1. Dr M A Panneerselvam, Professor, Anna University
UNIT 3 : GENERATION OF HIGH DC, AC AND IMPULSE VOLTAGES AND HIGH CURRENTS
Dr M A Panneerselvam, Professor, Anna University

2. Dr M A Panneerselvam, Professor, Anna University
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

3. Dr M A Panneerselvam, Professor, Anna University
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

4. Dr M A Panneerselvam, Professor, Anna University
development of dielectric materials, etc., )
Different forms of high voltages and currents mentioned earlier are classified as:
High DC voltages
ii)High AC voltages of power frequency
Dr M A Panneerselvam, Professor, Anna University

5. Dr M A Panneerselvam, Professor, Anna University
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

6. 3.1 GENERATION OF HIGH DC VOLTAGES Half wave rectifier circuit:
Dr M A Panneerselvam, Professor, Anna University

7. Dr M A Panneerselvam, Professor, Anna University
VOLTAGE AND CURRENT WAVEFORMS
Dr M A Panneerselvam, Professor, Anna University

8. Dr M A Panneerselvam, Professor, Anna University
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

9. Dr M A Panneerselvam, Professor, Anna University
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

10. Dr M A Panneerselvam, Professor, Anna University
FULL WAVE RECTIFIER CIRCUIT
Dr M A Panneerselvam, Professor, Anna University

11. Dr M A Panneerselvam, Professor, Anna University
SIMPLE VOLTAGE DOUBLER CIRCUIT
Dr M A Panneerselvam, Professor, Anna University

12. Dr M A Panneerselvam, Professor, Anna University
CASCADED VOLTAGE DOUBLER CIRCUIT
Dr M A Panneerselvam, Professor, Anna University

13. Dr M A Panneerselvam, Professor, Anna University
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

14. Dr M A Panneerselvam, Professor, Anna University
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

15. Dr M A Panneerselvam, Professor, Anna University
CASCADED RECTIFIER UNIT COCKROFT WALTON VOLTAGE WITH PULSE GENERATOR MULTIPLIER CIRCUIT
Dr M A Panneerselvam, Professor, Anna University

16. Dr M A Panneerselvam, Professor, Anna University
900 kV COCKROFT WALTON DC GENERATOR
Dr M A Panneerselvam, Professor, Anna University

17. Dr M A Panneerselvam, Professor, Anna University
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

18. Dr M A Panneerselvam, Professor, Anna University
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

19. Dr M A Panneerselvam, Professor, Anna University
discharging points which are at some 10 kV from earth potential. The belt ,having width varying between cm to metres, is driven at about 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

20. Dr M A Panneerselvam, Professor, Anna University
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

21. Dr M A Panneerselvam, Professor, Anna University
ELECTROSTATIC BELT DRIVEN GENERATOR
Dr M A Panneerselvam, Professor, Anna University

22. Dr M A Panneerselvam, Professor, Anna University
Voltage developed in Van de graff Generator:
Dr M A Panneerselvam, Professor, Anna University

23. Dr M A Panneerselvam, Professor, Anna University

24. 3.2 GENERATION OF HIGH AC VOLTAGES AT POWER FREQUENCY
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

25. Dr M A Panneerselvam, Professor, Anna University
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

26. Dr M A Panneerselvam, Professor, Anna University
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

27. Dr M A Panneerselvam, Professor, Anna University
CASCADED TRANSFORMER CONNECTION ( SCHEMATIC )
Dr M A Panneerselvam, Professor, Anna University

28. Dr M A Panneerselvam, Professor, Anna University
CASCADED TRANSFORMER UNIT ( IREQ, CANADA)
Dr M A Panneerselvam, Professor, Anna University

29. Dr M A Panneerselvam, Professor, Anna University
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

30. Dr M A Panneerselvam, Professor, Anna University
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

31. Dr M A Panneerselvam, Professor, Anna University
RESONANT TRANSFORMER EQUIVALENT CIRCUIT
Vc = -jVXc/ R+j(Xl- Xc) = XcV/R= V/ωCR
Dr M A Panneerselvam, Professor, Anna University

32. Dr M A Panneerselvam, Professor, Anna University
2.2 MV SERIES RESONANT CIRCUIT
Dr M A Panneerselvam, Professor, Anna University

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.
Dr M A Panneerselvam, Professor, Anna University

