course code: ee 2316 - saadawi1saadawi1.net/uploadedfiles/extra_files/08kjj31b6.pdf · for high...

High Voltage Engineering

Course Code: EE 2316

10/28/2017 Prof. Dr. Magdi El-Saadawi 1

Prof. Dr. Magdi M. El-Saadawi

www.saadawi1.net

E-mail : [email protected]

www.facebook.com/magdi.saadawi

ContentsChapter 1

Introduction to High Voltage Technology

Chapter 2

Generation of High Voltages and Currents

Chapter 3

Measurement of High Voltages and Currents

Chapter 4

Breakdown Mechanism of Gases, Liquid and

Solid Materials210/28/2017 Prof. Dr. Magdi El-Saadawi

Chapter 3

Measurement of High Voltages and Currents

3.1. Introduction

3.2. Measurement of High Direct Current Voltages3.2.1 High Ohmic Series Resistance with Microammeter

3.2.2 Resistance Potential Dividers for d.c. Voltages

3.2.3 Generating Voltmeters

3.3. Measurement of High A.C. and Impulse Voltages3.3.1 Series Impedance Voltmeters

3.3.2 Capacitance Potential Dividers and Capacitance Voltage Transformers

3.3.3 Electrostatic Voltmeters

3.3.4 Peak Reading a.c. Voltmeters

3.3.5 Spark Gaps

3.3.6 Potential Dividers

3.4. Measurement of High A.C. and Impulse Currents3.4.1 Measurement of High Direct Currents

3.4.2 Measurement of High Frequency and Impulse Currents

3.4.3 Cathode Ray Oscillographs for Impulse Measurements

3.5. Solved Examples

10/28/2017 Prof. Dr. Magdi El-Saadawi

3


Table 3.1 High voltage Measurement Techniques


Table 3.2 High Current Measurement Techniques

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➢Measurement of high a.c. voltages employ conventional

methods like series impedance voltmeters, potential

dividers, potential transformers, or electrostatic voltmeters.

But their designs are different from those of low voltage

meters, as the insulation design and source loading are the

important criteria.

➢When only peak value measurement is needed, peak

voltmeters and sphere gaps can be used. Often, sphere gaps

are used for calibration purposes.

➢ Impulse and high frequency a.c. measurements invariably

use potential dividers with a cathode ray oscillograph for

recording voltage waveforms.

3.3 Measurement of High A.C. and Impulse Voltages

Prof. Dr. Magdi El-Saadawi

10/28/2017 7

➢ For power frequency a.c. measurements the series

impedance may be a pure resistance or a reactance.

➢ In H.V. a capacitor is preferred as series reactance because:

➢ Resistances involve power losses,

➢ For high resistances, the variation of resistance with temperature

is a problem, and

➢ The residual inductance of the resistance gives rise to an

impedance different from its ohmic resistance.

High resistance units for

HV have stray capacitances

and have an equivalent

circuit as shown.

3.3.1 Series Impedance Voltmeters


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➢The entire resistor unit then has to be taken as a

transmission line equivalent, for calculating the

effective resistance.

➢Also, the ground or stray capacitance of each

element influences the current flowing in the unit,

and the indication of the meter results in an error.

➢The equivalent circuit of a high voltage resistor

neglecting inductance and the circuit of

compensated series resistor using guard and timing

resistors is shown in Figs. 3.5a and b respectively



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• In Fig. 3.5b 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.

• By tuning resistors Ra the shielding resistor end

potentials may be adjusted with respect to the

actual measuring resistor so that the resulting

compensation currents between the shield and the

measuring resistors provide a minimum phase

angle.



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3.3.2 Capacitance Potential Dividers and

Capacitance Voltage Transformers


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➢To avoid the drawbacks pointed

out earlier, a series capacitor is

used instead of a resistor for a.c.

high voltage measurements.

➢The schematic diagram is shown

➢The current Ic through the meter

is: Ic = jωCV

where,

• C = capacitance of series capacitor,

• ω = angular frequency, and

• V= applied a.c. voltage.

Capacitance Potential Dividers


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• Series capacitance voltmeters were used with

cascade transformers for measuring rms values up

to 1000 kV.

• The series capacitance was formed as a parallel

plate capacitor between the high voltage terminal of

the transformer and a ground plate suspended

above it.

• The meter was usually a 0-100 μA moving coil

meter and the overall error was about 2%.



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➢ Objective:

Measuring high A.C voltage using capacitive voltage

dividers

➢Components:

• An electrostatic voltmeter or a high impedance V.T.V.M.

