UNIT IV - High Voltage Engineering

MEASUREMENT OF HIGH VOLTAGES AND CURRENTS

ByAnish John paul. MHead of SchoolSchool of Electrical & Electronics Engineering

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Series resistance micrometer Resistance potential divider Generating voltmeter Sphere and other sphere gaps

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A very high resistance in series with a micrometer. V = IR The resistance is constructed from a large no. of wire

wound resistors in series. Limitations:

◦ Power dissipation◦ Temperature effects and long time stability,◦ Voltage dependence of resistive elements,◦ sensitivity to mechanical stresses.

Series resistance meters are built for 500 kV d.c. with an accuracy better than 0.2%.

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It uses electrostatic voltmeter or high impedance voltmeter.Let, V2-Voltage across R2

The influence of temperature and voltage on the elements is eliminated in the voltage divider arrangement.Sudden voltage changes during transients due to:

Switching operationFlashover of test objects

To avoid sudden changes in voltages, voltage controlling capacitors are connected across the elements

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2121

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R)R(R

VVmagnitude, voltage High

)R(RR

VV

A generating voltmeter is a variable capacitor electrostatic voltage generator.

It generates current proportional to the applied external voltage.

This arrangement provides loss free measurement of DC and AC voltages

The device is driven by an external synchronous or constant speed motor and does not absorb power or energy from the voltage measuring source.

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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,

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

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Generating voltmeters employ rotating sectors or vanes for variation of capacitance.

The high voltage source is connected to a disc electrode S3 which is kept at a fixed distance on the axis of the other low voltage electrodes S0, S1 and S2.

The rotor S0 is driven at a constant speed by a synchronous motor at a suitable speed (1500,1800,3000, or 3600 rpm).

The rotor vanes of S0 cause periodic change in capacitance between the insulated disc S2 and the h.v. electrode S3.

The shape and number of the vanes of S0 and S1 are so designed that they produce sinusoidal variation in the capacitance.

The generated a.c. current through the resistance R is rectified and read by a moving coil instrument.

If the current is small an amplifier may be used before the current is measured.

The instrument is calibrated using a potential divider or sphere gap.

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Advantages of Generating Voltmeters

◦ No direct connection to high voltage electrode,

◦ Scale is linear and extension of range is easy,

◦ A very convenient instrument for electrostatic devices such as Van de Graaff

Limitations of Generating Voltmeters

◦ They require calibration,

◦ Careful construction is needed

◦ Disturbance in position and mounting of the electrodes make the calibration

invalid.

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Series impedance voltmeter Potential dividers (resistance or capacitance type) Potential transformers (Electromagnetic or CVT) Electrostatic voltmeter Sphere gaps

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Electrostatic VoltmeterElectrostatic Voltmeter One of the direct methods of measuring high voltages is

by means of electro-static voltmeters. For voltages above 10 kV, generally the attracted disc

type of electrostatic voltmeter is used. When two parallel conducting plates (cross section area

‘A’ and spacing ‘s’) are charged q and have a potential difference V, then the energy stored in the is given by

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Newtons

VA

2

1F

s

A

ds

dC

s

AC e,capacitancfielduniformFor

Newtonds

dCV

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1FForce,

ds FdCVdWCVW

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2

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ε

εε

2

1

2

1

It is thus seen that the force of attraction is proportional to the square of the potential difference applied, so that the meter reads the square value (or can be marked to read the rms value).

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Electrostatic voltmeters of the attracted disc type may be connected across the high voltage circuit directly to measure up to about 200 kV, without the use of any potential divider or other reduction method. [The force in these electrostatic instruments can be used to measure both a.c. and d.c. voltages].

The right hand electrode forms the high voltage plate. The centre portion of the left hand disc is cut away and encloses a small disc which

is movable and is geared to the pointer of the instrument. This can be achieved by suspension of the moving electrode on one arm of a

balance or its suspension on a spring The small movement is generally transmitted and amplified by a spot light and

mirror system, but many other systems have also been used. An incident light beam will therefore be reflected toward a scale calibrated to read

the applied voltage magnitude. The electrostatic measuring device can be used for absolute voltage measurements

since the calibration can be made in terms of the fundamental quantities of the gap length and forces.

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Advantages:

i. Active power losses are negligibly small

ii. Voltages upto 600kV can be measured.

