10_chapter 4

91

4.3 DESCRIPTION OF APPARATUS

4.3.1 POINT PLANE ELECTRODE SYSTEM ARRANGEMENT

The plane electrode is made up of mild steel and has very smooth

finished surface [Fig. 4.1]. The point electrode is a hemispherically

capped cylinder [Fig. 4.2 ]. The different materials used for point

electrode are stainless steel, copper [Fig. 4.3], brass [Fig. 4.4 ], silver

plated brass [Fig. 4.5], chromium plated brass [Fig. 4.6 ] and nickel

plated brass[Fig. 4.7]. The diameter of point electrode used are 1mm,

1.5mm, 2mm, 3mm, 4mm and 5mm for copper ; 1mm, 1.5mm, 2mm and

3mm for stainless steel, 1mm, 1.6mm, 2mm, 2.5mm, 3mm, 4mm and

5mm for brass. The minimum length of hemispherically capped electrode

used in experiments is 130mm [ > 2.5 gap length ]. This electrode is

mounted horizontally using brass holders [Fig. 4.8] 50mm in length and

with one side rounded off to a radius of 12.5mm and all surfaces very

smooth finished. The length of the brass holder is 50mm [Fig4.9]. The

inner portion of the brass holder is threaded to accommodate an

aluminum rod which in turn is supported by vertically placed 8.27cm dia

stands. The horizontal axis of the point plane gap is at a height of 80cms.

This is achieved by use of vertical insulating end supports. [Fig.4.10] The

plane electrode is brazed on to a brass holder[Fig. 4.11] with ½ “BSW

threads in the inner portion, which in turn is supported by vertically

placed 8.27cm dia stands of length 80cm which in turn is placed on a

wooden table.

92

4.3.2 HIGH VOLTAGE D.C. SUPPLY

The high voltage D.C. supply is obtained from voltage doubler circuit

arrangement. The rating of HV transformer used is 240V / 100KV /

5KVA [Fig. 4.12]. The HV winding of transformer has small number of

turns near the ground end

HVfgryrrghthrtrhtrthhhhdndn4trRFGRYFGRGR of the HV winding which

is used for measurement of HVAC output voltage. The LV side of the HV

transformer is supplied through an oil filled 240 V / 0 – 240 V Variac

with all controls [Fig.4.13]. The circuit diagram of voltage doubler circuit

is shown in Fig. 4.26. A current limiting resistance of 30 kiloohms, 200

watts [ non inductive type ] has been used in series with the HV

electrodes in all the experiments [Fig. 4.12]. The connections to the

capacitors from HV rectifiers [Fig. 4.15] are made through corona free

arrangements made from aluminum material. A suitable set of corona

free connectors facilitate changeover of polarity of D.C supply to point

plane electrode system [Fig. 4.15 & 4.16].

The output voltage of HVDC test set is connected to an accurate

HVDC measuring arrangement. The HV arm of the HVDC measuring

arrangement is a compensated type HV resistance [Fig. 4.17]. The

measured resistance value is 29.3 Mega ohms. The HVDC resistor in

series with an accurate reading DC milliammeter [Fig. 4.18] was used for

accurate measurement of HVDC voltage applied to HV electrode of the

point plane experimental electrodes. As a verification procedure, a

93

100mm diameter sphere gap assembly[Fig. 4.19] has also been used for

checking the HVDC output voltages.

4.3.3 THE ULTRAVIOLET LIGHT SOURCE

A mercury discharge lamp [ Fig. 4.20 ] 125 watt, 200-250V (1N.HPL-

N BC) Philips make was used to provide ultraviolet radiation required to

release the initial photoelectric current IO from the cathode. The

ultraviolet lamp was mounted at a distance of 30 cm from horizontal axis

of the gap and was directed to the entire gap and point electrode. An

initial warming up period of 30 minutes was allowed for stabilization.