(vacuum-tube voltmeter) or an oscilloscope

• A standard compressed air or gas condenser, C1

• A large loss condenser (mica, paper, ..). C2

• A long cable for connecting the HV source to the meter

➢Wiring: as shown in Fig. 3.7

➢Procedure:



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➢Procedure:

• Measure the value of Cm

• Read the values of C1 and C2

• Take the required H.V. measurement cautions

• Wire the circuit components as shown in Figure

• Connect the H.V. source to the connected circuit

• Read the voltmeter reading V2

• Calculate the Value of V1 using

• Repeat the experiment and take the average



17

➢It is similar to series

capacitance voltmeter

➢A matching transformer is

connected between the load

or meter M and C2

➢The value of the tuning

choke L is chosen to make

the equivalent circuit of the

CVT purely resistive or to

bring resonance condition:L= inductance of the choke,

Capacitance Voltage Transformer - CVT

LT = equivalent inductance of the transformer referred to h.v. side.

18

➢The voltage ratio becomes:

(neglecting the voltage drop

ImXe which is very small

compared to the voltage VC1)

where

➢VRi is the voltage drop in the

transformer and choke windings



19

• The advantages of a CVT are:

• simple design and easy installation,

• can be used both as a voltage measuring device for meter and

relaying purposes.

• frequency independent voltage distribution along elements as

against conventional magnetic potential transformers which

require additional insulation design against surges, and

• provides isolation between the high voltage terminal and

low voltage metering.

• The disadvantages of a CVT are:

• the voltage ratio is susceptible to temperature variations, and

• the problem of inducing ferro-resonance in power systems.



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• A uniform field spark gap will always have a spark over

voltage within a known tolerance under constant

atmospheric conditions.

• Hence a spark gap can be used for measurement of the

peak value of the voltage, if the gap distance is known.

• Normally, only sphere gaps are used for voltage

measurements. In certain cases, uniform field gaps and

rod gaps are also used, but their accuracy is less

• Sphere gap breakdown is independent of the voltage

waveform and hence is suitable for measuring the peak

value of all H.V. types: d.c., a.c. and impulse voltages of

short rise times (rise time > 0.5 μs).

3.3.5 Spark Gaps


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• Sphere gaps can be arranged either

– Vertically with lower sphere grounded, or

– horizontally with both spheres connected to the source voltage or

one sphere grounded.

• The two spheres used are identical in size and shape.

• Spheres are generally made of copper, brass, or aluminum;

the latter is used due to low cost.

• One sphere is grounded and the other is connected to the

HV source

• A series resistance is usually connected between the source

and the sphere gap to: limit the breakdown current

• The standard diameters for the spheres are as shown in

tables: 2,5,6.25,10,12.5,15,25,50,75,100,150, and 200 cm.

3.3.5 Spark Gaps

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➢Procedure:

• The voltage to be measured is applied to the sphere

• The distance or spacing S between them is decreased

until the spark occur

• Ground the spheres to discharge the electrical charges

• Take the distance S between spheres and compute the

value of the measured voltage from the tables

• Repeat the experiment and take the average

3.3.5 Spark Gaps


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• nearby earthed objects,

• atmospheric conditions and humidity,

• irradiation, and

• polarity and rise time of voltage waveforms.

Factors Influencing the Sparkover Voltage of

Sphere Gaps


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(i) Effect of nearby earthed objects


(ii) Effect of atmospheric conditions

•Humidity effect increases with the size of spheres and is maximum for

uniform field gaps, and

• Sparkover voltage increases with the partial pressure of water vapor in

air, and for a given humidity condition, the change in sparkover voltage

increases with the gap length.

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• Illumination of sphere gaps with ultra-violet or x-

rays aids ionization in gaps easy. يساعد فى تسهيل عمليات التأين

• The effect of irradiation is pronounced واضح for

small gap spacings.

• Hence, irradiation is necessary for smaller sphere

gaps of gap spacing less than 1 cm for obtaining

consistent values.

(iii) Effect of Irradiation


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• It has been observed that the sparkover voltages for

positive and negative polarity impulses are

different.

• Experimental investigation showed that for sphere

gaps of 6.25 to 25 cm diameter, the difference

between positive and negative d.c. voltages is not

more than 1%.

• For smaller sphere gaps (2 cm diameter and less)

the difference was about 8% between negative and

positive impulses of 1/50 μs waveform.

(iv) Effect of polarity and waveform


10/28/2017 32

• A rod gap may be used to measure the peak value of power

frequency and impulse voltages.

• The gap usually consists of two 1.27 cm square rod

electrodes square in section at their end and are mounted

on insulating stands so that a length of rod equal to or

greater than one half of the gap spacing overhangs the

inner edge of the support.

• The arrangement consists of two hemispherically capped

rods of about 20 mm diameter as shown in Fig. 3.15.

• The accuracy of the above relation is better than 20% and,

therefore, provides better accuracy even as compared to a

sphere gap.

Rod Gaps


Video Links

https://www.youtube.com/watch?v=m925I3yapBc

https://www.youtube.com/watch?v=KVANbkI8AmM


https://www.youtube.com/watch?v=m925I3yapBc

https://www.youtube.com/watch?v=KVANbkI8AmM

course code: ee 2316 - saadawi1saadawi1.net/uploadedfiles/extra_files/08kjj31b6.pdf · for high...

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