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1 Introduction to High Voltage Measurement It is essential to measure the voltages and currents accurately ensuring perfect safety to the personnel and equipment in industrial testing and research laboratories. The location and layout of the devices are important as the person handling the equipment must be protected against overvoltages and also against any induced voltages due to stray coupling. The different used for high voltage measurements may be classified as in table and table Table High Voltage Measurement Techniques (a) D.C. Voltages (i) Series resistance microammeter (ii) Resistance potential divider (iii) Generating voltmeter (iv) Sphere and other spark gaps (b) A.C. Voltages (i) Series impedance ammeters (power frequency) (ii) Potential dividers (resistance or capacitance type) (iii) Potential transformers (electromagnetic or CVT) (iv) Electrostatic voltmeters (v) Sphere gaps (c) A.C. High frequency (i) Potential dividers with a cathode ray oscillograph (resistive or voltages, impulse (ii) Peak voltmeters and other rapidly (iii) Sphere gaps changing voltages Table High Current Measurement Techniques (a) Direct currents (i) Resistive shunts with milliammeter (ii) Hall effect generators (iii) Magnetic links (b) Alternating currents (i) Resistive shunts (power frequency) (ii) Electromagnetic current transformers (c) High frequency a.c. (i) Resistive shunts Impulse and rapidly (ii) Magnetic potentiometers or Rogowski coils changing currents (ii) Magnetic links (iv) Hall effect generators High Ohmic Series Resistance with Microammeter High d.c. voltages are usually measured by connecting a very high resistance (few hundreds of megaohms) in series with a microammeter as shown in fig. Only the current I flowing through the large calibrated resistance R is measured by the moving coil microammeter. The voltage of the source is given by V= IR. The voltage drop in the meter is negligible as the impedance of the meter is small in comparison to the series resistance. A protective device like a paper gap, neon glow tube, zener diode with a suitable series resistance is connected across the meter as a protection against high voltages in case series resistance fails or flashes over. The ohmic value of resistance R is so chosen such that a current of to 0 µa is allowed for full scale deflection. The limitations in the series resistance design are: i. Power dissipation and source loading.

2 ii. iii. iv. Temperature effects and long time stability. Voltage dependence or resistive elements Sensitivity to mechanical stresses. HV R µa Protective device Fig. Series resistance microammeter Resistance Potential Dividers for d.c. Voltages A resistance potential divider with an electrostatic voltmeter is shown in fig. The influence of temperature and voltage on the elements is eliminated in the voltage divider arrangement. The high voltage magnitude is given by [(R + R)/R]V where V is the d.c. voltage across the low voltage arm R. With sudden changes in voltages, such as switching operations, flashover of the test objects or source short circuits, flashover or damage may occur to the divider elements due to the stray capacitance across the elements and due to ground capacitances. To avoid these transient voltages, voltage controlling capacitors are connected across the elements. A corona free termination is necessary to avoid unnecessary discharges at high voltage ends. Potential dividers are made with 0.05% accuracy upto 00 kv, with 0.0% accuracy upto 300 kv HV R R P ESV Fig. Resistance Potential Divider ESV Electrostatic Voltmeter P Protective Device Generating Voltmeter The generating principle is employed where source loading is prohibited or when direct connection to the high voltage source is to be avoided. A generating voltmeter is a variable capacitor electrostatic voltage generator which generates current proportional to the applied external voltage. The device is driven by an external synchronous or constant speed motor and does not absorb power or energy from the voltage measuring source.

