1. UNIT 4 : MEASUREMENT OF VERY HIGH VOLTAGES AND CURRENTS
4.0 INTRODUCTION
The following table gives the different methods ( techniques )
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for measurement of very high voltages :
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HIGH CURRENT MEASUREMENT TECHNIQUES :
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5. 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 :
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RESITANCE IN SERIES WITH AMMETER
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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)
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8. RESISTANCE POTENTIAL DIVIDER WITH ELECTROSTATIC VOLTMETER
Resistance Potential Dividers:
RESISTANCE POTENTIAL DIVIDER WITH ELECTROSTATIC VOLTMETER
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300 kV DIVIDER FOR DC ( Ht.210 cm)
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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,
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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
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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’.
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13. SCHEMATIC DIAGRAM OF GENERATING VOLTMETER (ROTATING VANE TYPE )
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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
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of range is easy and
(4)A very convenient instrument for electrostatic device such as Van-de-graff generator and particle accelerators
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16. 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
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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
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18. SERIES CAPACITANCE WITH MILLIAMMETER FOR AC MEASUREMENT
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4.2.2 Capacitance potential dividers: V1 = V2 ( C1+ C2 +Cm)/ C1
CAPACITANCE POTENTIAL DIVIDER
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STANDARD (COMPRESSED GAS) CAPACITOR FOR 1000 kV RMS
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4.2.3 Capacitance voltage transformer:
SCHEMATIC REPRESENTATION OF ‘CVT’
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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,
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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.
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ABSOLUTE ELECTROSTATIC LIGHT BEAM
VOLTMETER ARRANGEMENT
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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
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SERIES CAPACITACE PEAK VOLTMETER
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4.2.6 Peak voltmeters with potential dividers:
PEAK VOLTMETER WITH CAPACITOR POTENTIAL DIVIDER AND ELECTROSTATIC VOLTMETER
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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.
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ELECTRODES FOR 300 kV (rms) BRUCE PROFILE
SPARK GAP (half contour)
UNIFORM FIELD ELECTRODE GAP
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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 .
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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 √ dS
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COMPARISON OF SPARKOVER VOLTAGES USING UNIFORM FIELD GAPS AND SPHERE GAP METHODS AT TEMP. 200 C AND PRESSURE 760 mm of Hg.
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33. 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
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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.
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SCHEMATIC DIAGRAM OF POTENTIAL DIVIDER WITH DELAY CABLE AND OSCILLOSCOPE
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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.
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RESISTANCE POTENTIAL DIVIDER WITH SURGE CABLE AND OSCILLOSCOPIC TERMINATION
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For correct compensation the impedances of the high voltage and low voltage arms are chosen as , R1C1=R2Cm
Potential dividers for high impulse voltages: Resistance Dividers :
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EQUIVALENT CIRCUIT OF A RESISTANCE POTENTIAL DIVIDER WITH SHIELD AND GUARD RINGS
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Capacitance voltage dividers:
CAPACITANCE VOLTAGE DIVIDER FOR VERY HIGH VOLTAGES AND ITS EQUIVALENT CIRCUIT
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CAPACITOR DIVIDER FOR 6 MV IMPULSE VOLTAGE
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Resistance –Capacitance Dividers:
RESISTANCE-CAPACITANCE CAPACITANCE
MIXED DIVIDER DIVIDER
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43. 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).
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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.
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VERTICAL SPHERE GAP
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HORIZONTAL SPHERE GAP
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The clearance around the spheres for various diameters are given below:
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48. 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
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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 .
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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.
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There are two methods to obtain the 50 % disruptive discharge voltage namely,
Average method and
Up and down method
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AVERAGE METHOD
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UP AND DOWN METHOD
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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.
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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.
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60. 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
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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
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LOW OHMIC SHUNT EQUIVALENT CIRCUIT OF
SHUNT
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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.
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BIFILAR FLAT STRIP RESISTIVE SHUNT
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SCHEMATIC ARRANGEMENT OF A COAXIAL OHMIC SHUNT
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