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UNIT 3 : GENERATION OF
HIGH DC, AC AND IMPULSE
VOLTAGES AND HIGH
CURRENTS
Dr M A Panneerselvam, Professor,
Anna University
1
3.0 INTRODUCTION
Generation of very high voltages
and high currents becomes
necessary for the following
reasons :
For use in applied physics,
electrostatic precipitators,
particle accelerators, etc.,
Dr M A Panneerselvam, Professor,
Anna University
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For testing power apparatus to be
used in high voltage systems
For testing surge diverters with
high impulse currents
For R & D purpose ( study of
breakdown mechanisms and
Dr M A Panneerselvam, Professor,
Anna University
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development of dielectric
materials, etc., )
Different forms of high voltages
and currents mentioned earlier
are classified as:
i) High DC voltages
ii)High AC voltages of power
frequency
Dr M A Panneerselvam, Professor,
Anna University
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iii) High AC voltages of high
frequency
iv) High impulse voltages
v) Long duration switching
surges
vi) High impulse currents used for
testing surge diverters
Dr M A Panneerselvam, Professor,
Anna University
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3.1 GENERATION OF HIGH DC
VOLTAGES
Half wave rectifier circuit:
HALF WAVE RECTIFIER CIRCUIT
Dr M A Panneerselvam, Professor,
Anna University
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VOLTAGE AND CURRENT WAVEFORMS
Dr M A Panneerselvam, Professor,
Anna University
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Full wave rectifier circuit:
Single phase full wave circuit can
only be used when transformer
HT winding is earthed at middle
point and DC output is earthed at
one end.
Dr M A Panneerselvam, Professor,
Anna University
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Voltage doubler ( multiplier )
circuit:
When high DC voltages are
needed , a voltage doubler or
cascaded rectifier doubler
circuits are used as shown be in
the next slides.
Dr M A Panneerselvam, Professor,
Anna University
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FULL WAVE RECTIFIER CIRCUIT
Dr M A Panneerselvam, Professor,
Anna University
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SIMPLE VOLTAGE DOUBLER CIRCUIT
Dr M A Panneerselvam, Professor,
Anna University
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CASCADED VOLTAGE DOUBLER CIRCUIT
Dr M A Panneerselvam, Professor,
Anna University
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COCKROFT-WALTON
CIRCUIT :
Cascaded voltage multiplier
circuits for higher voltages
becomes cumbersome and
require too many isolating
transformers. In such cases we
extend a simple voltage doubler
Dr M A Panneerselvam, Professor,
Anna University
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circuit using ‘Cockroft-Walton’
principle as shown in the next
figure.
ELECTROSTATIC MACHINES:
Electrostatic generators convert
mechanical energy directly into
electrical energy. In contrast to
Dr M A Panneerselvam, Professor,
Anna University
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CASCADED RECTIFIER UNIT
WITH PULSE GENERATOR
COCKROFT WALTON VOLTAGE
MULTIPLIER CIRCUIT
Dr M A Panneerselvam, Professor,
Anna University
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900 kV COCKROFT WALTON DC GENERATOR
Dr M A Panneerselvam, Professor,
Anna University
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electromechanical energy
conversion , electrical charges
are moved in this generator
against the force of electric field,
thus gaining higher potential
energies and
consuming
mechanical energy.
Dr M A Panneerselvam, Professor,
Anna University
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Based on the above principle , Van
de graff ,in 1931, succeeded with
the development of electrostatic
belt driven generators.
In the figure that follows , charge
is sprayed onto an insulating
moving belt by means of corona
Dr M A Panneerselvam, Professor,
Anna University
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discharging points which are at
some 10 kV from earth potential.
The belt ,having width varying
between cm to metres, is driven at
about 15-20 m/s by means of a
motor. The charge is conveyed to
the upper end where it is removed
Dr M A Panneerselvam, Professor,
Anna University
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from the belt by discharging
points connected to metal
electrode. The entire equipment
is usually enclosed in an earthed
metal tank filled with compressed
gas like air, air-freon and SF6 at
5 to 15 atm .