Disadvantage:

i. For constant distance ‘s’, F α V2, the sensitivity is small. This can

be overcome by varying the gap distance d in appropriate steps.

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Electrostatic VoltmeterElectrostatic Voltmeter

Absolute Electrostatic Voltmeter

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Series Impedance VoltmeterSeries Impedance Voltmeter For power frequency a.c. measurements the series impedance may be a pure

resistance or a reactance. But use of resistances yields the followings,

◦ Power losses◦ Temperature problem◦ Residual inductance of the resistance gives rise to an impedance different from its ohmic

resistance. High resistance units for high voltages have stray capacitances and hence a unit

resistance will have an equivalent circuit as shown in Fig. At any frequency ω of the a.c. voltage, R+jXL is connected in parallel with –jXC.

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CRj

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CRjLCSince

CRjLC

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CjLjR

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LanglePhasewhere

CRR

LjRCRjLjRZ

RC

LCRCRjLjRZ

CRj

CRj

CRj

LjRZ

ωω

tanφ,,

ωω

1ωω

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ω1

ω

1

2

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Extended Series Resistance neglecting inductance is shown in figures. Resistor unit then has to be taken as a transmission line equivalent, for calculating

the effective resistance. Ground or stray capacitance of each element influences the current flowing in the

unit, and the indication of the meter results in an error. Stray ground capacitance effects can be removed by shielding the resistor ‘R’ by a

second surrounding spiral RS which shunts the actual resistor but does not contribute to the current through the instrument.

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Series Capacitance VoltmeterSeries Capacitance Voltmeter To avoid the drawbacks pointed out Series impedance voltmeter, a series

capacitor is used instead of a resistor for a.c. high voltage measurements. Current through the instrument, Ic=V/Xc=jωCV The rms value of the voltage V with harmonics is given by,

where V1,V2 ,... ,Vn represent the rms value of the fundamental, second... and nth harmonics.

The currents due to these harmonics are

I1=ωCV1 , I2=2ωCV2 , ……In=nωCVn

Not recommended when a.c. voltages are not pure sinusoidal waves but contain considerable harmonics.

Used for measuring rms values up to 1000 kV.

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222

21 nrms VVVV

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21 2ω nrms nVVVCI

C – capacitor D1,D2 – Diodes

OP – Protective devicesI – indicating meterIc(t) – capacitor current

waveform

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Capacitance Potential DividersCapacitance Potential Dividers

Harmonic Effects can be eliminated by use of CPD with ESV.

Long Cable needs calibration Gas filled condensers C1 and C2 are used as

shown in figure. C1 is a three terminal capacitor, connected to

C2 by shielded cable.

C2 is shielded to avoid stray capacitance

Applied voltage V1 is given by,

where,◦ Cm - Capacitance of the meter and cable leads

◦ V2 - Reading of Voltmeter

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C1 - Standard Compressed Gas H.V. CondenserC2 - Standard Low Voltage CondenserESV- Electrostatic VoltmeterP -Protective GapC.C - Connecting Cable

1

2121 C

CCCVV m

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Capacitance Voltage TransformerCapacitance Voltage Transformer

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Capacitive Voltage Transformer: Capacitance divider with a suitable matching or isolating potential transformer tuned for resonance condition is often used in power systems for voltage measurements.

CPD can be connected only to high impedance meter or ESV. But, CVT can be connected to low impedance device like pressure coil of wattmeter or relay coil.

C1 is few units of HV capacitance, and the total capacitance will be around a few thousand picofarads

C2 is a non-inductive capacitance

A matching transformer is connected between the load or meter M and C2

Transformer ratings: HV side - 10 to 30 kV; LV side - 100 to 500 V Value of the tuning choke L is chosen to bring resonance condition. This condition

is satisfied when,

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21T CC

1LL

where,L - Inductance of the chokeLT - Equivalent inductance of the transformer referred to h.v. side

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Advantages:◦ simple design and easy installation,◦ can be used both as a voltage measuring device for meter◦ provides isolation between the high voltage terminal and low voltage metering.

Disadvantages:◦ the voltage ratio is susceptible to temperature variations, and◦ the problem of inducing ferro-resonance in power systems.

