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Mesurement Of High Voltages & High Currents Unit 4

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High Voltage Measurement Techniques 2

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Sphere Gaps Applicatios: Voltage Measurement (Peak) - Peak values of voltages may be measured from 2 kV up to about 2500 kV by means of spheres. Arrangements: 1. Vertically with lower sphere grounded (For Higher Voltages) 2. Horizontally with both spheres connected to the source voltage or one sphere grounded (For Lower Voltages). 3

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The arrangement is selected based on the relation between the peak voltage, determined by sparkover between the spheres, and the reading of a voltmeter on the primary or input side of the high-voltage source. This relation should be within 3% (IEC, 1973). Standard values of sphere diameter are 6.25, 12.5, 25, 50, 75, 100, 150, and 200 cm. The Clearance around the sphere gaps: 4 Sphere Gaps Fig C :Breakdown voltage characteristic of sphere gaps

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The effect of humidity is to increase the breakdown voltage of sphere gaps by up to 3%. Temperature and pressure, however, havea significant influenceo n breakdown voltage. Breakdown Voltage under normal atmospheric conditions is, V s =kV n where k is a factor related to the relative air density (RAD) δ. The relation between the RAD(δ) and the correction factor k: Under impulse voltages, the voltage at which there is a 50% breakdown probability is recognized as the breakdown level. 5 Sphere Gaps

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Factors Influencing the Sparkover Voltage of Sphere Gaps i. Nearby earthed objects, ii. Atmospheric conditions and humidity, iii. Irradiation, and iv. Polarity and rise time of voltage waveforms. The limits of accuracy are dependant on the ratio of the spacing d to the sphere diameter D, as follows: d < 0.5 D Accuracy = ± 3 % 0.75 D > d > 0.5 D Accuracy = ± 5 % For accurate measurement purposes, gap distances in excess of 0.75D are not used 6 Sphere Gaps

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High Ohmic Series Resistance with Microammeter Resistance (R) : Constructed with large wire wound Value: Few hundreds of Mega ohms –Selected to give (1-10μA) for FSD. Voltage drop in each element is chosen to avoid surface flashovers and discharges (5kV/cm in air, 20kV/cm in oil is allowed) Provided with corona free terminals. Material: Carbon alloy with temperature coefficient of / o C. Resistance chain located in air tight oil filled PVC tube for 100kV operation with good temp stability. Mircoammeter – MC type Voltage of source, V=IR Impedance of the meter is few ohms. i.e, very less compared to R so the drop across the meter is negligible. Protection: Paper gap, Neon Glow tube, a zener diode with series resistance – Gives protection when R fails. 7

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Maximum voltage: 500kV with 0.2% accuracy. Limitations: Power dissipation & source loading Temp effects & 8 High Ohmic Series Resistance with Microammeter

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Measurement Of High Dc Voltage 9 Series resistance micrometer Resistance potential divider Generating voltmeter Sphere and other gaps

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SERIES RESISTANCE MICROMETER 10 A very high resistance in series with a micrometer. Current through R is measured using micrometer. Voltage of source, V = IR The resistance is constructed from a large no. of wire wound resistors in series. Can be operated upto 500kV (D.C) Accuracy = ±0.2% Selection of R value: Current allowed: 1 to 10 A Corona free termination Temp. coefficient<10 -4 / 0 C : Carbon Alloy Placed in airtight, oil filled PVC tube to maintain temp. stability

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RESISTANCE POTENTIAL DIVIDER 11 It uses electrostatic voltmeter. Can be placed near the test object which might not always be confined to one location Let, V 2 -Voltage across R 2 Sudden voltage changes during transients due to: Switching operation Flashover of test objects Damage due to stray capacitance across the elements & ground capacitance

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GENERATING VOLTMETER 12 Generating principle is used where direct loading or direct connection is to be avoided. Generating voltmeter: A variable capacitor electrostatic voltage generator. It generates current proportional to voltage under measurement This arrangement provides loss free measurement of DC and AC voltages It is driven by synch. motor, so doesnt observe power from the voltage measuring source The high voltage electrode and the grounded electrode in fact constitute a capacitance system. The capacitance is a function of time as the area A varies with time and, therefore, the charge q(t) is given as,

