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UNIT 4 : MEASUREMENT OF VERY HIGH VOLTAGES AND CURRENTS 4.0 INTRODUCTION The following table gives the different methods ( techniques ) Dr M A Panneerselvam, Professor, Anna University 1 for measurement of very high voltages : Dr M A Panneerselvam, Professor, Anna University 2 Dr M A Panneerselvam, Professor, Anna University 3 HIGH CURRENT MEASUREMENT TECHNIQUES : Dr M A Panneerselvam, Professor, Anna University 4 4.1 MEASUREMENT OF HIGH DC VOLTAGES The various methods of measuring very high currents are explained through the following figures: 4.1.1 High resistance in series with micro ammeter : Dr M A Panneerselvam, Professor, Anna University 5 RESITANCE IN SERIES WITH AMMETER Dr M A Panneerselvam, Professor, Anna University 6 Referring to circuit (a) ,the voltage v(t) = R i(t) Referring to circuit (b), v(t) = v2 (t) ( R1 + R2 ) / R2 = v2 (t) (1 + R1/R2) V = V2 ( 1 + R1/R2) Dr M A Panneerselvam, Professor, Anna University 7 4.1.2 Resistance Potential Dividers: RESISTANCE POTENTIAL DIVIDER WITH ELECTROSTATIC VOLTMETER Dr M A Panneerselvam, Professor, Anna University 8 300 kV DIVIDER FOR DC ( Ht.210 cm) Dr M A Panneerselvam, Professor, Anna University 9 4.1.3 Generating Voltmeters: The charge stored in a capacitor C is given by, q = cv If capacitance varies with time , when connected to voltage source, the current through the capacitor, Dr M A Panneerselvam, Professor, Anna University 10 i = dq/dt = v dc/dt + c dv/dt For DC voltages dv/dt = 0 and hence , I = dq/dt = v dc/dt If capacitance varies between the limits C0 and (C0 + Cm) sinusoidally as,C = C0+ Cm sin ωt Dr M A Panneerselvam, Professor, Anna University 11 the current ‘i’ is given by, i = v dc/dt = v cm ω cos ωt I = Im cos ωt where Im = VωCm For a constant angular frequency ‘ω’, the current is proportional to the applied voltage ‘V’. Dr M A Panneerselvam, Professor, Anna University 12 SCHEMATIC DIAGRAM OF GENERATING VOLTMETER (ROTATING VANE TYPE ) Dr M A Panneerselvam, Professor, Anna University 13 The advantages of a generating voltmeters are : 1)No source loading by the meter 2)No direct connection to the HV electrode 3)Scale is linear and extension Dr M A Panneerselvam, Professor, Anna University 14 of range is easy and (4)A very convenient instrument for electrostatic device such as Van-de-graff generator and particle accelerators Dr M A Panneerselvam, Professor, Anna University 15 4.2 MEASUREMENT OF HIGH AC VOLTAGES For power frequency AC measurements series impedance like pure resistor or reactance can be used. Since resistances involve power losses, often capacitor is preferred. Resistance varies with Dr M A Panneerselvam, Professor, Anna University 16 temperature and also have stray capacitances. Hence series capacitance is mostly used. 4.2.1 Series capacitance voltmeter: This method is recommended only for pure sinusoidal voltages. i.e., Ic = jωcv Dr M A Panneerselvam, Professor, Anna University 17 SERIES CAPACITANCE WITH MILLIAMMETER FOR AC Dr M A Panneerselvam, Professor, MEASUREMENT Anna University 18 4.2.2 Capacitance potential dividers: V1 = V2 ( C1+ C2 +Cm)/ C1 CAPACITANCE POTENTIAL DIVIDER Dr M A Panneerselvam, Professor, Anna University 19 STANDARD (COMPRESSED GAS) CAPACITOR FOR 1000 kV RMS Dr M A Panneerselvam, Professor, Anna University 20 4.2.3 Capacitance voltage transformer: Dr MREPRESENTATION A Panneerselvam, Professor, OF ‘CVT’ SCHEMATIC Anna University 21 Resonance occurs when ω ( L1+L2) equals 1/ ω (C1+C2) 4.2.4 Electrostatic voltmeters: In electrostatic fields, the attractive force between the electrodes of parallel plate condensor is given by, Dr M A