3 Principle of operation The charge stored in a capacitor of capacitance C is given by q = CV. If the capacitance of the capacitor varies with time when connected to the source of voltage V, the current through the capacitor For d.c. voltages dv/dt = 0 Hence dq dc i = = V + C dt dt dq dc i = = V dt dt If the capacitance C varies between the limits C 0 and (C 0 + C m) sinusoidally as C = C 0 + C m sinωt and ωvcm the current is given by i = Im cosωt where Im = ωvcm and the rms value is given by irms = For a constant angular frequencyω, the current is proportional to the applied voltage V. The generated current is rectified and measured by a moving coil meter. Generating voltmeter can also be used for measuring a.c. voltage provided the angular frequency ω is the same or equal to half that of the supply frequency. A generating voltmeter with a rotating cylinder consists of two exciting field electrodes and a rotating two pole armature driven by a synchronous motor at a constant speed n. The a.c. current flowing between two halves of the armature is rectified by a commutator. The fig.3 shows a schematic diagram of a generating voltmeter. The high voltage source is connected to a disc electrode S 3 which is kept at a fixed distance on the axis of the other low voltage electrodes S 0, S and S. The rotor S 0 is driven at a constant speed by a synchronous motor at a suitable speed. The rotor vanes of S0 cause periodic change in capacitance between the insulated disc S and the H.V. electrode S 3. The shape and number of the vanes of S 0 and S are so designed that they produce sinusoidal variation in the capacitance. The generated a.c. current through the resistance R is rectified and read by a moving coil instrument. The instrument is calibrated using a potential divider or sphere gap. The calibration curves of a generating voltmeter are shown in fig.4 (a) and 4(b) S 3 S 0 S S dv dt Motor Fig.3 Schematic Diagram of Generating Voltmeter 3

4 S3 S0 S S 3 = High Voltage Electrode S 0 = Rotor S & S = Fixed Electrodes S Advantages of Generating Voltmeter The following are the advantages of a generating voltmeter: i. No source loading by the meter. ii. No direct connection is required with the high voltage electrode. iii. The scale is linear and extension of range is easy. iv. It is a very convenient instrument for electrostatic devices. Limitations of Generating Voltmeter The following are the limitations of a generating voltmeter: i. Calibration of the voltmeter is required. ii. It requires a careful construction and it is a cumbersome instrument requiring an auxiliary drive. Capacitance Voltage Transformer A capacitance voltage transformer (CVT) is basically a capacitor divider used with a suitable matching or isolating potential transformer tuned for resonance conditions. A CVT can be connected to a low impedance device like a wattmeter pressure coil or relay coil which is in contrast to a simple capacitor divider which requires a high impedance meter like a TVM or electrostatic voltmeter. The fig.4 shows the schematic diagram of CVT with its equivalent circuit as shown in fig.5 C C L T M Fig. 4 Schematic diagram of CVT The capacitor C is made of a few units of high voltage capacitors such that the total capacitance is of few thousand picofarads. A matching transformer is connected between the load or meter M and C. The transformer ratio is chosen on economic grounds and the H.V. winding rating maybe 0 or 30 kv with the L.V. winding rated from 00 to 500 V. The value of the tuning choke L is chosen to make the equivalent circuit of the CVT purely resistive. The resonance condition is achieved when the following condition is satisfied: ω ( L+ LT) = ωc + C ( ) where L = inductance of the choke 4