4.3.4 DETAILS OF DIGITAL STORAGE OSCILLOSCOPE

Yokogawa make (window based ) DSO [ Fig. 4.21] was used for

recording the corona pulses. The details of the oscilloscope are:

Model number : 9140L(701311)

Maximum sampling rate : 5 GS/s

Frequency bandwidth : 1 GHz

Input channels :4 (CH1 to CH4)

Input impedance :1 MΩ ±1.0% in parallel with 20 pF

Capacitance

Voltage axis sensitivity : 2mv/div to 5V/div (1-2-5 steps)

Vertical axis accuracy :For 1MΩ INPUT:±1.5% of 8 divTime base accuracy

:± 0.001%

Display : 8.4 inch(21.3cm) color TFT liquid

Display

94

Storage :Built-in memory media

Media type :Compact flash

Capacity :32 MB

USB peripheral interface connector

Connector type :USB type A connector(receptacle)

PB 500 (10:1 passive probe) :10MΩ *10

4.4 GENERAL DESCRIPTION OF LABORATORY CONDITIONS

The laboratory is situated about 4 KM from the main traffic road. The

area is almost totally free from disturbances and the atmospheric

conditions are steady without abrupt changes. Experiments have been

conducted during the time periods 11.0 a.m to 5.0 p.m of the day during

which disturbances and fluctuations are minimum. Only the days on

which fair weather conditions prevailed have been selected for carrying

out experimental investigations. However, as a precautionary measure,

atmospheric conditions- pressure, dry bulb temperature and wet bulb

temperature have been recorded at every two hours period while carrying

out experimental investigations.

4.5 MEASUREMENT OF INITIATORY CURRENT IO

An Electrometer (Keithley make) has been used for measurement of

initiatory current. A known gap was set up and the connection made

with input supply to UV lamp, HVDC supply and Electrometer and the

output voltage to point electrode was set at zero value. The whole system

was maintained under these conditions for 30 minutes for allowing time

95

duration for stabilization of the experimental setup. After stabilization,

initiatory current was measured by applying a voltage of 1KV to HV

electrode (i.e., point electrode). These measurements were carried out for

several gap spacings and several diameters of the point electrode. The

average value of measured current was same and the current did not

vary with gap spacing or change in radius of hemispherical electrode.

The magnitude of the measured average initial current is 1*10-12 Amp.

96

4.6 DESIGN AND CONSTRUCTION OF HV RESISTANCE WITH

VOLTAGE BALANCING CAPACITORS

It consists of 15 sections of (330KΩ *3 ) resistances connected in

parallel with 0.01 microfarad 2000V rated voltage balancing capacitors.

Component of each section is tested for 2000volts DC, one minute

withstand. Only good components, which did not vary in value more than

±1% are selected for building the voltage divider. Before testing, each

resistor is thoroughly cleaned using Trichloroethylene (to remove grease,

moisture and soluable materials ) and a coating of epoxy resin is applied

over external surface and allowed to set for 24 hours.

The complete HV resistance is built using above tested components

and assembled inside a insulating tube with leads suitably brought out.

The space inside the insulating tube is filled with good quality

transformer oil. The above HV resistor is used along with an accurate

reading D.C. milliammeter and with calibration resistors for accurate

measurement of HVDC voltages upto 15 KV. The schematic circuit

diagram of the arrangement is shown in Fig. 4.22

4.7 VERIFICATION OF OUTPUT VOLTAGE INDICATED BY HVDC

METER USING 100MM DIA STANDARD SPHERE GAP

ASSEMBLY

The connections were made as shown in Fig. 4.24. The surface of

both the spheres ( 100 mm dia ) were cleaned using a good clean dry

cloth. The surface of both the spheres were again cleaned using cloth

97

dipped in Trichloroethylene. The atmospheric pressure in mm of

mercury, dry bulb and wet bulb temperatures were noted. Using the

standard slip gauge ( Accurately machined thickness blocks ), the gap

distance between the two spheres was adjusted to 5 mm. A UV lamp was

used for irradiation of the gap and it was kept at a safe distance from the

spheres such that the mounting of the UV lamp does not influence the

sparkover characteristics of the standard sphere gap assembly. The

output voltage of the DC set was varied till the sparkover of the sphere

gap occurred. Just at sparkover the reading indicated by HVDC meter

was recorded. The experiment was repeated 5 times at intervals of 1

minute. The average value of 5 readings indicated by the meter was

compared with the sparkover value given in the standards corresponding

to the gap spacing after the correction factor [Table 4.1 ]. This showed

very good agreement within the accuracies of measurement.