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13 and, For d.c. Voltages, Hence If the capacitance varies linearly with time and reaches its peak value C m is time T c /2 and again reduces to zero linearly in time T c /2, the capacitance is given as If the capacitance C varies sinusoidally between the limits C 0 and (C 0 + C m ) then C = C 0 + C m sin ωt and the current i' is then given as, i(t) = i m cos ω t, where i m = VC m ω Here ω is the angular frequency of variation of the capacitance. Generally the current is rectified and measured by a moving coil meter Generating voltmeters can be used for a.c. voltage measurement also provided the angular frequency ω is the same or equal to half that of the voltage being measured. Above fig. shows the variations of C as a function of time together with a.c. voltage, the frequency of which is twice the frequency of C (t). GENERATING VOLTMETER

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Instantaneous value of current i(t) = C m f v V(t) where f v = 1/T v the frequency of voltage. Since f v = 2f c and f c = 60/n we obtain, I(t) = n/30 C m V(t) Fig. shows a schematic diagram of a generating voltmeter which employs rotating vanes for variation of capacitance High voltage electrode is connected to a disc electrode D 3 which is kept at a fixed distance on the axis of the other low voltage electrodes D 2, D 1, and D Generating Voltmeter The rotor D 0 is driven at a suitable constant speed by a synchronous motor. Rotor vanes of D 0 cause periodic change in capacitance between the insulated disc D 2 and the high voltage electrode D 3. Number and shape of vanes are so designed that a suitable variation of capacitance (sinusodial or linear) is achieved. The a.c. current is rectified and is measured using moving coil meters. If the current is small an amplifier may be used before the current is measured.

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Generating voltmeters are linear scale instruments and applicable over a wide range of voltages. The sensitivity can be increased by increasing the area of the pick up electrode and by using amplifier circuits Advantages: i. scale is linear and can be extrapolated ii. source loading is practically zero iii. no direct connection to the high voltage electrode. 15 Generating Voltmeter

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Measurement Of High Ac Voltage 16 Electrostatic voltmeter Series impedance voltmeter Potential dividers : Resistance or Capacitance type Potential transformers : Electromagnetic or CVT Sphere gaps

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Electrostatic Voltmeter One of the direct methods of measuring high voltages is by means of electro-static voltmeters. For voltages above 10 kV, generally the attracted disc type of electrostatic voltmeter is used. When two parallel conducting plates (cross section area A and spacing s) are charged q and have a potential difference V, then the energy stored in the is given by 17 It is thus seen that the force of attraction is proportional to the square of the potential difference applied, so that the meter reads the square value (or can be marked to read the rms value).

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Electrostatic Voltmeter Electrostatic voltmeters of the attracted disc type may be connected across the high voltage circuit directly to measure up to about 200 kV, without the use of any potential divider or other reduction method. [The force in these electrostatic instruments can be used to measure both a.c. and d.c. voltages]. The right hand electrode forms the high voltage plate. The centre portion of the left hand disc is cut away and encloses a small disc which is movable and is geared to the pointer of the instrument. The range of the instrument can be altered by setting the right hand disc at pre- marked distances. The force of attraction F(t) created by the applied voltage causes the movable part-to which a mirror is attached-to assume a position at which a balance of forces takes place. An incident light beam will therefore be reflected toward a scale calibrated to read the applied voltage magnitude. 18

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Advantages: i. Low loading effect ii. Active power losses are negligibly small iii. Voltage source loading is limited to the reactive power needed to charge the system capacitance.(i.e., For 1V Voltmeter- Capacitance is few Pico farad) iv. Voltages upto 600kV can be measured. Disadvantage: i. For constant distance s, F α V 2, the sensitivity is small. This can be overcome by varying the gap distance d in appropriate steps. 19 Electrostatic Voltmeter Absolute Electrostatic Voltmeter

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Series Impedance Voltmeter For power frequency a.c. measurements the series impedance may be a pure resistance or a reactance. But use of resistances yields the followings, Power losses Temperature problem Residual inductance of the resistance gives rise to an impedance different from its ohmic resistance. High resistance units for high voltages have stray capacitances and hence a unit resistance will have an equivalent circuit as shown in Fig. At any frequency ω of the a.c. voltage, R+jX L is connected in parallel with –jX C. 20

