The insulator is simultaneously subjected to electrical and mechanical stress. (i.e.
it shall be subjected to a power frequency voltage and a tensile force simultaneously. The voltage shall be 75%
of dry flash-over voltage of the unit. There should be no damage caused.
(d) Overvoltage test: The insulator shall be completely immersed in an insulating medium (oil), to prevent
external flashover occurring. The specified overvoltage must be reached without puncture. The voltage is then
gradually increased until puncture occurs.
(e) Porosity test: Freshly broken pieces of porcelain shall show no dye penetration after having been immersed
for 24 hours in an alcoholic mixture of fushing at a pressure of 2000 p.s.i.
9.5.3 Routine Tests
These are to be applied to all insulators and shall be commenced at a low voltage and shall be increased rapidly
until flash-over occurs every few seconds. The voltage shall be maintained at this value for a minimum of five
minutes, or if failures occur, for five minutes after the last punctured piece has been removed. At the conclusion
of the test the voltage shall be reduced to about one-third of the test voltage before switching off.
Mechanical Routine Test: A mechanical load of 20% in excess of the maximum working load of the insulator
is applied after suspending the insulator for one minute. There should be no mechanical failure of the insulator.