Measurement Of High DC Voltage • Series Resistance Micrometer • Resistance Potential Divider • Generating Voltmeter • Sphere and Other Gaps
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: • Vertically with lower sphere grounded (For Higher Voltages) • Horizontally with both spheres connected to the source voltage or one sphere grounded (For Lower Voltages).
Sphere Gaps • 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: Fig C :Breakdown voltage characteristic of sphere gaps
Sphere Gaps • 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, Vs=kVn 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.
Sphere Gaps • Factors Influencing the Sparkover Voltage of Sphere Gaps • Nearby earthed objects, • Atmospheric conditions and humidity, • Irradiation, and • 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
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 10-4/oC . • 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
High Ohmic Series Resistance with Microammeter • 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. • Maximum voltage: 500kV with 0.2% accuracy. • Limitations: • Power dissipation & source loading • Temp effects & long time stability • Voltage dependence of resistive elements • Sensitivity to mechanical stresses
Resistance Potential Divider • It uses electrostatic voltmeter or high impedance voltmeter. • Can be placed near the test object which might not always be confined to one location • Let, V2-Voltage across R2 • Sudden voltage changes during transients due to: • Switching operation • Flashover of test objects • Damage due to stray capacitance across the elements & ground capacitance • To avoid sudden changes in voltages, voltage controlling capacitors are connected across the elements
Resistance Potential Divider • At high voltage ends, corona free termination is used to avoid unnecessary discharges. • Accuracy: • 0.05% accuracy up to 100 kV • 0.1% accuracy up to 300 kV • 0.5% accuracy for 500 kV
Generating Voltmeter • 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 doesn’t 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,
Generating Voltmeter and, For d.c. Voltages, Hence If the capacitance C varies sinusoidally between the limits C0 and (C0 + Cm) then C = C0 + Cm sin ωt and the current ‘i' is then given as, i(t) = im cos ω t , where im = VCmω 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 • Instantaneous value of current i(t) = CmfvV(t) • where fv = 1/Tv the frequency of voltage. • Since fv = 2fc and fc =1/( 60/n) we obtain, I(t) = (n/30) CmV(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 D3 which is kept at a fixed distance on the axis of the other low voltage electrodes D2, D1, and D0. • The rotor D0 is driven at a suitable constant speed by a synchronous motor. • Rotor vanes of D0 cause periodic change in capacitance between the insulated disc D2 and the high voltage electrode D3. • 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.
Generating Voltmeter • 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: • Scale is linear and can be extrapolated • Source loading is practically zero • No direct connection to the high voltage electrode • Very convenient instrument for electrostatic devices • Limitations: • They require calibration • Careful construction is needed and is a cumbersome instrument requiring an auxiliary drive • Disturbance in position and mounting of the electrodes make the calibration invalid.