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Series Impedance Voltmeter Extended Series Resistance neglecting inductance is shown in figures. Resistor unit then has to be taken as a transmission line equivalent, for calculating the effective resistance. Ground or stray capacitance of each element influences the current flowing in the unit, and the indication of the meter results in an error. Stray ground capacitance effects can be removed by shielding the resistor R by a second surrounding spiral R S which shunts the actual resistor but does not contribute to the current through the instrument. 21

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By tuning the resistors R a the shielding resistor end potentials may be adjusted with respect to the actual measuring resistor so that the resulting compensation currents between the shield and the measuring resistors provide a minimum phase angle. 22 Series Impedance Voltmeter

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Series Capacitance Voltmeter To avoid the drawbacks pointed out Series impedance voltmeter, a series capacitor is used instead of a resistor for a.c. high voltage measurements. Current through the instrument, I c =jωCV The rms value of the voltage V with harmonics is given by, where V 1,V 2,...,V n represent the rms value of the fundamental, second... and n th harmonics. The currents due to these harmonics are I 1 =ωCV 1, I 2 =2ωCV 2, ……I n =nωCV n With a 10% fifth harmonic only, the current is 11.2% higher, and hence the error is 11.2% in the voltage measurement Not recommended when a.c. voltages are not pure sinusoidal waves but contain considerable harmonics. Used for measuring rms values up to 1000 kV. 23

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Series Capacitance Voltmeter A rectifier ammeter was used as an indicating instrument and was directly calibrated in high voltage rms value. The meter was usually a (0-100)μA moving coil meter and the over all error was about 2%. 24

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Peak Reading Voltmeters For Sine wave, Peak Value=RMS Value X 2 Maximum dielectric strength may be obtained by non-sine wave. In that case, Peak Value RMS Value X 2 Therefore, peak measurement is important. Types: Series Capacitance Peak Voltmeter (Chubb-Frotscue Method) Digital Peak Voltmeter Peak Voltmeter with potential divider 25

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Peak Reading Voltmeters Chubb Frotscue Method: Chubb and Fortescue suggested a simple and accurate method of measuring peak value of a.c. voltages. The basic circuit consists of a standard capacitor, two diodes and a current integrating ammeter (MC ammeter) as shown in Fig (a). The displacement current i c (t), Fig is given by the rate of change of the charge and hence the voltage V(t) to be measured flows through the high voltage capacitor C and is subdivided into positive and negative components by the back to back connected diodes The voltage drop across these diodes can be neglected (1 V for Si diodes) as compared with the voltage to be measured The measuring instrument (M.C. ammeter) is included in one of the branches. The ammeter reads the mean value of the current, An increased current would be obtained if the current reaches zero more than once during one half cycle 26

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27 (Chubb Frotscue Method Continued…) This means the wave shapes of the voltage would contain more than one maxima per half cycle. The standard a.c. voltages for testing should not contain any harmonics and, therefore, there could be very short and rapid voltages caused by the heavy predischarges, within the test circuit which could introduce errors in measurements. To eliminate this problem filtering of a.c. voltage is carried out by introducing a damping resistor in between the capacitor and the diode circuit, Fig (b). The measurement of symmetrical a.c. voltages using Chubb and Fortescue method is quite accurate and it can be used for calibration of other peak voltage measuring devices. Peak Reading Voltmeters

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28 Digital Peak Voltmeter: In contrast to the method discussed just now, the rectified current is not measured directly, instead a proportional analog voltage signal is derived which is then converted into a proportional medium frequency for using a voltage to frequency convertor (Block A in Fig. 4.13). The frequency ratio fm/f is measured with a gate circuit controlled by the a.c. power frequency (supply frequency f) and a counter that opens for an adjustable number of period Δt = p/f. The number of cycles n counted during this interval is where p is a constant of the instrument. Peak Reading Voltmeters

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29 By proper selection of R and P, Voltage can be measured immediately. Accuracy is less than 0.35% Digital Peak Voltmeter continued…. Peak Reading Voltmeters