34. Dr M A Panneerselvam, Professor, Anna University
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

35. Dr M A Panneerselvam, Professor, Anna University
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

36. Dr M A Panneerselvam, Professor, Anna University
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

37. Dr M A Panneerselvam, Professor, Anna University
EQUIVALENT CIRCUIT OUTPUT WAVEFORM
TESLA COIL
Dr M A Panneerselvam, Professor, Anna University

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 difference
of two exponential waveforms as
Dr M A Panneerselvam, Professor, Anna University

39. Dr M A Panneerselvam, Professor, Anna University
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

40. Dr M A Panneerselvam, Professor, Anna University
i) Full impulse:
Dr M A Panneerselvam, Professor, Anna University

41. Dr M A Panneerselvam, Professor, Anna University
ii) Chopped impulse:
Dr M A Panneerselvam, Professor, Anna University

42. Dr M A Panneerselvam, Professor, Anna University
iii) Front of wave impulse:
Dr M A Panneerselvam, Professor, Anna University

43. Dr M A Panneerselvam, Professor, Anna University
IMPULSE VOLTAGE WAVEFORM
Dr M A Panneerselvam, Professor, Anna University 43

44. Dr M A Panneerselvam, Professor, Anna University
SCHEMATIC DIAGRAM OF MARX CIRCUIT ARRANGEMENT FOR MULTISTAGE IMPULSE GENERATOR
Dr M A Panneerselvam, Professor, Anna University

45. Dr M A Panneerselvam, Professor, Anna University
MULTISTAGE IMPULSE GENERATOR INCORPORATING SERIES AND WAVE TAIL RESISTANCES WITHIN THE GENERATOR
Dr M A Panneerselvam, Professor, Anna University

46. Dr M A Panneerselvam, Professor, Anna University
MULTISTAGE IMPULSE GENERATOR CONNECTED TO POTENTIAL DIVIDER,MEASURING SPHERES AND LOAD
Dr M A Panneerselvam, Professor, Anna University

47. Dr M A Panneerselvam, Professor, 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

48. Dr M A Panneerselvam, Professor, Anna University
RESISTANCE ‘R2’ ON THE GENERATOR SIDE
Dr M A Panneerselvam, Professor, Anna University

49. Dr M A Panneerselvam, Professor, Anna University
Taking the circuit in Fig. (a) ,
Dr M A Panneerselvam, Professor, Anna University

50. Dr M A Panneerselvam, Professor, Anna University

51. Dr M A Panneerselvam, Professor, Anna University
IMPULSE WAVE AND ITS COMPONENTS
Dr M A Panneerselvam, Professor, Anna University

52. Dr M A Panneerselvam, Professor, Anna University
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

53. Dr M A Panneerselvam, Professor, Anna University
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

54. Dr M A Panneerselvam, Professor, Anna University
The following table shows the result for some selected wave shapes:
Dr M A Panneerselvam, Professor, Anna University

55. Dr M A Panneerselvam, Professor, Anna University
Triggering of impulse Generators:
TRIGATRON SPARK GAP
Dr M A Panneerselvam, Professor, Anna University

56. Tripping of Impulse Generator with three electrode gap:
TRIPPING OF IMPULSE GENERATOR WITH A THREE ELECTRODE GAP
Dr M A Panneerselvam, Professor, Anna University

57. Dr M A Panneerselvam, Professor, Anna University
2.4 MV IMPULSE GENERATOR
Dr M A Panneerselvam, Professor, Anna University

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.
Dr M A Panneerselvam, Professor, Anna University

59. Dr M A Panneerselvam, Professor, Anna University
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

60. Dr M A Panneerselvam, Professor, Anna University
CIRCUITS FOR GENERATING SWITCHING SURGE VOLTAGES WITH OUTPUT WAVEFORMS ACROSS THE LOAD CX
Dr M A Panneerselvam, Professor, Anna University

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
Dr M A Panneerselvam, Professor, Anna University .

62. Dr M A Panneerselvam, Professor, Anna University
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

63. Dr M A Panneerselvam, Professor, Anna University
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

64. Dr M A Panneerselvam, Professor, Anna University
BASIC CIRCUIT OF AN IMPULSE CURRENT GENERATOR
Dr M A Panneerselvam, Professor, Anna University

65. ARRANGEMENT OF CAPACITORS FOR HIGH IMPULSE CURRENT GENERATION
Dr M A Panneerselvam, Professor, Anna University