High AC Voltage Measurement

by

High AC voltage Measurement

Following methods can be used

  1. Electrostatic
  2. Potential divider
  3. Potential transformer (Voltage Transformer)
  4. Electrostatic
  5. Sphere gap

Impulse voltage Measurement

Following methods can be used :

1. Voltage dividers.

 (a) Resistance with CRO

 (b) Capacitances and current, voltage transformer

 (c) RC

2. Peak voltmeters

3. Sphere gap

For RMS value measurement methods used are :

  1. Electrostatic voltmeter.
  2. Potential dividers

For PEAK value measurement methods used are :

  1. Peak meter.
  2. Sphere gap.

For instantaneous value : CRO

Potential dividers (for impulse)
Voltage divider for ac and dc or impulse may consist of resistors capacitors or combinations. Inductors are not used as pure inductor without stray capacitors can not built, can form oscillatory circuit with inherent capacitances of the test object and may lead inaccuracy in measurement and high voltages in the measuring circuit.

For measurement of high voltage a potential divider is necessary for tapping a sample of the impulse voltage just sufficient to produce a suitable deflection on the CRO plates.

The potential divider is represented by two impedances Z1, Z2 connected in series and the sample voltage required for measurement is taken from across Z2.

V1 – Voltage to be measured.

V2 – Sample voltage.

V2 = ( Z2 / Z1 + Z2) V1

If the impedances are pure resistance.

V2 = (R2 / R1 + R2) V1

And in case of pure capacitances
V2 = (C2 / C1 + C2) V1

V2 – is in a few hundred volts

V1 – is in Mega Volts

Z1  – should high value with high length

Z2  – small value.

The large length of Z1 results inaccurate measurements because of stray capacitance associated with long length to especially with impulse voltage. Therefore precaution should be taken on L.V. side of potential divider. A screened cable of finite length is required for correction to the oscillograph. Other errors and distortion of wave shape can also occurs.

(a) Resistances potential dividers :

Circuit comprises of high voltage resistance R1 and R2.

Figure A

Resistances potential dividers
Figure B

Resistances potential dividers
Figure C

A delay cable of surge impedance Z and deflecting plates of the CRO. Extra resistances may be connected either in series with the delay cable or across the deflecting plates. The resistances are important from impedance matching view point to obtain given wave shape.

V1 / V2  = R1 / R2

Use :  =

Used for recording of standard impulse voltage.

Merits :

  1. Simplicity of connection.
  2. Less space required.
  3. Less weight.
  4. Easy portability.
  5. Can be placed near the test objects.

Demerits :

  1. Output of voltage divides in distorted for high frequency.
  2. Required a careful electrostatic shielding to minimize stray capacitance effect.
  3. Resistances value causes power loss.

Leave a Comment