The document discusses the relationship between lightning characteristics and standard impulse waveforms, emphasizing the importance of impulse voltage generation for testing electrical equipment against lightning surges. It details the design and simulation of an impulse voltage generator, including the configuration of single and multi-stage generators to achieve desired output voltages. The analysis identifies challenges in high voltage testing and suggests using modeling software to address these issues.

1. Deliberation of Characteristics and Generation of
Impulse Voltages
Dr.Md Raju Ahmed
Dhaka University of Engineering and Technology,Gazipur-1700,Dhaka
Email: - raju97eee@yahoo.com
Milton sarker
Dhaka University of Engineering and Technology,Gazipur-1700,Dhaka
Email:milton.duet@gmail.com
Abstract:
Lightning characteristics and standard impulse
waveform are related to each other. But the lack
of realization about the relation between them
would make the solution to produce better
protection against lightning surge becomes
harder. Natural lightning surge waveform has
been compared to standard impulse waveform as
evidence that there have similarity between
them. The standard impulse waveform could be
used to test the strength of electrical equipment
against the lightning. Therefore designing and
simulating the impulse generator are the purpose
of this project beside to get better understanding
about lightning characteristics. This project aims
to develop an impulse generator circuit. The
main objectives of this work are two folds: the
first is the characterization of impulse voltages
and the second is the designing of an impulse
voltage generator. Our working purpose is to
give a concept about Impulse voltages and
impulse generator to the students and
researchers.
Keyword: Lightning characteristics, impulse
voltage, generation of impulse voltage.
Introduction:
Electrical energy transmission and distribution
system involves transformers, isolators,
switchgears, lightning arrestors etc.. All of them
are high voltage apparatus and they are exposed
to high transient voltages due to internal and
external over voltages. Before commissioning
they are therefore tested for reliability with
standard impulse voltages or currents. So it is
essential to characterize the impulse voltages.
The electrical strength of high voltage apparatus
against external over voltages that can appear in
power supply system due to lightning strikes is
tested with lightning impulse voltages. A
standard full impulse voltages rises in a short
time less than a few micro seconds and fall
appreciably slower, ultimately zero. The rising
part of impulse wave is called wave front falling
part is called wave tail. International Electro-
technical Commission (IEC) has specified that
insulation of high voltage transmission line and
equipment‟s should withstand standard lightning
impulse voltage of wave shape 1.2/50µs. The
tolerances for lightning impulse voltage amount
to ±3% on the value of the test voltage.
During tests with switching impulse voltages,
the stressing of the power apparatus by internal
voltages consequent to switching operation in
the supply network is simulated. The time
constants here are appreciably larger. According
to IEC for higher voltages (220 kV and above)
the insulation of transmission line should
withstand 250/2500µs has a time to peak of 250
µs (tolerance: ±20 %) and a time to half-value
2500 µs (tolerance: ±60 %).
During tests on insulators very rapidly rising
voltages are used. With conventional impulse
voltage generator of low inductance (about 1µH)
per stage, maximum step size 2.5kV/ns can be
attained. But by appropriate design of circuit,
step-front impulse generator with step size up to