Panneerselvam, Professor, Anna University 22 F= - dWs/ds = d/ds ((1/2 )CV2 ) = ½ V2 dc/ds =1/2 ε0 A ( V/s)2 As the force is proportional to the square of the voltage , the measurement can be made for both AC and DC voltages. Dr M A Panneerselvam, Professor, Anna University 23 ABSOLUTE ELECTROSTATIC VOLTMETER LIGHT BEAM ARRANGEMENT Dr M A Panneerselvam, Professor, Anna University 24 4.2.5 Series capacitance peak voltmeter: ( Chubb-Frotscue method): In this method a half wave rectifier is connected in series with a capacitance and an ammeter as shown in the figure next . The rectified current reading,I = Vm ω C Dr M A Panneerselvam, Professor, Anna University 25 SERIES CAPACITACE PEAK VOLTMETER Dr M A Panneerselvam, Professor, Anna University 26 4.2.6 Peak voltmeters with potential dividers: PEAK VOLTMETER WITH CAPACITOR POTENTIAL DIVIDER AND ELECTROSTATIC VOLTMETER Dr M A Panneerselvam, Professor, Anna University 27 Discharge resistor Rd is used to permit variation of Vm when it is reduced. 4.2.7 Uniform field gaps: The arrangement of an uniform field gap is shown in the next slide. Dr M A Panneerselvam, Professor, Anna University 28 ELECTRODES FOR 300 kV (rms) SPARK GAP BRUCE PROFILE (half contour) UNIFORM FIELD ELECTRODE GAP Dr M A Panneerselvam, Professor, Anna University 29 Ragowski presented a design for uniform field electrodes for spark over voltages upto 600 kV and is given by, V= AS + B √S where ‘A’ and ‘B’ are constants and ‘S’ is the gap spacing . Dr M A Panneerselvam, Professor, Anna University 30 At a temperature of 250 C and pressure 760 mm of Hg , taking air density factor ‘d’ into account sparkover voltage ‘V’ is given as, V= 24.4 dS + 7.50 √ dS Dr M A Panneerselvam, Professor, Anna University 31 COMPARISON OF SPARKOVER VOLTAGES USING UNIFORM FIELD GAPS AND SPHERE GAP METHODS AT TEMP. 200 C AND PRESSURE 760 mm of Hg. Dr M A Panneerselvam, Professor, Anna University 32 4.3 MEASUREMENT OF HIGH IMPULSE VOLTAGES 4.3.1 Potential Dividers: Potential Dividers for high voltage impulse, high frequency AC and fast rising transient voltage Dr M A Panneerselvam, Professor, Anna University 33 measurements are either resistive or capacitive or mixed element type. The low voltage arm of the divider is usually connected to a fast recording oscilloscope or a peak reading instrument through a delay cable. Dr M A Panneerselvam, Professor, Anna University 34 SCHEMATIC DIAGRAM OF POTENTIAL DIVIDER WITH DELAY CABLE AND OSCILLOSCOPE Dr M A Panneerselvam, Professor, Anna University 35 4.3.1.1 Resistance potential dividers for low impulse voltages: The wave form of the output voltage measured across the low voltage arm should be a correct replica of the input wave shape. Dr M A Panneerselvam, Professor, Anna University 36 RESISTANCE POTENTIAL DIVIDER WITH SURGE CABLE AND OSCILLOSCOPIC TERMINATION Dr M A Panneerselvam, Professor, Anna University 37 For correct compensation the impedances of the high voltage and low voltage arms are chosen as , R1C1=R2Cm 4.3.1.2 Potential dividers for high impulse voltages: Resistance Dividers : Dr M A Panneerselvam, Professor, Anna University 38 EQUIVALENT CIRCUIT OF A RESISTANCE POTENTIAL DIVIDER WITH SHIELD AND GUARD RINGS Dr M A Panneerselvam, Professor, Anna University 39 4.3.2 Capacitance voltage dividers: CAPACITANCE VOLTAGE DIVIDER FOR VERY HIGH VOLTAGES AND ITS EQUIVALENT CIRCUIT Dr M A Panneerselvam, Professor, Anna University 40 CAPACITOR DIVIDER FOR 6 MV IMPULSE VOLTAGE Dr M A Panneerselvam, Professor, Anna University 41 4.3.3 Resistance –Capacitance Dividers: RESISTANCE-CAPACITANCE CAPACITANCE MIXED DIVIDER Dr M A Panneerselvam, Professor, Anna University DIVIDER 42 4.4 