Measurement Of High Measurement Of High Voltages Voltages & High Currents& High Currents Unit 4Unit 4

High Voltage Measurement High Voltage Measurement TechniquesTechniques

2

Measurement Of High DC VoltageMeasurement Of High DC Voltage

3

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

Sphere GapsSphere 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: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).

4

5

Sphere GapsSphere 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:

6

Sphere GapsSphere Gaps

Fig C :Breakdown voltage characteristic of 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.

7

Sphere GapsSphere Gaps

Factors Influencing the Sparkover Voltage of Sphere Gapsi. 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

8

Sphere GapsSphere Gaps

9

Sphere GapsSphere Gaps

High Ohmic Series Resistance High Ohmic Series Resistance with Microammeterwith 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

10

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

11

High Ohmic Series Resistance High Ohmic Series Resistance with Microammeterwith Microammeter

Resistance Potential Divider Resistance Potential Divider

12

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

2

2121

21

212

R)R(R

VVmagnitude, voltage High

)R(RR

VV

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

13

Resistance Potential Divider Resistance Potential Divider

Generating VoltmeterGenerating Voltmeter

14

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,

15

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 VoltmeterGenerating Voltmeterdt

tdCtV

dt

tdVtC

dt

tdqti

)()(

)()(

)()(

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.

16

Generating VoltmeterGenerating Voltmeter

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 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:

i. Scale is linear and can be extrapolated

ii. Source loading is practically zero

iii. No direct connection to the high voltage electrode

iv. Very convenient instrument for electrostatic devices

Limitations:i. They require calibration

ii. Careful construction is needed and is a cumbersome instrument requiring an auxiliary drive

iii. Disturbance in position and mounting of the electrodes make the calibration invalid.

17

Generating VoltmeterGenerating Voltmeter