TABLE 4.1

RELATION BETWEEN CORRECTION FACTOR K AND AIR DENSITY

FACTOR d

d 0.70 0.75 0.80 0.85 0.90 0.95 1.0 1.05 1.10 1.15

K 0.72 0.77 0.82 0.86 0.91 0.95 1.0 1.05 1.09 1.12

99

4.8 MEASUREMENT OF CORONA INCEPTION VOLTAGES

[ POSITIVE POLARITY]

The schematic diagram of connection for measurement of corona

inception voltages in point plane gaps is as shown in fig 4.23. For

measurement of current at inception, an accurate digital multimeter [ 4

½ digit, LCD ] with an accuracy of 0.5% was used. Initially, the point and

plane electrodes were cleaned using Trichloroethylene. Using corona free

connectors, the connections were made so as to connect positive DC

supply to point electrode Fig. 4.15. The laboratory dry bulb, wet bulb

temperatures and pressures were noted down. Using accurately

machined thickness blocks corresponding to required gap spacing, the

point plane gap was set to a desired value, say 10mm. The voltage

applied to point electrode was steadily increased untill the corona

current measurement meter indicated current of 1.0 microamperes.

The voltage corresponding to corona onset current of 1 microampere

has been considered as corona onset potential (In few cases corona onset

current was less than 1.0 μA. However, the difference between the

voltage at actual inception and that required for corona current to be 1.0

μA was within measurement accuracy ). At inception, oscillograph

showed current pulses of varying magnitude occuring at different

intervals of time. At near 1.0 µA current as indicated by the DC

microammeter, the pulsed behaviour of corona was relatively lower. For

each gap spacing, five readings of inception potentials were recorded at

100

one minute time interval. The average of five readings is taken as corona

inception potential magnitude.

The experiment was repeated for different gap spacing between point

and plane electrode. Further, the experiments were also conducted for

different radii of hemispherical tip of the point plane gap and for different

materials of point electrode. The experimental results are tabulated in

Table 4.2. The tabulated corona inception voltages are values at STP

which are values after applying air density and humidity correction

factors to measured values. If Va is the corona inception voltage at

prevailing atmospheric conditions, the corona inception voltage at STP is

given by Vs=Va*h/d where h is humidity correction factor and d is air

density correction factor given by d=0.386p/(273+Td). Humidity

correction factor „ h „ is obtained from the recorded temperatures of wet

and dry bulb thermometers and by using the graphs provided in the

standards which are shown in Fig.s 4.25 and 4.26.

4.9 MEASUREMENT OF CORONA INCEPTION VOLTAGES ( NEGATIVE

POLARITY )

Using the HVDC supply set up for negative polarity (Fig. 4.16 ), the

experiments were conducted for measurements of corona inception

voltages in a similar way as described in above section. However, the

experiment was repeated by irradiation of the gap with the help of a UV

lamp placed at a distance 30cm from the gap. At onset, negative corona

consisted of Trichel pulses. In case of negative point plane corona,

101

humidity correction factor h=1. The experimental results are tabulated in

Table 4.2.

102

Fig. 4.1 :- Plane Electrode

103

Fig. 4.2 Hemispherically capped cylinder

104

Fig. 4.3 Copper Electrodes

105

Fig. 4.4 :- Brass Electrode

106

Fig. 4.5 Silver Plated Brass Electrode

107

Fig. 4.6 Chromium Plated Brass electrode

108

Fig. 4.7 Nickel Plated Brass Electrode

109

Fig. 4.8 Brass holders

110

Fig. 4.9 Brass holder electrode assembly

111

Fig. 4.10 Point electrode support system

112

Fig. 4.11 Plane electrode support system

113

Fig. 4.12 100KV HV Transformer

114

Fig. 4.13 Control panel with oil filled variac

115

Fig. 4.14 A Passive probe across 100 Kilo ohm Resistor

116

Fig. 4.14b Digital microammeter for corona current measurement

117

Fig. 4.15 Rectifiers and Connections for positive polarity

118

Fig. 4.16 Rectifiers and connections for Negative polarity

119

Fig. 4.17 Compensated HV resistor

120

Fig. 4.18 KV Meter

121

Fig. 4.19 100mm dia sphere gap assembly

122

Fig. 4.20 Mercury discharge lamp

123

Fig. 4.21 Yokogawa 1000 MHz DSO

124

Fig 4.22 HV Resistance with voltage balancing capacitors

125

Fig. 4.23 SCHEMATIC DIAGRAM OF TEST SET FOR POINT PLANE ELECTRODES

126

TABLE 4.2

EXPERIMENTAL VALUES OF CORONA INCEPTION POTENTIALS

[ POSITIVE AND NEGATIVE POLARITY ]