MEASUREMENT OF HIGH VOLTAGES USING SPHERE GAPS Sphere gaps are used to measure peak values of all types of high voltages (DC,AC,Impulse and Switching surges). Dr M A Panneerselvam, Professor, Anna University 43 The accuracy with potential dividers is very high provided the divider ratio is estimated correctly. Whereas the measurement with sphere gaps are fool proof though the accuracy is less. Dr M A Panneerselvam, Professor, Anna University 44 Dr M A Panneerselvam, Professor, VERTICAL SPHERE GAP Anna University 45 HORIZONTAL SPHERE GAP Dr M A Panneerselvam, Professor, Anna University 46 The clearance around the spheres for various diameters are given below: Dr M A Panneerselvam, Professor, Anna University 47 50 % DISRUPTIVE DISCHARGE APPLICABLE TO IMPULSE VOLTAGE BREAKDOWN Unlike DC or AC voltages, the impulse voltage is applied only for microseconds duration. Provided we apply sufficient voltage to Dr M A Panneerselvam, Professor, Anna University 48 cause a disruptive discharge , the breakdown may occur once and may not occur the next time when the same level of voltage is applied. Hence we resort to statistical methods to obtain the disruptive discharge voltage . Dr M A Panneerselvam, Professor, Anna University 49 50% disruptive discharge voltage is that voltage which causes disruptive discharges for 50 % of the total number of applications . Higher the number of applications we get more accurate values. Dr M A Panneerselvam, Professor, Anna University 50 There are two methods to obtain the 50 % disruptive discharge voltage namely, Average method and Up and down method Dr M A Panneerselvam, Professor, Anna University 51 AVERAGE METHOD Dr M A Panneerselvam, Professor, Anna University 52 UP AND DOWN METHOD Dr M A Panneerselvam, Professor, Anna University 53 Disruptive discharge voltages: The peak disruptive discharge voltages(50 % disruptive discharge for impulse voltages) for AC voltage, negative polarity of both impulse and switching surge and DC voltage of both polarities are given in the following tables. Dr M A Panneerselvam, Professor, Anna University 54 Dr M A Panneerselvam, Professor, Anna University 55 Dr M A Panneerselvam, Professor, Anna University 56 Peak disruptive discharge voltages (50 % disruptive discharge for impulse voltages) for positive polarity of both impulse and switching surge voltages are given in the following tables at a temp. of 200 C and pressure 760 mm of Hg. Dr M A Panneerselvam, Professor, Anna University 57 Dr M A Panneerselvam, Professor, Anna University 58 Dr M A Panneerselvam, Professor, Anna University 59 4.5 MEASUREMENT OF HIGH FREQUENCY AND IMPULSE CURRENTS The most common method for high impulse current measurements is a low ohmic pure resistive shunt . The voltage drop Dr M A Panneerselvam, Professor, Anna University 60 across the shunt , v(t)= R i(t) The measuring circuit is shown in the next slide. There are two types of current shunts , namely (1) Bifilar flat strip shunt and (2) Tubular shunt . As the voltage drop across the shunt is measured through an Dr M A Panneerselvam, Professor, Anna University 61 LOW OHMIC SHUNT EQUIVALENT CIRCUIT OF SHUNT Dr M A Panneerselvam, Professor, Anna University 62 oscilloscope , the wave form should be a true replica of the current wave form. Hence special care is taken during the design of current shunts that they should be of pure resistance only without inductance or capacitance. Dr M A Panneerselvam, Professor, Anna University 63 BIFILAR FLAT STRIP RESISTIVE SHUNT Dr M A Panneerselvam, Professor, Anna University 64 SCHEMATIC ARRANGEMENT OF A COAXIAL OHMIC SHUNT Dr M A Panneerselvam, Professor, Anna University 65