Dr M A Panneerselvam, Professor,
Anna University
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ELECTROSTATIC
BELT DRIVEN
GENERATOR
Dr M A Panneerselvam,
Professor,
Anna University
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Voltage developed in Van de
graff Generator:
Dr M A Panneerselvam, Professor,
Anna University
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Dr M A Panneerselvam, Professor,
Anna University
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there are no losses in the system.
It generates very high voltages
with small output current.
3.2 GENERATION OF HIGH AC
VOLTAGES AT POWER
FREQUENCY
CASCADED TRANSFORMERS:
Dr M A Panneerselvam, Professor,
Anna University
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For voltages higher than about
300 to 500 kV cascading of
transformers have the following
advantages:
Flexibility in the output voltage
Lesser insulation
Easy transportation and erection
Dr M A Panneerselvam, Professor,
Anna University
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Easy maintenance and over
hauling
A prerequisite to apply this
technique is an exciting winding
within each transformer unit, as
shown in the following figure.
Dr M A Panneerselvam, Professor,
Anna University
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CASCADED TRANSFORMER CONNECTION ( SCHEMATIC )
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Anna University
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CASCADED TRANSFORMER UNIT ( IREQ, CANADA)
Dr M A Panneerselvam, Professor,
Anna University
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RESONANT TRANSFORMERS:
By means of
Resonant
transformers very high voltages
can be developed using the
principle of resonance. The high
voltage testing transformer
consists of leakage reactance of
windings , the magnetizing
Dr M A Panneerselvam, Professor,
Anna University
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reactance
and the shunt
capacitance across the output
due to bushing and also the test
object. During resonance the
inductive impedance equals the
capacitive impedance and
hence current is limited only by
the resistance of the circuit.
Dr M A Panneerselvam, Professor,
Anna University
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RESONANT TRANSFORMER
EQUIVALENT CIRCUIT
Vc = -jVXc/ R+j(Xl- Xc) = XcV/R= V/ωCR
Dr M A Panneerselvam, Professor,
Anna University
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2.2 MV SERIES RESONANT CIRCUIT
Dr M A Panneerselvam, Professor,
Anna University
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3.3 GENERATION OF HIGH
AC VOLTAGES AT HIGH
FREQUENCY
High frequency high voltages are
required for rectifier DC power
supplies and testing with high
frequency damped oscillations.
Dr M A Panneerselvam, Professor,
Anna University
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Advantages of HF transformers
are:
Absence of iron core, pure sine
wave output, slow building up of
voltage and uniform distribution
the voltage across the winding
coils.
Dr M A Panneerselvam, Professor,
Anna University
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The commonly used HF resonant
transformer is TESLA COIL as
shown in the next figure.
The primary and secondary
windings (L1 & L2) are wound on an
insulated former with no core and
immersed in oil. The windings
Dr M A Panneerselvam, Professor,
Anna University
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are tuned to a frequency of 10 to
100 kHz by means of condensers
C1 & C2 .
Using a simplified analysis based
on energy stored ,
W2 = η W1= η ½ C1 V12 = ½ C2 V22
From which, V2 = V1 √ η C1 / C2
Dr M A Panneerselvam, Professor,
Anna University
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EQUIVALENT CIRCUIT
OUTPUT WAVEFORM
TESLA COIL
Dr M A Panneerselvam, Professor,
Anna University
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3.4 GENERATION OF HIGH
IMPULSE VOLTAGES
Lightning impulse waveform is an
unidirectional impulse of nearly
double exponential in shape. It
can be shown to be the difference
of two exponential waveforms as
Dr M A Panneerselvam, Professor,
Anna University
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below:
v(t) = V ( exp (–άt) – exp (-βt) )
Three types of impulse voltage
wave forms can occur , namely,
i) full impulse
ii) chopped impulse and
iii) front of wave impulse
Dr M A Panneerselvam, Professor,
Anna University
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i) Full impulse:
Dr M A Panneerselvam, Professor,
Anna University
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ii) Chopped impulse:
Dr M A Panneerselvam, Professor,
Anna University
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iii) Front of wave impulse:
Dr M A Panneerselvam, Professor,
Anna University
