This document discusses the generation of high voltage impulses. It describes impulsive and oscillatory transients and their causes. A 1.2/50 μs, 1000 kV wave represents an impulse voltage wave with a 1.2 μs front time and 50 μs tail time. Modified Marx circuits are used to generate high voltage impulses, with capacitors charged in stages through high resistance and discharged through spark gaps. Wave shaping is controlled through resistors and capacitors. Commercial impulse generators typically have 6 sets of resistors to control the waveform and are rated by voltage, number of stages, and stored energy.

1. MODULE 3
GENERATION OF HIGH VOLTAGE
IMPULSE

2. TRANSIENT OVER VOLTAGES
• Impulsive Transients
• Oscillatory Transients
CAUSES OF IMPULSIVE TRANSIENTS
•External causes – Eg Lightning
CAUSES OF OSCILLATORY TRANSIENTS
•Internal Causes – Eg Power system switching operations

3. • Rise time of 0.5 to 10 µ s and decay time to
50% of the peak value of the order of 30 to
200 µ s.
•A 1.2/50 µ s, 1000 kV wave represents an impulse
voltage wave with a front time of 1.2 µ s , fall time
to 50% peak value of 50 µ s and a peak value of
1000 kV.

4. 1.25(O1t2-O1t1):Front time
O1t4 :tail time

5. • Tolerances in rise time:+/-30%
• Tolerances in tail time:+/-20%

7. Waveshape Control
• The following approximate analysis is used to calculate
the wave front and wave tail times.
• The resistance R2 will be large. Hence, the simplified
circuit shown in Fig. 6 is used for wave front time
calculation.
• Taking the circuit inductance to be negligible during
charging, C1 charges the load capacitance C2
through R1.
Then the time taken for charging is approximately three
times the time constant of the circuit and is given
by

8. Wave front time Wave tail time

9. • 1.Single capacitor and charging unit becomes
costlier for voltage>200kV
• Cost and size increases at the rate of V^2
• 2.high dc charging voltage is required
• 3.suppression of corona discharges from
structure and leads during charging period is
difficult
• 4.Switching of very high voltage spark gap-
difficult

11. Modified marx circuit

13. • Usually the charging resistance Rs is chosen to
limit the charging current to about 50 to
100mA, and the generator capacitance C is
chosen such that the product CRs is about 10 s
to 1 min.
• The gap spacing is chosen such that the
breakdown voltage of the gap G is greater than
the charging voltage V.

14. • Thus, all the capacitances are charged to the
voltage V in about 1 minute.
• When the impulse generator is to be
discharged, the gaps G are made to spark over
simultaneously by some external means.
• Thus, all the capacitors C get connected in
series and discharge into the load capacitance
or the test object.

15. • The discharge time constant C(R1/n) (for n
stages) will be very very small (microseconds),
compared to the charging time constant CRs
which will be few seconds.
• Hence, no discharge takes place through the
charging resistors Rs.

16. • In the Marx circuit ,the impulse wave shaping
circuit is connected externally to the capacitor
unit,
• In the modified Marx circuit is shown,
wherein the resistances R1 and R2 are
incorporated inside the unit.

17. • R1 is divided into n parts equal to R1/n and put in
series with the gap G.
• R2 is also divided into n parts and arranged
across each capacitor unit after the gap G.
• Advantages:
1.save space
2.cost saving
3.By distributing R1 and R2 inside the unit are that the
control resistors are smaller in size and the efficiency
(Vo/nV) is high.

18. • The nominal energy stored is given by
• V is the nominal maximum voltage (n times charging voltage).
• A 16-stage impulse generator having a stage capacitance of 0.280µ F
and a maximum charging voltage of 300 kV will have an energy rating
of 192 kW sec.
• The height of the generator will be about 15 m and will occupy a floor
area of about 3.25 x 3.00 m.
• The waveform of either polarity can be obtained by suitably changing
the charging unit polarity

19. Components of a Multistage Impulse
Generator
1. dc. Charging Set
The charging unit should be capable of giving a
variable d.c voltage to charge the generator
capacitors to the required value
2.Charging Resistors
• These will be non-inductive high value resistors of
about 10 to 100 kilo-ohms.
• Each resistor will be designed to have a maximum
voltage between 50 and 100 kV.

20. 3.Generator Capacitors and Spark
Gaps
• These are arranged vertically one over the other with all the
spark gaps aligned.
• The capacitors are designed for several charging and
discharging operations.
• On dead short circuit, the capacitors will be capable of giving
10 kA of current.
• The spark gaps will be usually spheres or hemispheres of 10 to
25 cm diameter.
• Sometimes spherical ended cylinders with a central support
may also be used.

21. 4.Wave-shaping Resistors and Capacitors
• Resistors will be non-inductive wound type and should be
capable of discharging impulse currents of 1000 A or more.
• Each resistor will be designed for a maximum voltage of 50
to 100 kV.
• The resistances are bifilar wound or non-inductive thin flat
insulating sheets.
In some cases, they are wound on thin cylindrical
formers and are completely enclosed.
The load capacitor may be of compressed gas or oil filled
with a capacitance of 1 to 10µF.

22. • A commercial impulse voltage generator uses six sets of
resistors ranging from 1.0 ohm to about 160 ohms with
different combinations (with a maximum of two resistors at a
time)
• The resistors used are usually resin cast with voltage and
energy ratings of 200 to 250 kV and 2.0 to 5.0 kWsec.
• The entire range of lightning and switching impulse voltages
can be covered using these resistors in series or in parallel
combination.

23. • 5.Triggering System
This consists of trigger spark gaps to cause spark
breakdown of the gaps
• 6.Voltage Dividers
• Voltage dividers of either damped capacitor or
resistor type and an oscilloscope with
recording arrangement are provided for
measurement of the voltages across the test
object.

24. • Impulse generators are nominally rated by the
total voltage (nominal), the number of stages,
and the gross energy stored.
• The nominal output voltage is the number of
stages multiplied by the charging voltage.

25. Efficiency(C1 and C2)

26. Generation of switching impulse
Controls the wave
shape

27. Time to crest value

28. Circuits for oscillating impulse

30. Impulse current generator