Breadown of Air P Dasanayake

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EE3092 -Laboratory Practice V

BREAKDOWN CHARACTERISTICS OF

AIR GAPS

Instructor:- Mr. L.S.N. Bambarawane

Name: - V.I.P. Dasanayake

Index Number: - 090075M

Group: - G3

Submission date: -03/04/2012



Calculations

Considering given equations;

Vn =

V-Breakdown voltage at given pressure and temperature

Vn-Breakdown voltage at standard pressure and temperature

– Gas density correction factor

Pressure (P)- 750 mmHg

Temperature (T)- 300C

= 0.9543

Specimen calculation to calculate real breaking down voltage;

Considering the sphere – rod to rod with 1.5 cm air gap;

Vn =

Gap

spacing

(cm)

Rod Plate Sphere

Observed-

230v

Actual-

100kv

Observed-

230v

Actual-

100kv

Observed-

230v

Actual-

100kv

0.5 15 6.834 16 7.290 22 10.023

1 24 10.934 28 12.757 42 19.135

1.5 31 14.124 39 17.769 60 27.336

2 37 16.857 57 25.969 76 34.626

2.5 39 17.769 73 33.259 87 39.638

3 42 19.135 75 34.170 111 50.572

3.5 55 25.058 93 42.371 124 56.495

4 59 26.881 102 46.472 136 61.962

4.5 61 27.792 111 50.572 - -



Breaking down of air gaps

Gap spacing (cm)Vn(kV)

Rod Plate Sphere

0.5 6.834 7.290 10.023

1 10.934 12.757 19.135

1.5 14.124 17.769 27.336

2 16.857 25.969 34.6262.5 17.769 33.259 39.638

3 19.135 34.170 50.572

3.5 25.058 42.371 56.495

4 26.881 46.472 61.962

4.5 27.792 50.572

0.000

10.000

20.000

30.000

40.000

50.000

60.000

70.000

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

B r e a k i n g d o w n v o l t a g e ( k V )

Gap spacing(cm)

Rod

Plate

Sphere



Discussion

1. High Voltage Lab arrangement

High voltage lab is consisted with two main parts, as controlling section

(figure 1) and testing area (figure 2) which is situated at upstairs and downstairs respectively.

When it comes to the ground floor, there are lots of safety precaution are being taken

to avoid any harm to any equipment in the room or to any live stock. Since this a place where deal with

high currents and flashovers from breakdown of air during the testing, it may dangerously affect to live

stock.( human body can withstand for only about 50V, but here we are dealing with kilo Volts). Some

of the safety precautions which are being taken are as follows.

Figure 1

Figure 2



Mainly the control room and the testing are separated, so that there is a huge distance from the

experiments doing in testing area, to ensure the operator’s safety.

All the testing area is being earthed for the protection of live stocks and all the equipments.

Experiment are done in the high voltage lab, huge currents may passed through air. To prevent

any harm from those to any live stock and avoid any other unnecessary touching, there is a

metal railing which is earthed perfectly. And it is made such that, a plug is set to front door of

the testing area to cut off the main power supply from the controlling panel, so we can make

sure that plug connection is done after everybody in the room left

There is a metal rod which covered with a insulation, and perfectly earthed. This is used to

discharge any charge on the conductors. (this is normally done , before practical is begin)

2. Method of HV AC generation

Normally in these types of transformers, need to be insulated for about 100kV. These are

normally step up transformers comes with cascade type. If the voltage is higher value than 100kV, then

low voltage winding is connected to the primary of the first transformer, which is earthed. Using a

tapping system to the secondary side of the first transformer, low value is given to the primary side of

the second transformer and this transformer is not earthed. The rest of the transformers are also not

earthed And this process is done continuously until meet the requirement ( Ex:-if want a higher

voltage). And that make the cascade system. This cascade system is done, so that the current in the

secondary will be low compared to the primary side. So the required cross section of the conductor

will be less, which means the cost is reduced.

Another advantage of this system is that, we don’t need to transport a huge transformer with a

high cost. Since the cascade system is using, no need to transport huge transformers, so cost for

transportation will be reduced. (When the transformer use higher ratio of voltage the price for that also

increase)

3. Reason to use a 2MΩ resistor

When the voltage is increasing, after some value breakdown will occur. In the breakdown the

discharging current will increase suddenly, and it may cause for overheat the winding of the

transformer. So 2MΩ additional resistor is connected series to the current flow, so that current will be

limited. So cost for improving insulation and limiting current will be reduced.



4. Factors affect for the breakdown voltage of the air for a given pair of electrodes

Pressure

At first when the pressure is increasing breaking down voltage is reduced since the

electron collision is much easier when the electrons are very close to each other. But for further

increasing of pressure, (for very high pressure) the collision is very difficult since electrons are very

close to each other.

Temperature

When the temperature in increasing, electrons absorbed the thermal energy

and they will be emitted from the valance band of the molecule. So with increasing temperature,

breaking down voltage also increased

Electrode separation.

With the increasing gap, the path required to spark to happen will be increasing, it is

like resistance increasing with length of a rod, when the gap between rods is increasing the breaking

down voltage is also increasing.

Area of the electrode

When the area of the electrode is increasing, breaking down voltage is reducing, because when

the area is increasing the probability of happening a discharge between rods are high.



Surface contaminationIf the surface contamination is high, breaking down voltage will be reduced

Surface finish.If the surface is rough or surface with edges, breaking down voltage will

be low. Since the probability if the emitting electrons from rough surfaces and pointed edges are high.

5. Measuring of HV (Sphere Gap method)

The basic idea of this measuring the breakdown voltage between two sphere for given

condition and for standard condition. Using Sphere Gap method for Measuring of HV break down is

the most common method since it is very reliable, so it can be use for calibration easily. In the standard

level there are even measured values for given gap spaces in tables.

The breakdown strength will depends on sphere size, distance between them .etc

normally spark gap id used for determine the voltage peak value, for the calibration of voltage

measuring devices.

The schematic diagram for air breakdown voltage test using standard sphere gap method is shown

below.

Following figure 3 shows the breakdown voltage characteristic of sphere gaps.

When the gap between two spheres is increased, the uniform field between two spheres is getting

distorted, so accuracy falls.

(P.T.O)

Figure 3

Schematic Circuit showing the source and Breakdown voltage measuring unit

using standard



Some of the graphs of breaking down voltage variation with some factors are given below.

References

http://www.allaboutcircuits.com/vol_1/chpt_12/8.html

A Thesis submitted in partial fulfillment of the requirements for the degree of Master of

Technology in Power Control and Drives by PARASELLI BHEEMA SANKAR (Roll No:

209EE2168)

Measurement of High Voltage lecture note by Prof. J.R. Lucas

Variation of sphere radius and air breakdown voltage Variation of sphere radius and electric fielddistribution




Breadown of Air P Dasanayake

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