dr m a panneerselvam, professor, anna university 1 unit 3 : generation of high dc, ac and impulse...
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Dr M A Panneerselvam, Professor, Anna University
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UNIT 3 : GENERATION OF HIGH DC, AC AND IMPULSE
VOLTAGES AND HIGH CURRENTS
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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.,
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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
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development of dielectric materials, etc., )Different forms of high voltages and currents mentioned earlier are classified as:i) High DC voltagesii)High AC voltages of power frequency
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iii) High AC voltages of high frequencyiv) High impulse voltages v) Long duration switching surges vi) High impulse currents used for testing surge diverters
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3.1 GENERATION OF HIGH DC VOLTAGES
Half wave rectifier circuit:
HALF WAVE RECTIFIER CIRCUIT
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VOLTAGE AND CURRENT WAVEFORMS
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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.
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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.
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FULL WAVE RECTIFIER CIRCUIT
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SIMPLE VOLTAGE DOUBLER CIRCUIT
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CASCADED VOLTAGE DOUBLER CIRCUIT
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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
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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
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CASCADED RECTIFIER UNIT COCKROFT WALTON VOLTAGE WITH PULSE GENERATOR MULTIPLIER CIRCUIT
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900 kV COCKROFT WALTON DC GENERATOR
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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.
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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
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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
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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 .
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Dr M A Panneerselvam, Professor, Anna University
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Voltage developed in Van de graff Generator:
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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:
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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
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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.
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CASCADED TRANSFORMER CONNECTION ( SCHEMATIC )
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CASCADED TRANSFORMER UNIT ( IREQ, CANADA)
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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
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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.
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Vc = -jVXc/ R+j(Xl- Xc) = XcV/R= V/ωCR
RESONANT TRANSFORMER EQUIVALENT CIRCUIT
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2.2 MV SERIES RESONANT CIRCUIT
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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.
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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.
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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
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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 V1
2 = ½ C2 V22
From which, V2 = V1 √ η C1 / C2
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TESLA COIL EQUIVALENT CIRCUIT OUTPUT WAVEFORM
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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 differenceof two exponential waveforms as
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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
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i) Full impulse:
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ii) Chopped impulse:
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iii) Front of wave impulse:
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IMPULSE VOLTAGE WAVEFORM
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SCHEMATIC DIAGRAM OF MARX CIRCUIT ARRANGEMENT FOR MULTISTAGE IMPULSE GENERATOR
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MULTISTAGE IMPULSE GENERATOR INCORPORATING SERIES AND WAVE TAIL RESISTANCES WITHIN THE GENERATOR
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MULTISTAGE IMPULSE GENERATOR CONNECTED TO POTENTIAL DIVIDER,MEASURING SPHERES AND LOAD
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Analysis of Impulse Generator circuit: Two basic circuits for single stage impulse generator are shown below:
RESISTANCE’ R2 ‘ ON THE LOAD SIDE
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RESISTANCE ‘R2’ ON THE GENERATOR SIDE
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Taking the circuit in Fig. (a) ,
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IMPULSE WAVE AND ITS COMPONENTS
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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
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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.
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The following table shows the result for some selected wave shapes:
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Triggering of impulse Generators:
TRIGATRON SPARK GAP
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Tripping of Impulse Generator with three electrode gap:
TRIPPING OF IMPULSE GENERATOR WITH A THREE ELECTRODE GAP
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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.
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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.
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CIRCUITS FOR GENERATING SWITCHING SURGE VOLTAGES WITH OUTPUT WAVEFORMS ACROSS THE LOAD CX
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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
.
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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
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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 :
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BASIC CIRCUIT OF AN IMPULSE CURRENT GENERATOR
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ARRANGEMENT OF CAPACITORS FOR HIGH IMPULSE CURRENT GENERATION