5 LT = equivalent inductance of the transformer referred to H.V. side The meter reactance Xm is neglected and is taken as a resistance load Rm when the load is connected to the voltage divider side. Advantages of CVT: The following are the advantages of a capacitance voltage transformer: i. Simple design and easy installation. ii. Can be used both as a voltage measuring device for meter and relaying purposes as well as coupling condenser for power line carrier communication & relaying. iii. The voltage distribution along the elements is frequency independent as against conventional potential transformers which requires additional insulation design against surges. iv. Provides isolation between the high voltage terminal and low voltage metering. Disadvantages of CVT: The following are the disadvantages of capacitor voltage transformer are: i. The voltage ratio is susceptible to temperature variations. ii. The problem of inducing ferro resonance in power systems. Electrostatic Voltmeter: It works on the principle that an attractive force develops between the electrodes of a parallel plate capacitor in the presence of electrostatic fields. The magnitude of the force is given by the expression as follows: F = W ds δ S δ = CV δs = V A V d V = ε 0V = ε0 gm. wt. = s s 85 s Where V = applied voltage between plates, C = capacitance between the plates, A = area of cross section of the plates, d = diameter of plates s = separation between the plates, ε0 = permittivity of the medium (air or free space) W s = work done in displacing a plate When one of the electrodes is free to move, the force on the plate can be measured by controlling it by a spring or balancing it with a counter weight. The force is proportional to the square of the applied voltage; the measurement can be made for a.c. or d.c. voltages. The electrostatic voltmeter measures the force based on the above equations and is arranged such that one of the plates is rigidly fixed whereas the other is allowed to move. Since it results in the disturbance of electric field, therefore even for high voltages the movable electrode is allowed to move by not more than a fraction of a millimetre to a few millimetres so that change in electric field is negligibly small. Principle of Operation of Electrostatic Voltmeter: The electric field is produced by voltage and therefore if the field force could be measured, the voltage can also be measured. The basic principle of an electrostatic voltmeter is that whenever a voltage is applied to a parallel plate electrode arrangement, an electric field is set up between the plates. Since the two plates are oppositely charged there is always a force of attraction between the plates. If the voltage is time dependent, the force developed is also time dependent. δc δs 5

6 Construction of electrostatic voltmeter: Various designs of the voltmeter have been developed which differ in the construction of electrode arrangement and in the use of different methods of restoring forces required to balance the electrostatic forces of attraction. Some of the methods are as follows: (i) Suspension of moving electrode on one arm of a balance. (ii) Suspension of moving electrode on a spring. (iii) Pendulous suspension of moving electrode. (iv) Torsional suspension of moving electrode. The fig. 5 shows a schematic diagram of an absolute electrostatic voltmeter. The hemispherical metal dome E encloses sensitive balance D which measures force of attraction between the movable disc which hangs from one of its arms and the lower plate B. HV G M D B R Light Source G F HV m M C H H Scale F (a) Absolute electrostatic voltmeter M Mounting plate G Guard plate F Fixed plate H Guard hoops or rings Fig. 5 Electrostatic Voltmeter (b) Light beam arrangement m Mirror B Balance C Capacitance divider D Dome R Balancing weight The movable electrode M hangs with a clearance of above 0.0 cm in a central opening in the upper plate which serves as a guard ring. The diameter of each plate is m. Light reflected from a mirror carried by the balance beam serves to magnify its motion. The uniformity of electric field is maintained by guard rings H which surround the space between the discs M and F. The guard rings are maintained at a constant potential in space by a capacitance divider ensuring a uniform special potential distribution. Usually the electrostatic voltmeters have a small capacitance (5 to 50 pf) and high insulation resistance (R 0 3 Ω). The area of the plates should be large, spacing between plates should be small and some dielectric medium other than air should be used to achieve higher force for a given voltage. The gap length cannot be made very small as this is limited by the breakdown strength of the dielectric medium between the plates. A limit is imposed on frequency range as the load inductance and 6

