1
Studies of transient disturbances on a transmission
system have shown that lightning strokes and switching
operations are followed by a traveling wave of a steep
wave front.
When a voltage wave of this type reaches a power
transformer it causes an unequal stress distribution along
its windings and may lead to breakdown of the insulation
system.
Therefore, it is necessary to study the insulation
behavior under impulse voltages.
Impulse Voltage
2
Wave Front: The duration of the wave front is the total time occupied by the
impulse voltage while rising from zero to peak value. The time is known as time
to peak value (t1)
Wave Tail: The duration of wave tail is the total time occupied by the impulse
voltage in rising to peak value and declining to half the peak value or the impulse
(t2)
Example: a 1000 KV, 1/50 impulse voltage has a peak value of 1000 KV, which
is attained in 1 S and 500 KV on the wave tail reached after 50 S.
Impulse Voltage
3
An impulse voltage is a unidirectional voltage which
rises rapidly to a maximum value and then decays slowly
to zero.
The wave shape is generally defined in terms of the
times t
1
and t
2
in microseconds,
where t
1
= time taken by the voltage wave to reach its
peak value
t
2
= total time from the start of wave to the instant
when it has declined to one-half of the peak value.
The wave is then referred to as a t
1
/t
2
wave.
Impulse Voltage
Some sort of standardization is required by
national/international standardization agencies
for the purpose of generating test voltages.
For lightening impulse the standard wave
shapes are
o 1.2/50 (Indian Standard)
o 1/50 (British Standard)
o 1.5/40 (American Standard)
4
Impulse Voltage
Standard Impulse
5
6
7
t
1
= 1.25 T
1
T
2
Where
OT
1
= time for the voltage wave to reach 10% of the peak voltage.
OT
2
= time for the voltage wave to reach 90% of the peak voltage.
The point O
1
where the line CD cuts the time axis is defined as the nominal
starting-point of the wave. The nominal wave tail t
2
is the time between O
1
and
the point on the wave tail where the voltage is one-half the peak value, i.e. t
2
=
O
I
T
4
.
Transient / Impulse Voltage
8
The wave is then referred to as a t
1
/ t
2
wave and
according to the standard specified in B.S. 923 a 1/50
sec wave is the standard wave. The specification
permits a tolerance of up to 50% on the duration of the
wave front and 20 % on the duration of the wave tail.
In the corresponding American specification, the nominal
wave front duration is defined as given by 1.5T
1
T
2
and
the standard wave is a 1.5/40 sec. The tolerances
allowed on the wave front and the wave tail are 0.5 sec
and 10 sec respectively.
Transient / Impulse Voltage
9
An impulse generator essentially consists of a capacitor
which is charged to the required voltage and discharged
through a circuit the constants of which can be adjusted
to give an impulse voltage of desired shape.
The basic circuit of a single-stage impulse generator is
shown in Fig. 5(a) where the capacitor C
1
is charged from
a direct current source until the spark-gap G breaks
down. A voltage is then impressed upon the object under
test of capacitance C
2
.
Single-stage Impulse Generator Circuit
10
Single-stage Impulse Generator Circuit
Fig:5
Transient / Impulse Voltage
11
12
Single-stage Impulse Generator Circuit
The wave-shaping resistors R
1
and R
2
control
respectively the front and the tail of the impulse voltage
available across C
2
. An analysis of the simple circuit,
presented by Draper is as follows.
Figure 5(b) represents the Laplace transform circuit of
the impulse generator of Fig. 5(a) and the output voltage
is given by the expression:
Transient / Impulse Voltage
13
Single-stage Impulse Generator Circuit
Transient Voltage
Fig: 5 (b)
14
a) Single-stage Impulse Generator Circuit
2. Transient / Impulse Voltage
15
Single-stage Impulse Generator Circuit
By Substitution:
Transient / Impulse Voltage
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Single-stage Impulse Generator Circuit
Transient Voltage
17
Single-stage Impulse Generator Circuit
Where S
1
and S
2
are the roots of equation s
2
+as+b = 0 and
both will be negative. From the transform tables
In a practical case R
2
is much greater than R
1
and C
1
much greater than C
2
and an approximate solution is
obtained by examining the auxiliary equation:
Transient / Impulse Voltage
18
Single-stage Impulse Generator Circuit
Where the value of (1/R
1
C
1
+ 1/R
2
C
2
) is much smaller than
1/R
1
C
2 .
2
1 2 1 2 1 2
2 1
2 1
1 1
1
1
.)
s s
RC R R C C
s Was addded as approximately zero
R
s
C
C
R
| |
+ + +
|
\ .
| |
| |
|
\ .
(
|
\
.
Transient / Impulse Voltage
19
20
Single-stage Impulse Generator Circuit
and the graph of the expression is shown in Fig. 6.
Transient / Impulse Voltage
Fig: 6
21
Single-stage Impulse Generator Circuit
Transient / Impulse Voltage
22
Typical Values of Load Capacitance