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IMPULSE VOLTAGE WAVEFORM
Dr M A Panneerselvam, Professor,
Anna University
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SCHEMATIC DIAGRAM OF MARX CIRCUIT ARRANGEMENT FOR
MULTISTAGE IMPULSE GENERATOR
Dr M A Panneerselvam, Professor,
Anna University
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MULTISTAGE IMPULSE GENERATOR INCORPORATING SERIES
AND WAVE TAIL RESISTANCES
WITHIN
Dr M A Panneerselvam,
Professor,THE GENERATOR
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Anna University
MULTISTAGE IMPULSE GENERATOR CONNECTED TO POTENTIAL
Dr M A Panneerselvam,
Professor,
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DIVIDER,MEASURING
SPHERES
AND LOAD
Anna University
Analysis of Impulse Generator
circuit: Two basic circuits for
single stage impulse generator
are shown below:
RESISTANCE’ R2 ‘ ON THE LOAD SIDE
Dr M A Panneerselvam, Professor,
Anna University
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RESISTANCE ‘R2’ ON THE GENERATOR SIDE
Dr M A Panneerselvam, Professor,
Anna University
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Taking the circuit in Fig. (a) ,
Dr M A Panneerselvam, Professor,
Anna University
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Dr M A Panneerselvam, Professor,
Anna University
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IMPULSE WAVE AND ITS COMPONENTS
Dr M A Panneerselvam, Professor,
Anna University
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Approximate values for ‘ t1 and t2’
are,
t1 = 3.0 R1Ce
where Ce = C1 C2 / ( C1+C2)
and t2 = 0.7 ( R1+R2)(C1+C2)
When resistances ‘ R1 and R2’ are
in ohms and capacitances ‘ C1
and C2’ are in microfarads the
Dr M A Panneerselvam, Professor,
Anna University
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time is in microseconds.
Depending upon the output
voltage requirement and to get
proper wave shape ,the no. of
stages of the impulse generator
can be connected in full series,
full parallel or series parallel.
Dr M A Panneerselvam, Professor,
Anna University
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The following table shows the result
for some selected wave shapes:
Dr M A Panneerselvam, Professor,
Anna University
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Triggering of impulse Generators:
TRIGATRON SPARK GAP
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Anna University
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Tripping of Impulse Generator
with three electrode gap:
TRIPPING OF IMPULSE GENERATOR WITH A THREE
Dr ELECTRODE
M A Panneerselvam,GAP
Professor,
Anna University
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2.4 MV IMPULSE GENERATOR
Dr M A Panneerselvam, Professor,
Anna University
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3.5 GENERATION OF
SWITCHING SURGES
Switching surges may
be
considered as equivalent to
impulse voltages of slow rising
front (0.1 to 10 ms) and a tail time
of several ms.
Dr M A Panneerselvam, Professor,
Anna University
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Impulse generator circuits can
be modified by choosing
suitable values for time to front
(t 1) and time to tail (t 2) to
produce switching surges as
shown in the next figure.
Dr M A Panneerselvam, Professor,
Anna University
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CIRCUITS FOR GENERATING SWITCHING SURGE VOLTAGES
WITH OUTPUT WAVEFORMS ACROSS THE LOAD CX
Dr M A Panneerselvam, Professor,
Anna University
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3.6 GENERATION OF HIGH
IMPULSE CURRENTS
Lightning discharges involve both
high voltage impulses and high
current impulses on transmission
lines. Surge diverters used for
protection have to discharge
Dr M A Panneerselvam, Professor,
Anna University
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high currents without damage.
Therefore generation of high
impulse currents becomes
necessary for testing surge
diverters , studies on arc and
electric plasmas. The impulse
currents used for testing surge
Dr M A Panneerselvam, Professor,
Anna University
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diverters are generally 4/10 and
8/20 μs with tolerances of ± 10 %
on both t1 and t2.
For producing impulse currents of
large values, a bank of capacitors
in parallel are charged to a
specific value and are discharged
through a series R-L circuit :
Dr M A Panneerselvam, Professor,
Anna University
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BASIC CIRCUIT OF AN IMPULSE CURRENT GENERATOR
Dr M A Panneerselvam, Professor,
Anna University
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ARRANGEMENT OF CAPACITORS FOR HIGH IMPULSE CURRENT
Dr M A Panneerselvam,
Professor,
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GENERATION
Anna University