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Peak Reading VoltmetersPeak Reading Voltmeters

For Sine wave,◦ Peak Value=RMS Value X 2◦ If the waveform is not sinusoidal. In that case,◦ Peak Value ≠ RMS Value X 2

Therefore, peak measurement is important. Types:

◦ Series Capacitance Peak Voltmeter (Chubb-Frotscue Method)◦ Digital Peak Voltmeter◦ Peak Voltmeter with potential divider

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Chubb Frotscue Method:◦ Chubb and Fortescue suggested a simple and accurate

method of measuring peak value of a.c. voltages.◦ The basic circuit consists of a standard capacitor, two

diodes and a current integrating ammeter (MC ammeter) as shown in Fig. 4.11 (a).

◦ The displacement current ic(t), is given by the rate of change of the charge and hence the voltage V(t) to be measured flows through the high voltage capacitor C and is subdivided into positive and negative components by the back to back connected diodes

The voltage drop across these diodes can be neglected (1 V for Si diodes) as compared with the voltage to be measured

The measuring instrument (M.C. ammeter) is included in one of the branches. The ammeter reads the mean value of the current,

An increased current would be obtained if the current reaches zero more than once during one half cycle

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(Chubb Frotscue Method Continued…)

This means the wave shapes of the voltage would contain more than one maxima per half cycle. The standard a.c. voltages for testing should not contain any harmonics and, therefore, there

could be very short and rapid voltages caused by the heavy predischarges, within the test circuit which could introduce errors in measurements.

To eliminate this problem filtering of a.c. voltage is carried out by introducing a damping resistor in between the capacitor and the diode circuit, Fig. 4.11 (b).

The measurement of symmetrical a.c. voltages using Chubb and Fortescue method is quite accurate and it can be used for calibration of other peak voltage measuring devices.


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Digital Peak Voltmeter: In contrast to the method discussed just now, the rectified current is not

measured directly, instead a proportional analog voltage signal is derived which is then converted into a proportional medium frequency for using a voltage to frequency convertor (Block A in Fig. 4.13).

The frequency ratio fm/f is measured with a gate circuit controlled by the a.c. power frequency (supply frequency f) and a counter that opens for an adjustable number of period Δt = p/f. The number of cycles n counted during this interval is

where ‘p’ is a constant of the instrument. Accuracy is less than 0.35%


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Peak voltmeter with Potential divider: Diode D is used for rectification Voltage across C2 is used to charge C3

Resistance Rd permits the variation of Vm when

V2 is reduced Electrostatic Voltmeter as indicating instrument Voltage across Cs Peak value to be measured

Discharge time constant=CsRd1 to 10 sec This arrangement gives discharge error. Discharge error depends on frequency of the supply


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Sphere GapsSphere Gaps

A spark gap can be used for measurement of the peak value of the voltage, if the gap distance is known.

Applications:◦ 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).

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3131


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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).

Standard values of sphere diameter are 6.25, 12.5, 25, 50, 75, 100, 150, and 200 cm.

The Clearance around the sphere gaps:

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Fig C :Breakdown voltage characteristic of sphere gaps

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The effect of humidity is to increase the breakdown voltage of sphere gaps by up to 3%.

Temperature and pressure, however, have a significant influence on breakdown voltage.

Breakdown Voltage under normal atmospheric conditions is, Vs=kVn where k is a factor related to the relative air density (RAD) δ.

Under impulse voltages, the voltage at which there is a 50% breakdown probability is recognized as the breakdown level.

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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

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Measurement of High CurrentsMeasurement of High Currents

Type of Current Method usedD.C Current 1. Resistant shunt

2. Hall Generator

High Power frequency A.C Current Transformer with electro-optical technique

High frequency and impulse currents 1. Resistive shunts2. Magnetic potentiometers or probes3. Magnetic links4. Hall generators5. Faraday Generators

Impulse Voltages and Currents Cathode Ray Oscilloscope

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Hall GeneratorsHall Generators

Hall effect is used to measure very high direct current.

Whenever electric current flows through a metal plate placed in a magnetic field perpendicular to it, Lorenz force will deflect the electrons in the metal structure in a direction perpendicular to the direction of both the magnetic field and the flow of current.

