High Voltage Engineering

Transcription

1 High Voltage Engineering Course Code: EE 2316 Prof. Dr. Magdi M. El-Saadawi saadawi1@gmail.com 9/23/2017 Prof. Dr. Magdi El-Saadawi 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 9/23/2017 2

3 Chapter 2 Generation of High Voltages and Currents 2.1. Introduction 2.2. Generation of High D.C. Voltages Half-Wave Rectifier Circuit Cascade circuits Electrostatic Generators 2.3. Generation of High A.C. Voltages Cascaded Transformers Series Resonant Circuit 2.4. Generation of Impulse Voltages and Currents Impulse Generator Circuits Multistage Impulse Generator Circuit Components of a Multistage Impulse Generator 2.5. Solved Examples 9/23/2017 Prof. Dr. Magdi El-Saadawi 3

4 2.1. Introduction High voltages (d.c., a.c., and impulse) are required for several applications. Electrostatic precipitators, الكهروستاتيكية particleالمرسبات accelerators in nuclear physics, etc. require high voltages (d.c.) of several kilovolts and even megavolts. High a.c. voltages of one mega volts or even more are required for testing power apparatus rated for extra high transmission voltages (400 kv system and above). High impulse voltages are required for testing purposes to simulate overvoltages that occur in power systems due to lightning or switching surges. 9/23/2017 Prof. Dr. Magdi El-Saadawi 4

5 2.1. Introduction The main concern of HV is for the insulation testing. Hence, generation of high voltages in laboratories for testing purposes is essential. Normally, in HV testing, the current under conditions of failure is limited to a small value (less than an ampere in the case of d.c. or a.c. voltages and few amperes in the case of impulse or transient voltages). But in certain cases, like the testing of surge diverters switchgear, orموجهات the short circuit testing of الصواعق high current testing with several hundreds of amperes is of importance. It may reach to several kiloamperes. 9/23/2017 Prof. Dr. Magdi El-Saadawi 5

6 2.2. Generation of High D.C. Voltages There are various applications of high d.c. voltages in industries, research medical sciences etc. HVDC transmission over both overhead lines and underground cables is becoming more and more popular. The most efficient method of generating high D.C. voltages is through : the process of rectification employing voltage multiplier circuits. Electrostatic generators 9/23/2017 Prof. Dr. Magdi El-Saadawi 6

7 2.2.1 Half-Wave Rectifier Circuits The simplest circuit for generation of high direct voltage is the half wave rectifier shown in Fig. 2.1 Here R L is the load resistance and C the capacitance to smoothen the d.c. output voltage If the capacitor is not connected, pulsating d.c. voltage is obtained at the output terminals whereas with the capacitance C, the pulsation at the output terminal are reduced. Assuming the ideal transformer and small internal resistance of the diode during conduction the capacitor C is charged to the maximum voltage V max during conduction of the diode D. 9/23/2017 Prof. Dr. Magdi El-Saadawi 7

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10 2.2.1 Half-Wave Rectifier Circuits 9/23/ Prof. Dr. Magdi El-Saadawi

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14 Voltage Multiplying Circuits When high d.c. voltages are to be generated, voltage doubler or cascaded voltage multiplier circuits are used. Vilard voltage doubler Circuit Greinacher voltage doubler circuit Cockroft-Walton Voltage Multiplier Circuit 9/23/2017 Prof. Dr. Magdi El-Saadawi 14

15 Vilard Circuit if the transformer is grounded at A instead of B as shown in Fig. 2.1 (a). Such a circuit is known as voltage doubler due to Villard for which the output voltage would be taken across D. This d.c. voltage, however, oscillates between zero and 2V max and is needed for the Cascade circuit. 9/23/

16 Greinacher voltage doubler circuit Suppose B is more positive with respect to A and the diode D 1 conducts thus charging the capacitor C 1 to Vmax with polarity as shown in Fig During the next half cycle terminal, A of the capacitor C 1 rises to Vmax and hence terminal M attains a potential of 2Vmax. 9/23/

17 Greinacher voltage doubler circuit Thus, the capacitor C 2 is charged to 2 Vmax through D 2. Normally the voltage across the load will be less than 2Vmax depending upon the time constant of the circuit C 2 R L. 9/23/

18 Cockroft-Walton Voltage Multiplier Circuit 9/23/

19 Cockroft-Walton at No Load Operation: The portion ABM MA is exactly identical to Greinacher voltage doubler circuit and the voltage across C becomes 2V max when M at voltage 2V max. During the next half cycle when B becomes positive with respect to A, potential of M falls and, therefore, potential of N also falls becoming less than potential at M hence C 2 is charged through D 2. Next half cycle A becomes more positive and potential of M and N rise thus charging C 2 through D 2. Finally, all the capacitors C 1, C 2, C 3, C 1, C 2, and C 3 are charged. 9/23/

20 Cockroft-Walton at No Load Operation: The voltage across the column of capacitors consisting of C 1, C 2, C 3, keeps on oscillating as the supply voltage alternates. This column, therefore, is known as oscillating column. However, the voltage across the capacitances C 1, C 2, C 3, remains constant and is known as smoothening column. The voltages at M, N, and O are 2 V max, 4 V max and 6 V max. Therefore, voltage across all the capacitors is 2V max except for C 1 where it is V max only. 9/23/

21 Cockroft-Walton at No Load Operation: The total output voltage is: 2nV max where n is the number of stages. Thus, the use of multistage arranged in the manner shown enables very high voltage to be obtained. The equal stress of the elements (both capacitors and diodes) used is very helpful and promotes a modular design of such generators. 9/23/

22 Cockroft-Walton Generator Loaded: 9/23/

23 Cockroft-Walton Generator Loaded: 9/23/

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25 Fig. 2.5 A Cockroft Walton d.c. generator for voltages up to 900 kv/10 ma with fast polarity reversal at ETH Zurich 9/23/2017 Prof. Dr. Magdi El-Saadawi 25

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28 Solved Examples p. 48 9/23/2017 Prof. Dr. Magdi El-Saadawi 28

29 Solved Examples p. 48 9/23/2017 Prof. Dr. Magdi El-Saadawi 29

30 Video Link 9/23/2017 Prof. Dr. Magdi El-Saadawi 30