2. 100kV/ns corresponding to arise time of 5ns per
500kV, can be generated.
Designing of an Impulse voltage generator:
In the design or use of impulse voltage
generators for research or testing, it is required
to evaluate the time variation of output voltage,
the nominal front and tail times and the voltage
efficiency for given circuit parameters. Also, it
needs to predict circuit parameters for producing
a given wave-shape, with a given source and
loading conditions. The loading can be inductive
or capacitive.
An impulse generator essentially consists of a
capacitor which is charged to the required
voltage and discharged through a circuit. The
circuit parameters can be adjusted to give an
impulse voltage of the desired shape. Basic
circuit of a single stage impulse generator is
shown in Fig. 1, where the capacitor Cs is
charged from a dc source until the spark gap G
breaks down. The voltage is then impressed
upon the object under test of capacitance Cb.
The wave shaping resistors Rd and Re control
the front and tail of the impulse voltage
available across Cb respectively. Overall, the
wave-shape is determined by the values of the
generator capacitance (Cs) and the load
capacitance (Cb), and the wave control
resistances Rd and Re.
Fig.1: Basic circuit of single stage impulse
generator.
Equation for the output voltage is;
Where v(t) - instantaneous output voltage; Vo-
DC charging voltage for the capacitor; α,β -
roots of the characteristics equation, which
depend on the parameters of the generator.
According to the fig.1,
The amount of energy stored in capacitor can
easily be calculated from the value of Cs and Vo
and the maximum energy stored in the impulse
capacitor is ;
But in this system by a single unit the generated
voltage value is not sufficient . So for producing
large amount of voltage multistage generator is
used. For a multi-stage generator, a group of
capacitors are charged in parallel and discharged
in series. The switch over of capacitors from a
parallel connection to series connection occurs
automatically when the intermediate spark gap
breaks down after the capacitors are charged to
the required potential Vo. The voltage at the
generator terminal is V(t) and is equal to nVo
where „n‟ is the number of stages. Fig.2, shows
the principle of a multistage lightning impulse
voltage generator, built up of a number of „n‟
identical stages. Then the value of Re, Rd, Cs
are changed as;
Re = nRe‫׳‬
Rd = nRd‫׳‬
Cs = Cs‫⁄׳‬n
Thus by increasing the number of stages the
output voltage can easily be increased at an
expected large value.

3. Fig.2: Multi stage impulse
generator.
Numerical analysis:
In a multi stage impulse generator the system
equation may be put in the following form,
Result and discussion:
An impulse generator circuit simulated on Orcad
simulation software is shown below in figure.3.
Here an impulse generator with 4 stages is
shown. The input of the circuit is 20 kV dc
voltage as shown in figure. Figure.4 shows the
output of the impulse generator.
Fig.3: A 4-stage impulse voltage generator
Fig.4: output wave shape of impulse generator.
R1
33
21
R2
33
21
R3
134
21 R4
134
21
C1
2u1
2
C2
2u
1
2
0
U1
TCLOSE = 0
1 2 U2
TCLOSE = 0
1 2
V1
20kV
R6
33
21
R7
33
21
R8
134
21 R9
134
21
C4
2u
1
2
C5
2u
1
2
U3
TCLOSE = 0
1 2 U4
TCLOSE = 0
1 2
R10
33
21
C6
1n
1
2
0
V
Time
0s 20us 40us 60us 80us 100us
V(R6:2)
-80KV
-40KV
0V
40KV

4. Problem associated with impulse generator:
In the case of a 15 stages 3MV multi-stage, the
capacitors are charged to 200 kV from a
regulator and charging supply section. After the
capacitors are charged, the first gap G1, is
triggered from a pulse triggering circuit, which
in turn causes a breakdown of all other gaps. To
obtain the desired waveform, the choice of Rd
and Re is critical. Changing these high voltage
resistors in the laboratory can be in conveni-ent
as the components are too bulky and the
processes are too time consuming. Also, the
generator has to be triggered each time to obtain
the desired output wave-form, which at times
could become risky for the personnel
involvement. To remove this problem modeling
with SIMULINK is used.
Conclusion:
In high voltage field it is very important to know
about different types of high voltages. I think my
analysis helps them who are searching about
high voltages. I try to give an overall description
on high voltage, their classes and generation
process. The major problem observed in high
voltage field is the testing of HV equipment in
the lab. The voltage ratings of the different
equipment are different there by it requires
different rating high voltage impulse generators
and the associated equipment. It is very difficult
to setup those practically. In this paper only the
definition and samples are given.
Literature cited:
[1] IEC 60060-1: High-voltage test techniques—
Part 1: General definitions and test
requirements(2010).
[2] IEC 60060-2: High-voltage test techniques—
Part 2: Measuring systems (2010).
[3] Naidu, M.S. and Kamaraju, “High Voltage
Engineering” 2nd. Ed.
[4] M. Jayaraju, P. Nair, B. Prabhakar. Design
and simulation of marx impulse voltage
generator. Journal of the Instrument Society of
India, 1999.
[5] M. Jayaraju. Performance of high voltage
insulation systems with non-standard switching
surges. Ph.d. thesis, University of Kerala, India,
2003.
[6] A. Carrus, L. Funes. Very short tailed
lightning double exponential wave generation
technique based on marx circuit standard
configuration. IEEE Transactions on Power
Apparatus and Systems, 1984.