The change in displacement generates an e.m.f called “Hall Voltage”

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Hall Voltage,

where, B-Magnetic Flux densityI-Currentd-Thickness of the metal plate

R-Hall Coefficient (depends on Material of the plate & temperature)

R is small for metals and High for semiconductors

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dBI

RV

dBI

αV

H

H

When large d.c. currents are to be measured the current carrying conductor is passed through an iron cored magnetic circuit

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The magnetic field intensity produced by the conductor in the air gap at a depth ‘d’ is given by,

The Hall element is placed in the air gap and a small constant d.c. current is passed through the element.

The voltage developed across the Hall element is measured and by using the expression for Hall voltage the flux density B is calculated and hence the value of current I is obtained.

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d21

H


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Faraday Generator or Magneto Optic Faraday Generator or Magneto Optic MethodMethod These methods of current measurement use the rotation of the plane

of polarisation in materials by the magnetic field which is proportional to the current (Faraday effect).

When a linearly polarised light beam passes through a transparent crystal in the presence of a magnetic field, the plane of polarisation of the light beam undergoes rotation. The angle of rotation is given by,

θ = α Bl

where,

α = A constant of the cyrstal which is a function of the wave length of the light.

B = Magnetic flux density due to the current to be measured in this case.

l = Length of the crystal.

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Fig. shows a schematic diagram of Magneto-optic method. Crystal C is placed parallel to the magnetic field produced by the

current to be measured. A beam of light from a stabilised light source is made incident on the

crystal C after it is passed through the polariser P1. The light beam undergoes rotation of its plane of polarisation. After the beam passes through the analyser P2, the beam is focussed on

a photomultiplier, the output of which is fed to a CRO.41

Faraday Generator or Magneto Optic Faraday Generator or Magneto Optic MethodMethod

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The filter F allows only the monochromatic light to pass through it. Photoluminescent diodes too, the momentary light emission of which is proportional to the current flowing through them, can be used for current measurement.

Advantages:1. It provides isolation of the measuring set up from the main current circuit. 2. It is insensitive to overloading. 3. As the signal transmission is through an optical system no insulation problem

is faced. However, this device does not operate for D.C current.

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Faraday Generator or Magneto Optic Faraday Generator or Magneto Optic MethodMethod

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Magnetic Potentiometer(Rogowski Coil)Magnetic Potentiometer(Rogowski Coil)

If the current to be measured is flowing through a conductor which is surrounded by a coil as shown in Fig.

and M is the mutual inductance between the coil and the conductor, the voltage across the coil terminals will be:

Usually the coil is wound on a non-magnetic former in the form of a toroid and has a large number of turns, to have sufficient voltage induced which could be recorded.

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dtdi

Mv(t)

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The coil is wound cross-cross to reduce the leakage inductance. If N is the number of turns of the coil, A the coil area and lm its mean

length, the mutual inductance is given by

Usually an integrating circuit RC is employed as shown in Fig to obtain the output voltage proportional to the current to be measured. The output voltage is given by

The frequency response of the Rogowski coil is flat upto 100 MHz but beyond that it is affected by the stray electric and magnetic fields and also by the skin effect.

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m

0

lNAμ

M

i(t)RCM

diRCM

dtdtdi

MRC1

v(t)dtRC1

(t)vt

00

Magnetic Potentiometer(Rogowski Coil)Magnetic Potentiometer(Rogowski Coil)

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Resistive ShuntResistive Shunt

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Used for high impulse current measurements is a low ohmic pure resistive shunt.

Current through the resistive element R produces a voltage drop v(t)=i(t)R. v(t) is transmitted to a CRO through a coaxial cable of surge impedance Z0.

Cable at oscilloscope end is terminated by a resistance Ri = Z0 to avoid reflections.

(a) Ohmic shunt (b) Equivalent circuit of the shunt

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Large dimension resistance will have a residual inductance L and a terminal capacitance C.

L may be neglected for low frequencies (), but becomes appreciable at higher frequencies when L is of the order of R.

C has to be considered when the reactance 1/ C is of comparable value L and C are important above 1MHz Frequency. Resistance: 10µ to few milliohms makes few volts drop. Resistance value is determined by the thermal capacity and heat dissipation

of the shunt. Voltage drop is given by,

where, V(s) and I(s) are the transformed quantities of the signals v(t) and i(t)

s- Laplace Operator or Complex Frequency

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)(1

)(2

sILCssRC

sLRsV

)()( sIsLRsV

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Types:1. Bifilar flat strip design,2. Coaxial tube or Park's shunt design, and3. Coaxial squirrel cage design

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It consists of resistor elements wound in opposite directions and folded back, with both ends insulated by a teflon or other high quality insulation.

