OBSERVATION SHEET

Observed waveform:

Name :- Kaushalya K.H.A

Index No :- 090254L

Practical :- High Voltage Impulse Generator

Group :- G 7

Date of Prac: :- 08.11.2013

Instructed By :- Mr.Chinthaka Kodithuwakku

Sphere Gap (mm) Observed Voltage (kV)05 0.510 4.515 6.020 2025 2430 3035 3140 34

Graph

Sphere Gap (mm) Observed Voltage (kV) Actual Voltage (kV) (Observed Voltage × 6)

5 0.5 3

10 4.5 27

15 6.0 36

20 20 120

25 24 144

30 30 180

35 31 186

40 34 204

Breakdown Voltage (kV) Vs. Sphere Gap (mm)

0 5 10 15 20 25 30 35 40 450

50

100

150

200

250

Sphere Gap (mm)

Brea

kdow

n Vo

ltage

(kV)

Calculations

CS – Surge Capacitance = 0.25µF

C0 – Shunt Capacitance = 0.003µF

R1D – Internal Damping Resistance

R1 – Charging Resistance

Rout –Wave Tail Resistance

Defining C1 (as there are 6 capacitors):

C1=C s6

=0.25 μF6

=0.042 μF

Defining C2:

C2=Co=0.003 μF

Defining R1:

R1=6×15+180=270Ω

Defining Rout:

1Rout

= 12000

+ 15000

Rout=¿

Efficiency (η)

η =C1

C1+C2

= 0. 0420 .042 + 0 . 003

¿0.933=93.3%

Wave front time (Tf)

Wave front is considered from 30% to 90%

T r = 3 . 243⋅η⋅R1⋅C2

= 3 .243×0 . 933×2 70×(0 . 003×10−6 )

¿2.45 μs

Wave tail time (Tt)

A=π

¿44.57 μs

Stored Energy at Maximum Voltage

Tt = 0 .693⋅Rout⋅C1

η

= 0 .693×1428 . 6×(0 . 042×10−6 )0 .933

1. Maximum Voltage

Emax=Vmax⋅C1

C1+C2

=300× 0 . 0420 .042+0 .003

¿280kV

2. Maximum Energy

Pmax=12C sV max

2+ 12C0 Emax

2

¿2007.6 J

Peak inverse voltage of the diode

Peak inverse voltage of the diode = 30√2 kV

= 51.96kV

Discussion

1. Charging and discharging processes of impulse generator

Impulse generators basically consist of an array of capacitors (accompanied by resistors and spark gaps) which are charged in a parallelconfiguration to a voltage "E" and then discharged in series with a voltage of "nE" where "n" in the number of capacitors charged.

The initial charging of the capacitors is done via a controlled current source by the use of a high voltage step up transformer and a full bridge rectifier. These capacitors once charged would be discharged in a series configuration through special precisely-spaced spark gap switches for each capacitor. The breakdown of the controlling sphere gap occurs first and it initiates the triggering of the other sphere gap. By changing the gap distance between the controlling spheres, it is possible to change the magnitude of the breakdown voltage.Selection of the capacitors will determine the peak current and rate of current rise during the discharge cycle. Not only the value of capacity and voltage is selected, but the discharge loop inductance and peak current handling is also considered.

The operation of the impulse generator could be explained in two ways according to the breakdown of sphere gaps.

Uncontrolled operationIn the uncontrolled operation, the break down voltage of the sphere gap is less than the peak value of the supply, so that it effectively closes when the voltage across the gap builds up above its breakdown value. The capacitorwould then discharge through the impulse generator circuit producing an impulse waveform. The impedance ofthe impulse generator charging circuit is much higher than that of the impulse generator circuit so that during theimpulse the rectifier and other related components can be disregarded.

Controlled operationIn the controlled mode of operation, the same basic circuit is used, but the capacitor is allowed to reach the full charging voltage without the sphere gap breaking down. The spark over voltage is set at slightly higher than thecharging voltage. In this case, at the sphere gap a special arrangement is used, such as a third sphere betweenthe other two, to be able to initiate breakdown of the gap.The potential across the main gap is divided into two by means of 2 equal resistors R, each of about 100 MΩ. By this means, half the applied voltage V appears across each of the two auxiliary gaps.Once the capacitor C1 has charged up to the full value, a small pulse voltage v is applied (about 20 %) at the third electrode (also known as the trigger electrode). This pulse raises the voltage across one of the auxiliarygaps to more than half the charging voltage (½ V + v)

so that it would be just sufficient to breakdown the gap.As this auxiliary gap breaks down, the full voltage would be applied across the remaining auxiliary gap causing it also to breakdown.Once both auxiliary gaps have broken down, the ionisation present in the region would cause the main gap also to breakdown almost simultaneously and thus the impulse voltage would be applied.

2. Diagram of the charging unit and details of its operation

This consists of a high voltage step-up transformer with a rectifier assembly connected to form a full wave bridge circuit. So it does not require any capacitors for voltage doubling. The circuit uses compensated selenium rectifiers which are selected to withstand high current surges, voltage reversals and flashovers. Protective resistors are used to limit the current to a safer limit of rectifiers in case of a short circuit.

3. Layout of the control panel of the impulse generator

The control panel for the impulse generator mainly consists of a dial and ammeter and voltmeter interfaces. The input voltage to impulse generator until the point of breaking down is controlled by the dial on the control panel. By slowly turning the dial clockwise the applied input voltage to the impulse generator could be gradually increased. For input voltages and input currents, readings are taken from the voltmeter and ammeter. The buildup of voltage at the sphere gap could be observed in the voltmeter and at the point of breakdown a sudden drop of voltage could also be observed.

4. Important features of oscilloscopes used for the study of fast transient phenomena in the work on high voltage and on spark breakdown in small gaps

Ability to capture wave forms which appear in substantially short time period. This feature is useful in measuring impulse voltages as the occurrence of an impulse takes place in microsecond range, it is impossible to measure the voltage values using ordinary measuring equipment.

The sampling frequency of the oscilloscope could be adjusted to be high enough to capture the waveform with a greater accuracy.

Oscilloscopes have the capability of saving the waveforms to an external storage device for further analysis and later reference.

5. Layout of the potential divider used in the laboratory and various types of potential dividers

Potential dividers are important when the oscilloscope is used to monitor the high voltage transients as it is essential to reduce the impulse voltage. Potential dividers can be found as capacitive or resistive.

Resistive potential dividerIn this method, a high resistance potential divider is connected across the high-voltage winding, and a definite fraction of the total voltage is measured by means of a low voltage voltmeter. The ratio of the divider is determined by the sensitivity of the oscilloscope.

Capacitive potential dividerIn this method, two capacitances C1and C2 are used in series, the electrostatic voltmeter being connected across the lower capacitor and connected to the oscilloscope.

EE 4192: Laboratory Practice VIII

High Voltage Impulse Generator

Name : K.H.A.Kaushalya

Index No : 090254L

Group : G-07

Date of Practical : 08.11.2013

Date of Submission : 23.01.2014

Instructed By : Mr. Chinthaka Kodithuwakku