Measurement of High Voltage(Naseer Ahmed)

8/4/2019 Measurement of High Voltage(Naseer Ahmed)

http://slidepdf.com/reader/full/measurement-of-high-voltagenaseer-ahmed 1/28

Measurement of high voltage

Presentation

By:-

Electrical Batch(2007 _ 2011)

UNDER THE GUIDANCE

Dr. Abdul Qadir Chang



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



Types Of High Voltages

System maximum voltage

lightning over voltages

Impulse high voltages

Switching (transient) over voltage s

Temporary over voltages



Classification w.r.t Measurement

Measurement of high direct current voltages

Measurement of ac high voltages(system frequency)

Impulse over voltages



Measurement Of High Direct Current

Voltages

Series Resistance micro ammeter

Resistance potential divider

Generating Voltmeters

Sphere Gaps



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



Drawbacks

Power dissipation

Temperature effects

Leakage currents

Limitations of voltage stress

This meter is built up to 500KV DC with 0.2% accuracy



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



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



Generating Voltmeter

Principle :-

Sinusoidal varying capacitance causes

production of current that is proportional to

applied voltage







The scale of Generating

voltmeter is linear as in

fig

The calibration is done

by potential divider or by

sphere gap




Advantages:-

No source loading

No direct connection with high voltage source

It is a linear scale instrument and scale can be

extrapolated



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



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.



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.



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



Measurement of ac high

voltages(system frequency

Series impedance voltmeter

Electrostatic Voltmeters

Potential Dividers

Sphere gaps



Measurement of ac high

voltages(system frequency)

Resistive Potential Divider

Potential divider ratio DR = U/V



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




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.







PrincipleF=1/2 €V2A/D2

Electrostatic force is directly proportional to V²(non

linear scale ) ,it can measure both ac and dc




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



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




Litchenberg patternsKlydonograph




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



End Of Presentation

Thank you

Measurement of High Voltage(Naseer Ahmed)

Documents