7 the measuring system capacitance forms a series resonance circuit. The greatest advantage of electrostatic voltmeter is its extremely low loading effect and negligibly small power loss. Sphere Gap: It is considered as one of the standard methods for the measurement of peak value of d.c., a.c. and impulse voltages and is used for checking the voltmeters and other voltage measuring devices used in H.V. test circuits. When an electric field across a gap exceeds the static breakdown strength of the gap it results in complete breakdown of the gaseous gap having uniform field. A uniform electric field is created in the gaseous gap between two spherical electrodes of equal diameter, if the electrodes are separated by a distance much smaller than the electrode radius. It can be used for measurement of impulse voltage of either polarity provided that the impulse is of standard waveform having wavefront time atleast µs and wavetail time 5 µs. The sphere gaps can be arranged either vertically with lower sphere grounded, or horizontally with both spheres connected to source voltage or one sphere grounded. The two spheres used are identical in shape and size. The fig. 6(a) and 6(b) shows the schematic arrangement. B d 0.5d P S A Fig. 6(a) Horizontal Arrangement of Sphere gap The horizontal arrangement is usually preferred for sphere diameters d < 50 cm. This arrangement is used for measurement at lower voltage ranges. With larger spheres the vertical arrangement is chosen where the lower electrode is earthed. In both the arrangements one of the spheres is static and the other is movable so that the spacing between them can be adjusted. A minimum clearance around the spheres must be available within which no external objects such as walls, ceilings, transformer tanks, impulse generators or supporting framework for the spheres are allowed. The minimum clearance is dependent on the gap spacing. The height of the sparking point P above the horizontal ground plane A, minimum clearance B are related to sphere diameter d and gap spacing S respectively. The voltage to be measured is applied between the two spheres and the distance between the gap gives a measure of the sparkover voltage. A series resistance is usually connected between the source and the sphere gap to (i) limit the breakdown current and (ii) to suppress unwanted oscillations in source voltage when breakdown occurs. The value of the series may vary from 00 Ω to 000 kω for a.c. and d.c. voltages but not more than 500 Ω in the case of impulse voltages. 7

8 B P d S A Insulating Support High Voltage Connection with series resistor A Height of sparking point P above the ground plane B Radius of space free from external structures Fig. 6(b) Vertical Arrangement of Sphere gap The spheres are made of copper, brass or aluminium. The standard diameters for the spheres are, 5, 6.5, 0,.5, 5, 5, 50, 75, 00, 50 and 00 cm. The spheres are carefully designed and fabricated so that their surfaces are smooth and the curvature is uniform. The surfaces should be free from dust, grease or any other coating. Irradiation of gap is needed when measurements of voltage are less than 50 kv are made with sphere gaps of 0 cm diameter or less. Factors Influencing Sparkover Voltage of Sphere Gaps: There are various factors that affect the sparkover voltage of a sphere gap like nearby earthed objects, atmospheric conditions and humidity, irradiation, polarity and rise time of voltage waveforms. (a) Influence of nearby earthed objects: The effect of nearby objects was investigated by Kuffel by enclosing the earthed sphere inside an earthed cylinder. It was observed that there was reduction in breakdown voltage given by the empirical formula. B V = mln + C D Where V = reduction in breakdown voltage B = diameter of earthed enclosing cylinder D = diameter of spheres m and C = factors depending on S/D ratio S = spacing between spheres The relation was less than % for S/D 0.5 and B/D 0.8. The reduction was only 3% for S/D.0 and B/D.0 (b) Influence of humidity: Kuffel studied the effect of humidity on breakdown voltage by using spheres of cm to 5 cm of diameter and uniform field electrodes. It was concluded that the sparkover increases with the partial pressure of water vapour in air, and for a given humidity condition, the change in breakdown voltage increases with the gap length. This is due to water particles which readily attach with free electrons thus forming negative ions. These ions therefore 8