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30 Peak voltmeter with Potential divider: Diode D is used for rectification Voltage across C 2 is used to charge C 3 Resistance R d permits the variation of V m when V 2 is reduced Electrostatic Voltmeter as indicating instrument Voltage across C s Peak value to be measured Discharge time constant=C s R d 1 to 10 sec This arrangement gives discharge error. Discharge error depends on frequency of the supply Peak Reading Voltmeters

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Measurement of High Currents Type of CurrentMethod used D.C Current1.Resistant shunt 2.Hall Generator High Power frequency A.CCurrent Transformer with electro-optical technique High frequency and impulse currents1.Resistive shunts 2.Magnetic potentiometers or probes 3.Magnetic links 4.Hall generators 5.Faraday Generators Impulse Voltages and CurrentsCathode Ray Oscilloscope 31

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Hall Generators Hall effect is used to measure very high direct current. Whenever electric current flows through a metal plate placed in a magnetic field perpendicular to it, Lorenz force will deflect the electrons in the metal structure in a direction perpendicular to the direction of both the magnetic field and the flow of current. The change in displacement generates an e.m.f called Hall Voltage 32

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Hall Voltage, where, B-Magnetic Flux density I-Current d-Thickness of the metal plate R-Hall Coefficient (depends on Material of the plate & temperature) R is small for metals and High for semiconductors 33 Hall Generators When large d.c. currents are to be measured the current carrying conductor is passed through an iron cored magnetic circuit

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The magnetic field intensity produced by the conductor in the air gap at a depth d is given by, The Hall element is placed in the air gap and a small constant d.c. current is passed through the element. The voltage developed across the Hall element is measured and by using the expression for Hall voltage the flux density B is calculated and hence the value of current I is obtained. 34 Hall Generators

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Faraday Generator or Magneto Optic Method These methods of current measurement use the rotation of the plane of polarisation in materials by the magnetic field which is proportional to the current (Faraday effect). When a linearly polarised light beam passes through a transparent crystal in the presence of a magnetic field, the plane of polarisation of the light beam undergoes rotation. The angle of rotation is given by, θ = α Bl where, α = A constant of the cyrstal which is a function of the wave length of the light. B = Magnetic flux density due to the current to be measured in this case. l = Length of the crystal. 35

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Fig. shows a schematic diagram of Magneto-optic method. Crystal C is placed parallel to the magnetic field produced by the current to be measured. A beam of light from a stabilised light source is made incident on the crystal C after it is passed through the polariser P 1. The light beam undergoes rotation of its plane of polarisation. After the beam passes through the analyser P 2, the beamis focussed on a photomultiplier, the output of which is fed to a CRO. 36 Faraday Generator or Magneto Optic Method

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The filter F allows only the monochromatic light to pass through it. Photoluminescent diodes too, the momentary light emission of which is proportional to the current flowing through them, can be used for current measurement. Advantages: 1. It provides isolation of the measuring set up from the main current circuit. 2. It is insensitive to overloading. 3. As the signal transmission is through an optical system no insulation problem is faced. However, this device does not operate for D.C current. 37 Faraday Generator or Magneto Optic Method

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Magnetic Potentiometer(Rogowski Coil) If the current to be measured is flowing through a conductor which is surrounded by a coil as shown in Fig. and M is the mutual inductance between the coil and the conductor, the voltage across the coil terminals will be: Usually the coil is wound on a non-magnetic former in the form of a toroid and has a large number of turns, to have sufficient voltage induced which could be recorded. 38

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The coil is wound cross-cross to reduce the leakage inductance. If N is the number of turns of the coil, A the coil area and l m its mean length, the mutual inductance is given by Usually an integrating circuit RC is employed as shown in Fig to obtain the output voltage proportional to the current to be measured. The output voltage is given by The frequency response of the Rogowski coil is flat upto 100 MHz but beyond that it is affected by the stray electric and magnetic fields and also by the skin effect. 39 Magnetic Potentiometer(Rogowski Coil)

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Resistive Shunt 40

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SERIES IMPEDANCE VOLTMETER 41 Extended series impedance with inductance neglected