Sl.no MATERIAL DIAMETER

IN MM

GAP

LENGTH

IN MM

POSITIVE

CORONA

INCEPTION

VOLTAGE IN

KV

NEGATIVE

CORONA

INCEPTION

VOLTAGE

IN KV

1 Brass 1mm 20 7.672 7.659

30 8.1668 8.3407

50 9.5418 9.966

1.6 20 9.73778 9.745

30 10.4037 10.466

50 11.871 11.94

2.0 20 11.234 11.74

30 12.163 12.824

50 13.078 13.841

2.5 20 11.78 12.643

30 13.36 13.885

50 15.043 15.048

3.0 20 13.077 13.4929

30 14.499 14.2948

127

50 16.315 15.604

4.0 20 16.25

30 16.4344 17.76

50 19.91158 19.49

5.0 20 18.2449

30 20.22 20.933

50 23.0529 21.38

COPPER 1.0 20 7.732 7.33

30 8.6928 7.913

50 9.9876 9.798

1.5 20 9.8289 10.315

30 10.9332 11.1047

50 12.0319 12.635

2.0 20 11.4553 11.599

30 12.554 12.929

50 14.4087 13.94

3.0 20 14.1156 14.208

30 15.0593 15.6

50 16.1831 16.71

4.0 20 16.658

30 18.1513 18.014

50 20.6189 20.162

128

5.0 20 18.505

30 20.079 19.6446

50 22.5 21.64

STAINLESS

STEEL

1.0 20 8 8.157

30 8.319 9.041

50 9.222 11.425

1.5 20 9.7338 12.01

30 10.7012 12.338

50 11.5165 13.9546

2.0 20 11.3078 12.776

30 12.362 13.37

50 13.696 14.189

3.0 20 13.79 15.129

30 15.032 16.95

50 16.846 18.78

NICKEL

PLATED

BRASS

1.0 20 7.9746 8.188

30 8.3494 8.792

50 9.389 9.574

1.5 20 9.8107 9.834

129

30 10.6557 10.689

50 12.1933 12.1058

2.0 20 10.866 11.34

30 12.113 12.522

50 13.354 13.7997

2.5 20 12.274 13.0856

30 13.609 14.1779

50 15.333 15.704

3.0 20 13.637

30 15.2436 14.98

50 17.856 16.445

4.0 20 14.7018

30 18.585

50 19.005 20.223

5.0 20 16.65

30 20.68 20.493

50 21.248 22.33

CHROMIUM

PLATED

BRASS

1.0 20 8.293 7.7898

30 8.84 8.5351

50 9.875 9.7544

130

1.5 20 9.977 9.933

30 10.9927 10.92027

50 12.35 12.0697

2.0 20 10.129 11.87

30 12.084 12.77

50 13.422 13.969

3.0 20 14.04 15.184

30 15.359 16.40849

50 16.836 18.0057

4.0 20 16.3426

30 18.6026 18.516

50 19.198 19.89

5.0 20 21.08

30 20.6225 20.856

50 23.2936 23.06

SILVER

PLATED

BRASS

1.0 20 7.9835 7.862

30 8.4938 8.825

50 9.8052 9.904

1.5 20 9.3923 10.446

30 10.3974 11.317

131

50 12.088 12.412

2.0 20 11.1818 11.613

30 12.129 12.062

50 13.7815 12.137

2.5 20 12.213 12.935

30 13.342 13.848

50 15.468 4.96

3.0 20 13.788 15.16

30 15.86 15.797

50 17.9179 17.2

4.0 20 16.148 17.129

30 17.8109 17.164

50 18.019 18.77

5.0 20 18.9 18.939

30 20.336 19.4

50 22.86 22.247

4.10 POLARITY EFFECTS ON CORONA INCEPTION POTENTIALS

On observations of results reported in Tables 4.2 for positive and

negative polarity corona inception potentials for same type of electrode

material and geometry, we observe that the negative polarity inception

voltages are generally higher as compared to positive polarity corona

inception voltages. In order to analyse this phenomena in a general way,

132

Fig. 4.74-4.75 are drawn to indicate movement of electrons which are

primary charge carriers that cause ionization as they move in electric

field regions for which the ratio E/P is greater than (E/P)critical. The

critical E/P is the value at which α=η for air and in the Fig. the distance

dc is the value of distance from point electrode to the position on the axis

where (E/P)= (E/P)critical. Viewing the phenomena in this manner, we

observe from the Fig. that near corona inception, for the positive polarity

of the point electrode electrons at distance dc from point electrode move

towards the +ve point electrode causing ionization in the path of

movement. During this path of movement, electrons travel into regions of

progressively increasing electric fields ( higher electric field regions ) and

thus can cause greater number of ionization collissions in their path

before reaching the anode. However, for the case of -ve polarity of the

point electrode, the situation is the converse. That is, electrons move

from point electrode into regions of progressively relatively lower values