9 slow down and are unable to ionise neutral molecules under field conditions in which electrons will readily ionize. It has been observed that within the humidity range of 4 to 7 g/m 3 the relative increase of breakdown voltage is found to be between 0. to 0.35 % per gm/m 3 for the largest sphere of diameter 00 cm and gap length upto 50 cm. (c) Influence of dust particles: The presence of dust particles between the gap results in erratic breakdown in homogeneous or slightly homogeneous electrode configurations. The dust particle comes in contact with one electrode getting charged to polarity of that electrode when d.c. voltage is applied. It then gets attracted by the opposite electrode due to field forces triggering early breakdown. The gaps subjected to a.c. voltages are sensitive to dust particles but the probability of erratic breakdown is less. (d) Influence of atmospheric conditions: The breakdown voltage of a spark gap depends on the air density which varies with the changes in both temperature and pressure. If the breakdown voltage is V under test conditions of temperature T and pressure p, if sparkover voltage is VO under standard conditions of temperature and pressure then V = k V O where k is a function of d where p 93 d = T The following table gives the relation between k and d d k (e) Influence of irradiation: The illumination of sphere gaps with ultraviolet or X rays aids easy ionization in gaps. It was observed that for spacings of 0.D to 0.3D for a.3 cm sphere gap with d.c. voltages there was reduction of 0% in breakdown voltage. The reduction in breakdown voltage is less than 5% for gap spacings more than cm and for gap spacings of cm or more it is about.5% Thus irradiation is necessary for smaller sphere gaps of gap spacing less than cm for obtaining consistent values. (f) Influence of polarity and waveform: It has been observed that the breakdown voltages for positive and negative polarity impulses are different. It has been experimentally investigated that for sphere gaps of 6.5 cm to 5 cm diameter, the difference between positive and negative d.c. voltages is not more than %. For smaller sphere gaps ( cm or less diameter) the difference is 8% between negative and positive impulses of /50 µs waveform. For the wavefronts of less than 0.5 µs and wavetails less than 5 µs the breakdown voltages are not consistent. Hence the use of sphere gap is not recommended for voltage measurement in such cases. Cathode Ray Oscilloscope for Impulse Measurements: The modern oscilloscopes are sealed tube hot cathode oscilloscopes provided with photographic arrangement for recording the waveforms. The CRO for impulse work normally has input voltage range from 5 mv/cm to about 0 V/cm. The bandwidth and rise time of the oscilloscope should be adequate. Rise time of 5 ns and bandwidth as high as 500 MHz maybe necessary. Sometimes high voltage surge test oscilloscopes do not have vertical amplifier and directly require an input voltage of 0 V. They can take a maximum signal of about 00 V (peak to peak) but require suitable attenuators for large signals. It is necessary to start the oscilloscope time base before the signal reaches the oscilloscope deflection plates in case of rapidly changing signals otherwise a portion of the signal maybe missed. Such measurements require an accurate initiation of horizontal time base which is known as triggering. The oscilloscopes are normally provided with both internal and external triggering facility. In case of external triggering, the signal is directly fed to start the time base and then applied to the vertical deflecting plates through a delay line. The delay is usually 0. to 0.5 µs. The fig. 7(a) and 7(b) shows the block diagram of surge test oscilloscope where measuring signal is transmitted to the CRO by a normal coaxial 9

10 cable. The delay is obtained by an externally connected coaxial long cable. Another method to obtain delay is using an electronic tripping device to trigger the impulse generator and CRO time base. A first pulse from the device starts the CRO time base and after a predetermined a second pulse triggers the impulse generator. c ZO 3 b a Fig. 7(a) Surge Test Oscilloscope Block Dig. Trigger amplifier a Vertical amplifier input Sweep generator b Input to delay line 3 External delay line c output of delay line to CRO Y plates 3 V(t) Fig. 7(b) Surge Test Oscilloscope Block Dig. Plug in amplifier 4 Trigger amplifier Y amplifier 5 Sweep generator 3 Internal delay line 6 X amplifier The electromagnetic interference is to be avoided and therefore it is essential that leads, layout and connections from the signal sources to CRO are to be arranged in such a manner which avoids induced voltages and stray pick-ups. The connecting cables behave as either capacitive or inductive depending on the load at the end of the cable. In case of fast rising signals to avoid unnecessary reflections at the cable ends, it has to be terminated properly by connecting a resistance equal to surge impedance of cable. The oscilloscopes have finite input impedance usually about to 0 MΩ resistance in parallel with a 0 to 50 pf capacitance thus acting as a load at the end of a surge cable and it attenuates the signal at CRO end. To eliminate noise voltages due to ground loop currents, electromagnetic coupling, cable shields, multiple shielding arrangement as shown in fig. 8 may have to be used. 0

11 HV 4 CRO 3 Potential divider Triple shielded cable 3 Inner shielded enclosure 4 Terminating impedance Fig.8 Multiple shielding arrangement for noise voltage eliminaion