28-09-2012, 05:38 PM
Impulse voltage generator modelling using MATLAB
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Abstract.
MATLAB is specifically designed for simulating dynamic systems. This paper describes a method
of modelling impulse voltage generator using Simulink, an extension of MATLAB. The equations for modelling
have been developed and a corresponding Simulink model has been constructed. It shows that Simulink
program becomes very useful in studying the effect of parameter changes in the design to obtain the desired
impulse voltages and waveshapes from an impulse generator.
Introduction
International Electrotechnical Commission (IEC) has specified that the insulation of transmission line
and other equipments should withstand standard lightning impulse voltage of wave shape 1.2/50 s and for
higher voltages (220 kV and above) it should withstand standard switching impulse voltage of waveshape
250/2500 μs[1].
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 waveshape, with a given source
and loading conditions. The loading can be inductive or capacitive. The waveshapes to be produced may be
standard impulse, steep fronted impulse, short tailed impulse or steep front short tailed impulse.
Expressions and curves have been already developed to predict the parameters of impulse voltage generator
circuits required to reproduce the wave forms of required shape for the testing of transformers[12]. An
analysis of standard Marx circuit, with an inductance in series with the tail resistance for the production of
short tailed impulses was done by Carrus[5]. An algorithm workable in a personal computer to evaluate the
time variation of the output voltage, nominal front and tail times and voltage efficiency for given circuit parameters
or to predict the circuit parameters for given waveshape, source and loading conditions was reported
in [6].
Spice versus simulink
SPICE is a user friendly simulator widely used in circuit analysis. SPICE consists of a group of device
files, one for each active circuit element, and one executetable file, SPICE has a rich library for integrated
circuits such as operational amplifiers, comparators etc. The user can create a model for part of a circuit and
save it as a sub circuit and then use this sub-circuit later. The SPICE executable file[8, 10, 11] will read the user
circuit file and then execute the simulation. The output is produced either in a graphical or text form which
shows the current and voltage of different nodes. Apart from circuit analysis and analogue electronics, SPICE
can also be used to study the time - domain steady state behaviour of power electronics circuits. However
SPICE cannot handle the dynamic behavior of switching converters because of the inherent switching nature
of their circuits.
Impulse voltage generator
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 waveshape is determined by the values of the generator
capacitance (Cs) and the load capacitance (Cb), and the wave control resistances Rd and Re.
Conclusion
A modelling technique has been developed for impulse voltage generator using SIMULINK. Mathematical
equations for the elements and the switches of the system are used to construct a SIMULINK dynamic
model of the circuit.
In the case of a larger system, it would be possible to break up the overall system into a number of smaller
subsystems, simulate and debug each of the subsystems and put together the complete system. This modelling
technique could be extended to some other applications in the area of power electronics, power systems, etc.