Measurement Of High Voltages

1
Measurement Of High Voltages High Currents

• Unit 4

2
High Voltage Measurement Techniques
3
Measurement Of High DC Voltage

• Series Resistance Micrometer
• Resistance Potential Divider
• Generating Voltmeter
• Sphere and Other Gaps

4
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).

5
Sphere Gaps
6
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
7
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, VskVn where k is a factor related
  to the relative air density (RAD) d.
• The relation between the RAD(d) and the
  correction factor k
• Under impulse voltages, the voltage at which
  there is a 50 breakdown probability is
  recognized as the breakdown level.

8
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 lt 0.5 D ?Accuracy 3
• 0.75 D gt d gt 0.5 D ?Accuracy 5
• For accurate measurement purposes, gap distances
  in excess of 0.75D are not used

9
Sphere Gaps
10
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, VIR

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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

12
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

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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

14
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 doesnt 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,

15
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 ?t
• and the current i' is then given as, i(t) im
  cos ? t , where im VCm?
• Here ? 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 ? 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).

16
Generating Voltmeter

• Instantaneous value of current i(t) Cm fvV(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.

17
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.