of electric fields in their path as they reach towards point of distance dc.

Therefore, there can be much lower number of ionization collisions for

this case of negative polarity as electrons travel from point electrode

towards the critical distance and subsequently towards the cathode.

Hence higher voltage has to be applied to point electrode to cause

corona. It may be also noted here that for negative polarity the electrons

can get attached to neutral molecules forming negative ions which do not

cause ionization. Also, the negative ions being heavy move slowly in low

133

field regions which situation is same for movement of positive ions in low

field regions for the case of positive polarity of point electrode. These are

possible considerations for a higher voltage required to be applied for

negative polarity of point electrode to cause corona inception as

compared to the voltage that is required to be applied to cause corona

inception for positive polarity of the electrode system.

4.11 EFFECT OF MATERIAL OF ELECTRODE ON CORONA

INCEPTION VOLTAGES

POSITIVE POLARITY

From experimental results we observe that for same diameter of point

electrode and for different gap distances the lowest corona inception

voltages are obtained for pure brass point electrode (not plated). By

change of material of point electrode whether using electroplated brass

electrodes or different material for point electrode the values of corona

inception potentials do not increase considerably for 1mm diameter of

the gap. The maximum increase in corona inception potential is about

7% in this case. Except for only few values of corona inception potentials

for nickel plated point electrode the trend is same for experimentally

determined corona inception potentials for other diameters of the

hemispherically capped point electrode upto diameter of 5.0mm.

However, the maximum difference in corona inception voltage for point

electrodes of other materials (for diameter of point electrode in range of

134

1.5mm to 5.0mm ) is approximately 10 % above the value measured for

pure brass electrode of same electrode geometry.

NEGATIVE POLARITY

For the case of negative polarity, we do not observe any consistent

behaviour of corona inception potentials as observed for the case of

positive polarity (Refer Table 4.2 ). With change of material of point

electrode whether electroplated brass or other material for point electrode

different from brass, the corona inception potentials are higher in certain

cases as compared to value obtained for pure brass point electrode and

certain other cases, the corona inception potentials are lower.

4.12 TRICHEL PULSE FREQUENCIES WITH NEGATIVE POLARITY

CORONA

The negative polarity corona consists of Trichel pulse behaviour from

occurrence of corona inception. As voltage is further increased after

corona inception, the DC microammeter records increase in current and

corresponding to increase in current there is increase in frequency of

Trichel frequencies. Even with UV rays shining directed to the gap of the

point plane electrode there were small variations in value of Trichel pulse

frequencies oscillographically recorded for a given value of DC current. In

all our experiments, in order to avoid possible damage to point electrode

the magnitude of corona current was limited to 5 microamperes. In

general for all the cases studied, the average value of Trichel pulse

frequencies increased with increase in DC corona current [ Maximum

135

corona current limited to 5 microamperes ]. For same value of DC corona

current, Trichel pulse frequencies decrease with increase in gap length.

Also, the average Trichel pulse frequencies decrease with increase in

diameter for same value of corona current. The results are shown in Fig.

4.68-4.70.

