Impulse testing of transformer

TESTING OF TRANSFORMER AND SURGE ARRESTER
Preet Patel(151310109032)
HIGH VOLTAGE ENGINEERING

CONTENTS
 Introduction
 Induced Voltage Test
 Partial Discharge Test
 Impulse Test
 Neutral Current Method
 Transferred Surge Current Method
 Testing of Surge Arrestor
 Spark Over Test
 Residual Voltage Test
 100% Standard Impulse Spark Over Test

INTRODUCTION
 Transformers are very important and costly apparatus in power systems
and so great care has to be exercised to see that the transformers are
not damaged due to over voltage.
 There are many test carried out on transformer to determine if insulation
can withstand high voltages or not.
 The main two test carried out are:
i. Induced Overvoltage Test
ii. Partial Discharge Test

INDUCED OVERVOLTAGE TEST
 Transformers are tested for over voltage by exciting the secondary of
the transformer.
 Induced Over Voltage Test Set is used for testing the strength of
insulation between turns and between other points of the
Transformers
 A high frequency AC source (100 to 400 Hz) to about twice the rated
voltage is connected to LT terminals.
 It is also used to detect the airgaps between the layers of winding.

PROCEDURE
1. Apply the excitation voltage to the terminals of the low voltage winding.
2. Left the other windings open circuited
3. Normally, we use high frequency voltage source is used and twice the
rated voltage is applied.
4. We can calculate the duration of the test from this relation:
T test = [rated frequency / test frequency] x 120 seconds = 40 s (min
15 sec, max 60 sec)
5. Repeat the test at each tap changer point of transformer.
6. The test is successful if no collapse of the test occurs.

PARTIAL DISCHARGE TEST
 Partial discharge tests on the windings are done to assess the
discharge magnitudes. The transformer is connected in manner similar
to any other equipment and the discharge measurements are made.
 Partial discharge test is carried out to determine the deterioration taking
place inside the insulating material
 The location of the fault or void is sometimes done by using the
travelling wave technique. So far, no method has been standardized as
to where the discharge is to be measured.

• The location of the fault or void is sometimes done by using the travelling wave technique.
So far, no method has been standardized as to where the discharge is to be measured.
• The root cause of partial discharge is deforming and defects formed during
manufacturing at design and development level.

IMPULSE TESTING OF TRANSFORMER
• Purpose of this test is to check the ability of the insulation of a transformer to withstand the
transient voltages due to lightening etc.
• Since the transients are impulses of short rise time, the voltage distribution along the
transformer winding will not be uniform.
• Equivalent circuit is shown.
Here,
L = Inductance (Series)
Cs=series capacitances
Cg= Shunt capacitances.

• If we apply an impulse wave to such a network then the voltage distribution along the
element will be uneven and oscillation will be set in producing voltages much higher
than the applied voltage.
Procedure For Impulse Testing :
Arrangement of transformer for test is shown in figure.

 Sequences for the test are as follow :
 Apply impulse voltage of magnitude 75% of the basic impulse level
(BIL) of the transformer under test.
 One full wave voltage of 100% BIL
 Two chopped waves of 100% BIL
 It is very important to see that the grounding is properly done and the
windings not under test are properly terminated.

NEUTRAL CURRENT METHOD
 In this method, a record of the impulse current flowing through a resistive shunt
between the neutral and ground point is used for detecting the fault.
 Here neutral current oscillogram is used. It consists high frequency oscillation, a low
frequency disturbance and a current rise due to reflection from ground end of the
windings.
 When a fault occurs such as arcing between the turns or from turn to ground a train of
high frequency pulses similar to that fault is observed in current oscillogram and
changes in shape.

TRANSFERRED SURGE CURRENT METHOD
 In this method, voltage across a resistive shunt connected between the low
voltage winding and the ground is used for fault location.
 A short high frequency discharge oscillation is transferred at the event of
failure and is recorded.
 Hence fault at further distance from the neutral is also recorded.
 The waveshape is distorted depending on the location and type of the fault and
so can be more clearly detected.

TESTING OF SURGE ARRESTER
 ‘‘A protective device for limiting surge voltages on equipment by
diverting surge current and returning the device to its original status.
It is capable of repeating these functions as specified.’’
 So as we know that surge arrester are the most reliable apparatus to
protect the power system against transient voltage due to lightning
and switching surge

POWER FREQUENCY SPARK OVER TEST
 It is a routine test
 This test is conduct using a series resistance to limit the
current in case a spark over occurs
 The arrester has to withstand at least 1.5 times the rated
value of the voltage for five successive application
 The test is generally done under dey and wet condition

FRONT OF WAVE SPARKOVER TEST
 In order to ensure that the surge arrester flashes over for very steep
fronted waves of high peaks, this test is conducted using an
overvoltage having a rate of rise of 100KV/µs, per 12KV of the rating.
 The estimated maximum steepness of the waves are specified in std.
and specifications. The test is done by conducting 100% sparkover
voltage test for increasing magnitude of the std. impulse wave.
 The volt-time characteristic of the arrester is plotted, and the
intersection of the V-t characteristics and the line with slope of the
virtual steepness of the front gives the front of a sparkover voltage.

RESIDUAL VOLTAGE TEST
 This test is conducted on pro-rated diverters of rating in the range 3
to 12KV only. The voltage developed across the Non-Linear Resistor
units (NLR) during the flow of surge currents through the arrester is
called the RESIDUAL VOLTAGE.
 Standard impulse current of the rated magnitude are applied, and the
voltage developed across the diverter is recorded using suitable
voltage divider and CRO.

 The magnitude of the current are approx.. 0.5, 1, and 2 times
the rated currents.
 From the oscillogram, a graph is drawn between the current
magnitude and the voltage developed across the diverter pro-
rated unit.
 From the graph, the residual voltage corresponding to the
exact rated current is obtained.

 Let V1=rating of the complete unit.
 V2=rating of the pro-rated unit tested.
 Vr1 =residual voltage of the complete unit.
 Vr2 =residual voltage of the pro-rated unit.
 Then, V1/V2= Vr1 / Vr2
 Let the Vrm be the maximum permissible residual voltage for a
complete unit. The ratio Vrm/V1=r, is defined as a multiplying factor of
the rating for the residual voltage test, which depends on V1. The
diverter is said to pass the test, if Vr2< rV2

HUNDRED PERCENT STANDARD IMPULSE
SPARKOVER TEST
 This test is conducted to ensure that the diverter operates positively when over voltages
of impulse nature occur. The impulse generator is adjusted to give the standard impulse
voltage of a preset magnitude specific in the specifications.
 The arrester has to spark over every time in each of the successive applications. The
test is done with both positive and negative polarity waveforms.
 Sometimes, the test is done by starting at a voltage level that does not give flashover at
all, and is repeated in increasing steps of voltage till hundred per cent flashover occurs.
 The magnitude of the voltage at which hundred per cent flashover occurs is the required
spark -over voltage

FRONT OF WAVE SPARKOVER TEST
 In order to ensure that the surge diverter flashes over for very steep fronted
waves of high peaks, this test is conducted using an overvoltage having a rate of
rise of 100 kv/ us, per 12 kV of the rating.
 The estimated maximum steepness of the waves are specified in standards and
specifications. The test is done by conducting hundred per cent spark over
voltage test for increasing magnitudes of the standard impulse wave.
 The time to spark over is measured. The volt-amp characteristic of the diverter is
plotted, and the intersection of the l characteristic and the line with slope of the
virtual steepness of the front gives the front of a wave sparkover voltage.

Impulse testing of transformer

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Impulse testing of transformer