The voltage signal is picked up through a ultra high frequency (UHF) coaxial connector.

The shunt suffers from stray inductance associated with the resistance element

Its potential leads are linked to a small part of the magnetic flux generated by the current that is measured.

To overcome these problems, coaxial shunts are chosen.

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In the coaxial design the current is made to enter through an inner cylinder or resistive element and is made to return through an outer conducting cylinder of copper or brass.

The voltage drop across the resistive element is measured between the potential pick-up point and the outer case.

The space between the inner and the outer cylinder is air and hence acts like a pure insulator.

The maximum frequency limit is about 1000 MHz and the response time is a few nanoseconds.

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In post arc current measurements, high ohmic value shunts which can dissipate larger energy are required.

In such cases tubular shunts are not suitable due to their limitations of heat dissipation, larger wall thickness, and the skin effect.

To overcome these problems, the resistive cylinder is replaced by thick rods or strips, and the structure resembles the rotor construction of double squirrel cage induction motor.

The equivalent circuit for squirrel cage construction is different, and complex.

The shunts show peaky response for step input, and a compensating network has to be designed to get optimum response.

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Potential Dividers for Impulse Voltage Potential Dividers for Impulse Voltage MeasurementsMeasurements Resistive or capacative or mixed

element type potential dividers are used for high voltage impulse measurements, high frequency a.c measurements, or for fast rising transient voltage measurements.

The low voltage arm of the divider is usually connected to a fast recording oscillograph or a peak reading instrument through a delay cable.

In high voltage dividers, Each element has a self resistance or capacitance. In addition, the resistive elements have residual inductances, a terminal stray capacitance to ground, and terminal to terminal capacitances.

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Fig. a. Schematic diagram of a potential divider with a delay cable and oscilloscope

Z1-Resistor or Series of resistors in Resistor Dividers (or) Capacitor or No. of Capacitors in Capacitance divider

Z2-A resistor or a capacitor or an R-C impedance depending upon the type of the divider

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The equivalent circuit of the Resistance divider with inductance neglected have been discussed already.

A capacitance potential divider also has the same equivalent where CS will be the capacitance of each elemental capacitor, Cg will be the terminal capacitance to ground, and R will be the equivalent leakage resistance and resistance due to dielectric loss in the element.

When a step or fast rising voltage is applied at the high voltage terminal, the voltage developed across the element Z2 will not have the true waveform as that of the applied voltage.

The cable can also introduce distortion in the waveshape.

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Potential Dividers for Impulse Voltage Potential Dividers for Impulse Voltage MeasurementsMeasurements

Eq. Circuit of resistive element

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The following elements mainly constitute the different errors in the measurement:

i. Residual inductance in the elements;

ii. Stray capacitance occurringa. between the elements,

b. from sections and terminals of the elements to ground, and

c. from the high voltage lead to the elements or sections;

iii. The impedance errors due toa. connecting leads between the divider and the test objects, and

b. ground return leads and current in ground leads; and

iv. Parasitic oscillations due to lead and cable inductances and capacitance of high voltage terminal to ground.

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The effect to residual and lead inductances becomes pronounced when fast rising impulses of less than one microsecond are to be measured.

The residual inductances damp and slow down the fast rising pulses. Secondly, the layout of the test objects, the impulse generator, and the

ground leads also require special attention to minimize recording errors.

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HALL GENERATORS FOR D.C CURRENT MEASUREMENTS Hall effect principle is used.If an electric current flows

through a metal plate located in a magnetic field perpendicular to it,Lorenz forces will deflect the electrons in the metal structure in a direction normal to the direction of both the current and magnetic field.

The charge displacement generates an emf in the normal direction (Hall voltage).

VH=RBi/d H=I/δ

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Current transformer is used.it uses electro optical technique.

A voltage signal proportional to the measuring current is generated and it is transmitted to the ground side through electro optical device.

Light pulses proportional to the voltage signal are transmitted by a glass optical fibre bundle to a photo detector and converted back into an analog voltage signal.

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Documents

UNIT IV - High Voltage Engineering