The average value of Trichel pulse frequencies for given corona

current changed with change in material of the cathode from brass to

nickel, silver and chromium plated brass electrode.[Fig 4.71-4.73]

4.13 CORONA PULSE RISE TIME MEASUREMENTS

The rise time measurements of corona pulse at inception for +ve

polarity was measured by recording the voltage pulse across 100KΩ

resistor using yokogawa make DSO. During studies on corona inception

phenomena, it was observed that for +ve polarity of the point plane

electrode geometry, the rise time of corona pulses just at corona

inception was very fast and in range of approximately 50 nanoseconds to

150 nanoseconds. In addition, extremely fast rise HV pulses have

applications in engineering topics such as electrostatic discharges (ESD),

electromagnetic interference (EMI), and also in medical sciences for study

of electroporation effects on biological cells[ 4.1 to 4.3 ]. From these

considerations, investigations were carried out to achieve extremely fast

rise time electric pulses. The oscilloscopically recorded waveforms of

corona pulses for positive polarity are shown in Fig.s 4.27 to 4.36 for

different materials, for different diameters and different gap lengths.

136

The lowest rise time have been observed for pure brasss and silver

plated brass material. These are shown in Fig. 4.54 to 4.63. For pure

brass material, generally, for a given value of DC corona current, the rise

time of corona pulse is decreasing with gap spacing. For pure brass

electrode, the minimum rise time observed for 1.0mm diameter is 61

nanoseconds for DC corona current and for a gap spacing of 50mm.

Experimental investigations were carried out for achieving possible

improvements in rise time of corona pulses by sudden application of over

voltage to a pre stressed point plane gap. The pre-stress voltage selected

was 65% (approximately) of the magnitude of steady state corona

inception voltage in air under fair weather conditions. The circuit

diagram is shown in Fig. 4.76. For these investigations point plane

electrode of 1 mm dia for point and 10mm gap distance was selected. The

oscillographic record of voltage pulse obtained under these conditions is

shown in Fig. 4.77. We observe from the Fig., the rise time of pulse

recorded is 29.5 nano seconds.

For further study of the phenomena, the voltages corresponding to

108 multiplication [This value corresponds to threshold value for

streamer development and critical distance is the distance

correspondimg to (E/P)CRI in air under fair weather condition for α= η]

and the critical distances from centre of point electrodes for various gap

distances and different diameters of hemispherical tip of point electrode

were calcluted. These are shown in Fig.s 4.78 to 4.83.

137

When the voltage corresponding to 108 multiplication (within about

+1%) was suddenly applied, there was a sound and this was

accompanied by an approximately small spot luminosity moving from tip

of HV electrode to the ground. It was clearly a transitory conduction of

gap and not at all a permanent conduction across the gap. Repetition of

this experiment for various gap distances indicated, that the phenomena

of transitory conduction (very short duration time conduction for times of

order of about few tens of nanoseconds) appeared very close to the

voltage corresponding to 108 multiplication for all gap spacings.

4.14 CONCLUSIONS

Point plane electrode geometry for experimental investigations has been

constructed.

1. A High voltage DC measuring device is built which uses accurately

measured RC components in order to measure the HVDC voltages

to good accuracy by measuring the DC current flowing through the

resistor ( ±1% accuracy ).

2. Experimentally determined values of corona inception potentials

for the -ve polarity of point electrode are higher than the values

obtained for the positive polarity.

3. The effect of material on corona inception voltage magnitudes is

not considerable and is observed to be consistently higher

(Maximum increase of 7%) for the positive polarity as compared to

corona inception voltages determined for brass electrode.

138

4. Even with UV light directed into the gap of electrodes, there are

small variations in Trichel pulse frequencies for a given value of DC

corona current.

5. Average value of Trichel pulse frequencies decrease with increase

in diameter.

6. Average value of Trichel pulse frequencies decrease when the

material is changed from brass to silver, nickel and chromium.

7. The lowest rise time for positive polarity corona inception voltage is

observed for silver plated brass electrode for steady state voltage

applications.

8. With suitable arrangement for sudden overvoltage application on

prestressed point plane gap (prestress voltage being 65% steady

state corona inception voltage) with magnitude of voltage

corresponding to 108 multiplication, it is possible to reduce the rise

time of positive polarity corona pulse voltage.

139

Figure. 4.24 Circuit Diagram indicating arrangement for application of

positive polarity and negative polarity voltages to point elctrodes.

