dr m a panneerselvam, professor, anna university 1 unit 3 : generation of high dc, ac and impulse...

Dr M A Panneerselvam, Professor, Anna University

1

UNIT 3 : GENERATION OF HIGH DC, AC AND IMPULSE

VOLTAGES AND HIGH CURRENTS


2

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.,


3

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


4

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


5

iii) High AC voltages of high frequencyiv) High impulse voltages v) Long duration switching surges vi) High impulse currents used for testing surge diverters


6

3.1 GENERATION OF HIGH DC VOLTAGES

Half wave rectifier circuit:

HALF WAVE RECTIFIER CIRCUIT


7

VOLTAGE AND CURRENT WAVEFORMS


8

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.


9

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.


10

FULL WAVE RECTIFIER CIRCUIT


11

SIMPLE VOLTAGE DOUBLER CIRCUIT


12

CASCADED VOLTAGE DOUBLER CIRCUIT


13

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


14

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


15

CASCADED RECTIFIER UNIT COCKROFT WALTON VOLTAGE WITH PULSE GENERATOR MULTIPLIER CIRCUIT


16

900 kV COCKROFT WALTON DC GENERATOR


17

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.


18

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


19

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


20

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 .


21 ELECTROSTATIC BELT DRIVEN GENERATOR


22

Voltage developed in Van de graff Generator:


23


24

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:


25

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


26

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.


27

CASCADED TRANSFORMER CONNECTION ( SCHEMATIC )


28

CASCADED TRANSFORMER UNIT ( IREQ, CANADA)


29

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


30

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.


31

Vc = -jVXc/ R+j(Xl- Xc) = XcV/R= V/ωCR

RESONANT TRANSFORMER EQUIVALENT CIRCUIT


32

2.2 MV SERIES RESONANT CIRCUIT


33

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.


34

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.


35

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


36

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


37

TESLA COIL EQUIVALENT CIRCUIT OUTPUT WAVEFORM


38

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


39

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


40

i) Full impulse:


41

ii) Chopped impulse:


42

iii) Front of wave impulse:


43

IMPULSE VOLTAGE WAVEFORM


44

SCHEMATIC DIAGRAM OF MARX CIRCUIT ARRANGEMENT FOR MULTISTAGE IMPULSE GENERATOR


45

MULTISTAGE IMPULSE GENERATOR INCORPORATING SERIES AND WAVE TAIL RESISTANCES WITHIN THE GENERATOR


46

MULTISTAGE IMPULSE GENERATOR CONNECTED TO POTENTIAL DIVIDER,MEASURING SPHERES AND LOAD


47

Analysis of Impulse Generator circuit: Two basic circuits for single stage impulse generator are shown below:

RESISTANCE’ R2 ‘ ON THE LOAD SIDE


48

RESISTANCE ‘R2’ ON THE GENERATOR SIDE


49

Taking the circuit in Fig. (a) ,


50


51

IMPULSE WAVE AND ITS COMPONENTS


52

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


53

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.


54

The following table shows the result for some selected wave shapes:


55

Triggering of impulse Generators:

TRIGATRON SPARK GAP


56

Tripping of Impulse Generator with three electrode gap:

TRIPPING OF IMPULSE GENERATOR WITH A THREE ELECTRODE GAP


57 2.4 MV IMPULSE GENERATOR


58

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.


59

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.


60

CIRCUITS FOR GENERATING SWITCHING SURGE VOLTAGES WITH OUTPUT WAVEFORMS ACROSS THE LOAD CX


61

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

.


62

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


63

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 :


64

BASIC CIRCUIT OF AN IMPULSE CURRENT GENERATOR


65

ARRANGEMENT OF CAPACITORS FOR HIGH IMPULSE CURRENT GENERATION

dr m a panneerselvam, professor, anna university 1 unit 3 : generation of high dc, ac and impulse...

Documents

anna universitydr

high currents dr

wave circuit

higher voltages

cockroftwalton circuit

half wave rectifier

simple voltage doublerdr

testing surge diverters