Transcript Document

Measurement Of High Voltages
& High Currents
Unit 4
High Voltage Measurement Techniques
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Measurement Of High DC Voltage
Series Resistance Micrometer
Resistance Potential Divider
Generating Voltmeter
Sphere and Other Gaps
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Sphere Gaps
Applicatios:
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Voltage Measurement (Peak) - Peak values of voltages may be measured from 2 kV up
to about 2500 kV by means of spheres.
Arrangements:
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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).
Sphere Gaps
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Sphere Gaps
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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).
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Standard values of sphere diameter are 6.25, 12.5, 25, 50, 75, 100, 150, and 200
cm.
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The Clearance around the sphere gaps:
Fig C :Breakdown voltage characteristic of
sphere gaps
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Sphere Gaps
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The effect of humidity is to increase the breakdown voltage of sphere gaps by
up to 3%.
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Temperature and pressure, however, havea significant influenceo n breakdown
voltage.
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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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The relation between the RAD(δ) and the correction factor k:
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Under impulse voltages, the voltage at which there is a 50% breakdown
probability is recognized as the breakdown level.
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Sphere Gaps
Factors Influencing the Sparkover Voltage of Sphere Gaps
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Nearby earthed objects,
ii.
Atmospheric conditions and humidity,
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Irradiation, and
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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:
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d < 0.5 D
Accuracy = ± 3 %
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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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Sphere Gaps
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High Ohmic Series Resistance with Microammeter
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Resistance (R) :
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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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High Ohmic Series Resistance with Microammeter
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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:
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Power dissipation & source loading
Temp effects & long time stability
Voltage dependence of resistive elements
Sensitivity to mechanical stresses
Resistance Potential Divider
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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
V2  V1 
R2
(R 1  R 2 )
High voltagemagnitude, V1  V2 
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Sudden voltage changes during transients due to:
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(R 1  R 2 )
R2
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
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At high voltage ends, corona free termination is used to avoid
unnecessary discharges.
Accuracy:
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0.05% accuracy up to 100 kV
0.1% accuracy up to 300 kV
0.5% accuracy for 500 kV
Generating Voltmeter
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Generating voltmeter: A variable
electrostatic voltage generator.
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It generates current proportional to voltage under
measurement.
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This arrangement provides loss free measurement
of DC and AC voltages
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It is driven by synch. motor, so doesn’t observe
power from the voltage measuring source
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The high voltage electrode and the grounded
electrode in fact constitute a capacitance system.
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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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capacitor
Generating Voltmeter
and, i(t ) 
dq (t )
dV (t )
dC (t )
 C (t )
 V (t )
dt
dt
dt
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.
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Generally the current is rectified and measured by a moving coil meter
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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.
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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).
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Generating Voltmeter
Instantaneous value of current i(t) = Cm fvV(t)
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where fv = 1/Tv the frequency of voltage.
Since fv = 2fc and fc =1/( 60/n) we obtain,
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I(t) = (n/30) CmV(t)
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Fig. shows a schematic diagram of a generating
voltmeter which employs rotating vanes for variation of
capacitance
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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.
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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.
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Generating Voltmeter
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Generating voltmeters are linear scale instruments and applicable over a wide range of
voltages.
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The sensitivity can be increased by increasing the area of the pick up electrode and by
using amplifier circuits
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Advantages:
i.
Scale is linear and can be extrapolated
ii.
Source loading is practically zero
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No direct connection to the high voltage electrode
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Very convenient instrument for electrostatic devices
Limitations:
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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.
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