This document discusses several methods for measuring high DC voltages: 1. Series resistance micrometers measure voltage by passing a known small current through a high-value resistor and measuring the voltage drop, allowing measurement up to 500kV. 2. Resistance potential dividers use two high-value resistors to proportionally step down a high voltage to a measurable level. 3. Generating voltmeters induce a small current proportional to the measured voltage without a direct connection. 4. Sphere gaps measure peak voltages up to 2500kV by measuring the sparkover voltage between two conductive spheres. Atmospheric conditions and spacing accuracy affect measurements.

1. Measurement Of High Dc Voltage
Contents
•Series resistance micrometer
•Resistance potential divider
•Generating voltmeter
•Sphere gaps
•Conclusion and Reference
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3. Series resistance micrometer
• 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
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4. • 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
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5. • 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 up to 500kV (D.C)
• Accuracy = ±0.2%
• Selection of R value:
– Current allowed: 1 to 10A
– Corona free termination
– Temp. coefficient<10-4/0C : Carbon Alloy
– Placed in airtight, oil filled PVC tube to maintain temp. stability
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6. Resistance potential divider
• It uses electrostatic 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
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8. Generating voltmeter
• 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 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,
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9. Schematic of generating voltmeter
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10. • 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 (sinusoidal 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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11. • 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.
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12. Sphere Gaps
• Applications:
– 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).
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13. • The arrangement is selected based on the relation
between the peak voltage, determined by spark over
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 effect of humidity is to increase the breakdown
voltage of sphere gaps by up to 3%.
• Temperature and pressure, however, have significant
influence in breakdown voltage.
• Breakdown Voltage under normal atmospheric
conditions is, Vs=kVn where k is a factor related to the
relative air density (RAD) δ.
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14. • 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.
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15. • Factors Influencing the Spark over 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
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16. • Conclusion
From these we conclude that how to measure
varies measurements of how to generate high
DC voltage in power system engineering.
• References
“High voltage engineering ” by M S Naidu and
V Kamaraju, Tata McGraw Hill Education, 5th
edition.
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