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SERIES CAPACITOR PEAK VOLTMETER 42 C – capacitor D 1,D 2 – Diodes OP – Protective devices I – indicating meter V(t) – voltage waveform I c (t) – capacitor current waveform T – period

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PEAK READING AC VOLTMETER 43

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PEAK READING AC VOLTMETER 44

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SPHERE GAPS MEASUREMENT 45 U b = k d U b0

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SPHERE GAPS 46

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Potential divider for impulse voltage measurement 47

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MEASUREMENT OF HIGH DIRECT CURRENTS 48 HALL GENERATORS FOR D.C CURRENT MEASUREMENTS Hall effect principle is used.If an electric current flows through a metal plate located in a magnetic field perpendicular to it,Lorenz forces will deflect the electrons in the metal structure in a direction normal to the direction of both the current and magnetic field. The charge displacement generates an emf in the normal direction (Hall voltage). V H =RB i /d H=I/δ

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MEASUREMENT OF HIGH POWER FREQUENCY ALTERNATING CURRENTS 49 Current transformer is used.it uses electro optical technique. A voltage signal proportional to the measuring current is generated and it is transmitted to the ground side through electro optical device. Light pulses proportional to the voltage signal are transmitted by a glass optical fibre bundle to a photo detector and converted back into an analog voltage signal.

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SPHERE GAP The sphere gap method of measuring high voltage is the most reliable and is used as the standard for calibration purposes. breakdown strength of a gas depends on the ionisation of the gas molecules, and on the density of the gas. As such, the breakdown voltage varies with the gap spacing; and for a uniform field gap, a high consistency could be obtained, so that the sphere gap is very useful as a measuring device. In the measuring device, two metal spheres are used, separated by a gas-gap. The potential difference between the spheres is raised until a spark passes between them. The breakdown strength of a gas depends on the size of the spheres, their distance apart and a number of other factors. A spark gap may be used for the determination of the peak value of a voltage wave, and for the checking and calibrating of voltmeters and other voltage measuring devices. The density of the gas (generally air) affects the spark-over voltage for a given gap setting. Thus the correction for any air density change must be made. The air density correction factor δ must be used. 50

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The spark over voltage for a given gap setting under the standard conditions (760 torr pressure and at 20 o C) must be multiplied by the correction factor to obtain the actual spark-over voltage. The breakdown voltage of the sphere gap (Figure:a) is almost independent of humidity of the atmosphere, but the presence of dew on the surface lowers the breakdown voltage and hence invalidates the calibrations. The breakdown voltage characteristic (figure 6.3) has been determined for similar pairs of spheres (diameters 62.5 mm, 125 mm, 250 mm, 500 mm, 1 m and 2 m) 51 Fig.b:Breakdown voltage characteristic of sphere gaps Figure. a:- Measuring spheres SPHERE GAP

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In sphere gaps used in measurement, to obtain high accuracy, the minimum clearance to be maintained between the spheres and the neighbouring bodies and the diameter of shafts are also specified, since these also affect the accuracy (Figure:d). There is also a tolerance specified for the radius of curvature of the spheres. Peak values of voltages may be measured from 2 kV up to about 2500 kV by means of spheres. One sphere may be earthed with the other being the high voltage electrode, or both may be supplied with equal positive and negative voltages with respect to earth (symmetrical gap). Needle gaps may also be used in the measurement of voltages up to about 50 kV, but errors are caused by the variation of the sharpness of the needle gaps, and by the corona forming at the points before the gap actually sparks over. Also the effect of the variation of the humidity of the atmosphere on such gaps is much greater. 52 SPHERE GAP Figure. d: Sphere gap

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When the gap distance is increased, the uniform field between the spheres becomes distorted, and accuracy falls. The limits of accuracy are dependant on the ratio of the spacing d to the sphere diameter D, as follows: d < 0.5 D Accuracy = ± 3 % 0.75 D > d > 0.5 D Accuracy = ± 5 % For accurate measurement purposes, gap distances in excess of 0.75D are not used Breakdown voltage characteristic is also dependant on the polarity of the high voltage sphere in the case of asymmetrical gaps, in a symmetrical gap, then the polarity has no effect. 53 Figure. C:- Breakdown voltage characteristics

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