140

Fig. 4.25

141

Fig. 4.26

142

Fig. :- 4.27 Recorded waveforms of corona pulse

Brass, diameter 1mm gap distance 30mm

143


Brass, diameter 5mm gap distance 20mm

144


Copper, diameter 1mm gap distance 20mm

145

Fig. 4.30 Recorded waveforms of corona pulse

Copper Diameter 4mm gap distance 20mm

146


Stainless steel, Diameter 1.5mm gap distance 30mm

147


Nickel plated brass, Diameter 2mm gap distance 30mm

148


Silver plated brass, Diameter 1mm gap distance 50mm

149


Nickel plated brass, diameter 2.5mm gap distance 20mm

150


Chromium plated brass, Diameter 2mm gap distance 50mm

151



152

Fig. 4.37 Recorded waveforms of Trichel pulse frequencies

Brass, Diameter 1mm gap distance 20mm

153


Brass, Diameter 3.1mm gap distance 50mm

154


Copper, Diameter 1mm gap distance 20mm

155

Fig. :- 4.40 Recorded waveforms of Trichel pulse frequencies

Copper , Diameter 1mm gap distance 20mm

156


Stainless steel, Diameter 1mm gap distance 20mm

157


Stainless steel, Diameter 1.5mm gap distance 30mm

158


Nickel plated brass, Diameter 1mm gap distance 20mm

159


Nickel plated brass, Diameter 1.5mm gap distance 50mm

160



161


Chromium plated brass, Diameter 2.5mm gap distance 20mm

162



163



164



165

Fig. 4.50 Comparison of positive corona inception voltage for Brass

and Nickel plated Brass.

166


and Chromium plated Brass.

167


and Silver plated Brass.

168

Fig. 4.53 Comparison of positive and negative Corona Inception

Voltages

169

Fig. 4.54 Positive corona: Plot of rise time vs gap distance for Brass,

Diameter 1mm

170

Fig. 4.55 Positive corona: Plot of rise time vs gap distance for Brass,

Diameter 2mm

171

Fig. 4.56 Positive corona: Plot of rise time vs gap distance for

Stainless steel, Diameter 1mm

172


Stainless steel, Diameter 1.5mm

173

Fig. 4.58 Positive corona: Plot of rise time vs gap distance for Silver

plated Brass, Diameter 1mm

174


plated Brass, Diameter 1.5mm

175



177



179





180


Chromium plated Brass, Diameter 1.5mm

181


Chromium plated Brass, Diameter 2mm

182


Chromium plated Brass, Diameter 5mm

183

Fig. 4.67 Positive corona: Plot of rise time vs gap distance for Nickel


184

Fig. 4.68 Plot of Average Trichel Pulse frequencies in KHz vs Corona

current for Brass, Diameter 1mm

186

Fig. 4.69 Plot of Average Trichel Pulse frequencies in KHz vs gap

length in mm

188

Fig. 4.70 Plot of Average Trichel Pulse frequencies in KHz vs

Diameter in mm

Fig. 4.71 Plot of Average Trichel Pulse frequencies in KHz for Brass

and Silver plated Brass vs gap length in mm

189


and Chromium plated Brass vs Gap length

190


and Nickel plated Brass vs Gap length

191

Fig 4.74 Pictorial Representation of polarity effect (Positive)

Fig. 4.75 Pictorial Representation of polarity effect (Negative)

192

Fig. 4.76

193

Fig. 4.77 Oscillographic record for Rise time in over voltage gap

194

Fig. 4.78 Plot of critical distance vs gap length corresponding to

108 Multiplication D=1.5mm

195

Fig. 4.79 Plot of critical distance vs gap length corresponding to 108

Multiplication D=2.0mm

196

Fig. 4.80 Plot of voltage in KV on tip corresponding to

108Multiplicatio arround diameter 1.0mm

197

Fig. 4.81 Plot of voltage in KV on tip coresponding to

108Mutiplication Diameter=1.5mm

198

Fig. 4.82 Plot of critical distance vs gap length corresponding to 108

Multiplication D=2.5mm

4.15 REFERENCES

1. IEEE standard surge withstand capability (swc) tests for protective relays

and relay systems P472/D9, c37.90.1-198X. Draft document of the

power system relaying committee,june 8, 1987.

2. Recommended transient voltage tests applicable to transistorized relays,

The British Electrical and Allied manufacturer‟s association (inc)

publication number 219, november 1966.

3. M.Behrend et al, “Pulse Generators for pulse electric field exposures of

Biological cells and tissues”, IEEE Transactions on Dielectrics and

Electrical insulation, vol 10, no. 5, october 2003,pp 820-825.

199

Fig. 4.83 Experimental Setup for Over voltage Application

10_chapter 4

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