High Voltage Engineering

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1 High Voltage Engineering Course Code: EE 2316 Prof. Dr. Magdi M. El-Saadawi saadawi1@gmail.com 1

2 Contents Chapter 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 Materials 2

3 Chapter 3 Measurement of High Voltages and Currents 3.1. Introduction 3.2. Measurement of High Direct Current Voltages High Ohmic Series Resistance with Microammeter Resistance Potential Dividers for d.c. Voltages Generating Voltmeters 3.3. Measurement of High A.C. and Impulse Voltages Series Impedance Voltmeters Capacitance Potential Dividers and Capacitance Voltage Transformers Electrostatic Voltmeters Peak Reading a.c. Voltmeters Spark Gaps Potential Dividers for Impulse Voltage Measurements 3.4. Measurement of High A.C. and Impulse Currents Measurement of High Direct Currents Measurement of High Frequency and Impulse Currents Cathode Ray Oscillographs for Impulse Measurements 3.5. Solved Examples 3

4 Table 3.1 High voltage Measurement Techniques 4

5 Table 3.2 High Current Measurement Techniques 5

6 3.3.3 Electrostatic Voltmeters The first version of electrostatic voltammeter 6

7 3.3.3 Electrostatic Voltmeters Principle of Operation When one of the electrodes is free to move, the force on the plate can be measured by controlling it by a spring or balancing it with a counterweight. For high voltage measurements, a small displacement of one of the electrodes by a fraction of a millimeter to a few millimeters is usually sufficient for voltage measurements. As the force is proportional to the square of the applied voltage, the measurement can be made for a.c. or d.c. voltage. 7

8 3.3.3 Electrostatic Voltmeters 8

9 9

10 Construction of Electrostatic Voltmeters Electrostatic voltmeter is made with parallel discs M &F configuration using guard rings G to avoid corona and field fringing at the edges The space between the two electrodes must be large enough to avoid breaking the air between them (about 100 cms for a voltage of about 300 kv). As the spacing between the two electrodes is large, the uniformity of the electric field is maintained by the guard rings H which surround the space between the discs M &F The guard rings H are maintained at a constant potential in space by a capacitance divider C ensuring a uniform special potential distribution. 11/12/

11 Construction of Electrostatic Voltmeters The limitation on the gap distance is the safe working stress (V/s) allowed in air is normally 5 kv/cm or less. The cap D encloses a sensitive balance B, one arm of which carries the suspension ايقاف of the moving disc. The movement of the disc is thereby magnified using a mirror which reflects a beam of light. Some instruments are constructed in an enclosed structure containing compressed air, carbon dioxide, SF6 gas or vacuum as medium between the two discs, in this case the meter is compact and much smaller in size. Using SF6 gas the voltmeter can measure voltages up-to 1000 kv and accuracy is of the order of 0.1% 11

12 Construction of Electrostatic Voltmeters The arrangement of this instrument may cause resonance. An upper frequency limit of about one MHz is achieved in careful designs. The accuracy for a.c. voltage measurements is better than ±0.25%, and for d.c. voltage measurements it may be ±0.1% or less. 12

13 3.3.4 Peak Reading a.c. Voltmeters In some occasions, the peak value of an a.c. waveform is more important to obtain the maximum dielectric strength of insulating solids, etc. When the waveform is not sinusoidal, rms value of the voltage multiplied by 2 is not correct. Hence a separate peak value instrument is desirable in high voltage applications 13

14 Series Capacitor Peak Voltmeter 14

15 Series Capacitor Peak Voltmeter Fig /12/2017 Prof. Dr. Magdi El-Saadawi 15

16 Digital peak reading meter Instead of directly measuring the rectified charging current, a proportional analog voltage signal is derived which is then converted into a proportional medium frequency f m 16

17 Digital peak reading meter 17

18 Peak Voltmeters with Potential Dividers 18

19 3.3.6 Potential Dividers for Impulse Voltage Measurements Potential or voltage dividers for high voltage impulse measurements, high frequency a.c. measurements, or for fast rising transient voltage measurements are usually either resistive or capacitive or mixed كما سبق فى الجهد العالى المتردد type. element The low voltage arm of the divider is usually connected to a fast recording oscillograph or a peak reading instrument through a delay cable. A schematic diagram of a potential divider with its terminating equipment is given in Fig

20 3.3.6 Potential Dividers for Impulse Voltage Measurements 20

21 3.3.6 Potential Dividers for Impulse Voltage Measurements The mainly errors in the measurement: Residual inductance in the elements; Stray capacitance occurring between the elements, from sections and terminals of the elements to ground, and from the high voltage lead to the elements or sections; The impedance errors due to connecting leads between the divider and the test objects, and ground return leads and extraneous current in ground leads; Parasitic oscillations التذبذبات الطفيلية due to lead and cable inductances and capacitance of high voltage 11/12/ terminal to ground.

22 Solved Examples pp

23 Solved Examples pp

2. Analog measurement of Electrical Quantities

2. Analog measurement of Electrical Quantities 2.1. Classification of Analog Instruments Definition and concept of Measurement The analog instruments can be classified on the basis of various parameters. Analog Instruments On the basis of measuring