Measurement of High Voltage(Naseer Ahmed)
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Transcript of Measurement of High Voltage(Naseer Ahmed)
8/4/2019 Measurement of High Voltage(Naseer Ahmed)
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Measurement of high voltage
Presentation
By:-
Electrical Batch(2007 _ 2011)
UNDER THE GUIDANCE
Dr. Abdul Qadir Chang
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Why High Voltages Are Measured
For industrial tests
In research laboratories work
To determine the over voltages of the system
To measure the electric and magnetic fields associated with
high voltage lines that may have effect on nearby living people
To determine insulation level for safety purposes the
insulations are tested under system & under high voltage
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Types Of High Voltages
System maximum voltage
lightning over voltages
Impulse high voltages
Switching (transient) over voltage s
Temporary over voltages
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Classification w.r.t Measurement
Measurement of high direct current voltages
Measurement of ac high voltages(system frequency)
Impulse over voltages
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Measurement Of High Direct Current
Voltages
Series Resistance micro ammeter
Resistance potential divider
Generating Voltmeters
Sphere Gaps
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Series Resistance Micro Ammeter
Series resistance in hundreds of MΩconstructed from wire wound resisters
that permit a current in microampere
for full scale deflection o micro
ammeter
The potential drop across resistor
should not exceed the insulation
breakdown limit The material of resistor is carbon alloy
with T.C.O .R of (1/10000) / celieus± 0.1 carbon resistor located in air tight
transformer oil PVC tube for 100KV
measurement is used
Zener
diode or
neon gas
V =IR , I = 50µA
V = Measured voltage
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Drawbacks
Power dissipation
Temperature effects
Leakage currents
Limitations of voltage stress
This meter is built up to 500KV DC with 0.2% accuracy
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2.Resistance potential divider
Circuit diagram
DC voltages are measured using resistance
voltage divider
R2 <<< R1
High DC voltage applied.
Drop across R1
Measure the voltage directly across R2
ESV
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Improvements & Drawbacks
Negligible source loading
Limited voltage stress
Capacitors are used to linear the stress across the
resistor elements
Power dissipation is still a drawback of this technique
Flash over takes place due to transient voltages
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Generating Voltmeter
Principle :-
Sinusoidal varying capacitance causes
production of current that is proportional to
applied voltage
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Generating Voltmeter
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Generating Voltmeter
The scale of Generating
voltmeter is linear as in
fig
The calibration is done
by potential divider or by
sphere gap
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Generating Voltmeter
Advantages:-
No source loading
No direct connection with high voltage source
It is a linear scale instrument and scale can be
extrapolated
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Sphere Gaps
The sphere gap method of measuring high voltage is the
most reliable and is used as the standard for calibration
purposes.
The breakdown strength of a gas depends:-
Ionization of the gas molecules
The density of the gas
The size of the spheres
Their distance
Air density correction factor
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Sphere Gaps
The accuracy depends upon d/D ratio
If d/D ratio is 0.5 then accuracy is ±3% (BS :358 IEC
PUB 52)
If d / D is 0.75 then accuracy is ± 5 (BS :358, IEC 358 )
For accurate measurement purposes, gap
distances in excess of 0.75D are not used.
Usually, a resistance is used in series with the sphere gap, of
about 1Ω /v so as to limit the current under spark over conditions
to about a maximum of 1 A.
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Sphere Gaps
In measuring impulse voltages, since the breakdown does not occur at exactly the
same value of voltage each time, what is generally specified is the 50 %breakdown value.
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Sphere Gaps
In case of ac peak value and dc voltage measurement
, the voltage is uniformly increased unto spark over
The mean of five readings is take, if they agree with
±3 then this is declared as measured readingFor impulse voltages two limits are taken not
differing by ±2 ,for lower limit 4 flashovers take
place while for upper limit 8 or 6 flashovers take
place, the mean of these voltages is taken as 50%
flashover voltage
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Measurement of ac high
voltages(system frequency
Series impedance voltmeter
Electrostatic Voltmeters
Potential Dividers
Sphere gaps
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Measurement of ac high
voltages(system frequency)
Resistive Potential Divider
Potential divider ratio DR = U/V
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Capacitive potential divider method
In this two capacitances C1 and C2 are used in series,
The electrostatic voltmeter being connected across the lower
capacitor.
The effective capacitance of C1
and C2 in series is C1C2/(C1+C2),
and since the charge is the same,
Voltage across C2 = C1 / (C1 + C2)V
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Capacitive potential divider method
The capacitance of h.v. standard capacitor must be accurately
known, and the capacitance must be free from dielectric
losses . For this reason, air capacitances are always used for
this purpose.
This method also measures the r.m.s. value.
If the shape of the voltage waveform is known, the peak
voltage may be obtained from the r.m.s. voltage.
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Capacitive potential divider method
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Electrostatic Voltmeters
PrincipleF=1/2 €V2A/D2
Electrostatic force is directly proportional to V²(non
linear scale ) ,it can measure both ac and dc
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Electrostatic Voltmeters
Small capacitance 5 t0 50 µf
High insulation resistance (R >10¹³ )
Accuracy for ac = ±0.25% dc = ±0.1
A movement in millimeter can measure large voltages
The limitation on gap distance is the v/d ratio which must not
exceed break down limit
The enclosed structure inctruments containing compressed
air, CO₂ & NO₂ with high stress of 100KV/cm can beused,these meters have compact shapesss
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Measurement of Surges
• Klydonograph
Lightning is probably the most special of the high voltage
phenomena
it is not possible to create lightning or to obtain a lightningstrike when and where we please
The phenomena of the lightning could be studied using
Litchenberg patterns obtained
The maximum voltage that can be measured using aKlydonograph is dependant on the thickness of the dielectric
material
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Measurement of Surges
Litchenberg patternsKlydonograph
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Measurement of Surges
This print on the photographic plate is not due to normal photographic
action, and occurs even through there is no visible discharge between the
electrodes. If flashover of the insulator or a visible discharge occurs, then
the film would become exposed and no patterns would be obtained
The vlotgae value depends upon radius of pattern
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End Of Presentation
Thank you