Breakdown Cha of Air Gaps Complete

BREAKDOWN CHARACTERISTICS OF AIR GAPS

Name

: T. M. N. P. Jayawardana Jayawardana

Course

: IESL

Group

:G2

Field

: Electrical

Instructed by

: Mr. H. L. R. Gunawardana

Date of performance: 28/01/2012

AIM: 1.To study the Ferranti HVAC generators 2.To determine the breakdown characteristics of sphere-sphere ,plate-plate & rod-rod gaps under AC high voltages

APPARATUS: 1.2MΩ Resistor  2.Foster & Ferranti HV generators 3. Sphere-sphere ,plate-plate , rod-rod gaps

PROCEDURE:  

Studied about the control panel & the methods of operation of the high voltage generators. Adjusted the gap setting of the plate  – plate gap from 0 .5cm to 4.5cm in steps of 0.5cm & obtained the breakdown voltage for positive d.c high voltages.



Repeated the experiment for negative d.c & a.c high voltages.



Followed the same procedure for plate-plate & sphere-sphere gaps.

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Noted down the atmospheric pressure & temperature.

Correction For temperature & pressure

The readings obtained must be corrected for standard temperature & pressure. V- Breakdown voltage at a pressure P & temperature T Vn-Breakdown voltage at s.t.p V =δVn δ – gas density correction factor

δ=





 

 

P is in mm Hg And t in deg.centigrate

SPECIMAN CALCULATION : δ=

 



 



= 0.9543

For the Rod at 0.5 Gap spacing : 0

Breakdown Voltage at pressure 750 Hgmm and Temperature 30 C at Primary voltage 20 V V= 0.9543x 20x 

 



=8.298 kV

TABULATION : Primary Breakdown voltage (v)

Secondary Breakdown voltage (kV)

Gap Spacing (cm)

Rod

Plate

Sphere

Rod

Plate

Sphere

0.50

20

11

19

8.298

4.564

7.883

1.00

24

20

46

9.958

8.298

19.086

1.50

26

39

58

10.788

16.182

24.065

2.00

31

55

76

12.862

22.820

31.533

2.50

34

72

90

14.107

29.874

37.342

3.00

42

80

102

17.426

33.193

42.321

3.50

53

94

124

21.990

39.002

51.449

4.00

59

105

138

24.480

43.566

57.258

4.50

63

115

158

26.140

47.715

65.556

Breakdown voltage Vs Gap distance for Rod, Plate and Sphere

kV 70.000

60.000

50.000

40.000

Rod Plate

30.000

Sphere

20.000

10.000

0.000 0.00

1.00

2.00

3.00

4.00

5.00

cm

DISCUSSION : 1.HV Lab Arrangement.

Ferranti A. C. Equipment

Isolator Closed

Interlocks complete

L. V. Volts

L. V. AMPS

Contactor Closed

Peak  RMS kV H V On

H V Off 

Isolator High

Low Lover volts

Rise Volts

The lab is located in two floors. The control panel and protection equipment are located in the upstairs. The high voltage test equipment are located at the down floor. This arrangement is made in order to give adequate air gap space between livestock and high voltage source during the tests. The high voltage area is covered by a metal net which is grounded in order to stop HV sparks discharging outside and to avoid entrance during the test period. The door of entrance door is protected with a protective plug which is a relay connected to the HV equipment with a relay normally open. The HV equipment can only be started by closing this plug which mean closing the entrance door. 2.Safety precautions 







All the HV generators are protected with safety relays in order to give the maximum protection. It automatically disconnect immediately after a HV discharge during an experiment. Also the plug connected to the entrance door to the test arena is connected to a safety relay which will open circuit the HV power supply when the door is open. The earthing rod is always kept across the entrance door. After entering the arena, the test equipment is earthed using this earthing rod to neutralize the stray static charges which may cause static electric shock if not neutralized. The test arena is covered with an earthed metal net in order to avoid discharges outside the test arena. This earthed metal net produces an shield which blocks out external static and non-static electric fields. The control room is situated in upstairs in order to provide adequate air gap between test area and the control room. This is essential to provide protection from electric discharges causing damage to the control area.

3. HV AC generation.

Arrangement of transformers in Ferranti test circuit. The Ferranti test circuit arrangement is using two 230V/100kV voltage transformers in series mode. By using these two transformers a total HV of 200kV is achieved. Isolation transformer is used to isolate the HV side from the primary side. By isolating HV side, an electric shock is only occurs if both phase and neutral ends of the HV side are touched. No electric shock due to earth leakage is avoided. 4. Need of 2MΩ resistor. The voltages related with the gaseous breakdown are very large, typically about 30kV/cm stress. Since the uniform increase in voltage used in the test, it is more probable to occur avalanche breakdown which develops over relatively long periods of time. It is typically more than 1us. The discharge current at the breakdown is suddenly increased due to avalanche effect. This current should be controlled for the safety of the equipment. Hence the 2MΩ resistor is used in order to limit the current at the breakdown.

5. Factors that affect the breakdown voltage of air for a given pair of electrodes. i.

Electrode separation.

For the gaps less than 1mm the breakdown voltage is nearly proportional to the length of the air gap. The break down stress is high about 1MV/cm. For the gaps greater than 1mm it is the breakdown voltage is approximately proportional to the square root of the gap length and the breakdown stress relatively low about 10kV/cm. Generally it can be expressed as,

    Where

x=1 for d<1mm x=0.5 for d>1mm

d- air gap length V- breakdown voltage k-constant. ii.

Electrode conditioning

At the beginning of a new electrode flash over, the breakdown voltages are relatively low and increase gradually. Then they attain a constant value of breakdown voltage. This is known as conditioning of  the electrode. This is shown in the following graph.

This occurs because the irregularities and impurities of the electrode reduces with the flash overs. The starting breakdown voltage can be about half of the conditioned breakdown voltage. iii.

Material and Surface finish. The smoother electrode surface finish results in high breakdown voltages and rough surface finish results in low breakdown voltages. The material of the electrode also directly affects the breakdown voltage.

iv.

Surface contamination The surface contamination of electrode can reduce the breakdown voltage about 50%.

v.

Area and configuration of electrodes.

The increase in the area of the electrode slightly reduces the breakdown voltage. Up to 1 mm gap, the more convex electrodes have higher breakdown voltage than the more nearly plane electrodes even though at the same voltage they carried a higher electric field at the surface. vi.

Temperature

Cooling the electrodes increases the breakdown voltage. But for iron and nickel electrode the o breakdown voltage remains constant for about 500 C. vii.

Pressure.

For small air gaps the reduction in pressure increases the breakdown voltage. Below a certain pressure There is no change in breakdown voltage. For large air gaps about 200mm below certain pressure limit, the breakdown voltage starts to decrease again.

6. Sphere gap method of measuring high voltages. Ionization of gas molecules and the gas density affects the breakdown strength of the gas. The breakdown voltage also varies with the gap spacing. Because of the high consistency of breakdown voltage for uniform air gap, the sphere gap can be used as a measuring device. Very accurate charts have prepared based on the experiments for different diameters and distances of air gap. Two metal spheres separated by a gas gap is used as a measuring device. Voltage difference between two spheres is increased until a spark passes among the spheres. This device can be used to measure the peak value of a voltage wave and for checking and calibrating of voltmeters.

The accuracy of the of the reading varies with the ratio between d and D as follows.

Hence to obtain more accurate results gap distance lower than 0.75D are used.