High Voltage Engineering

Course Code: EE 2316

9/23/2017 Prof. Dr. Magdi El-Saadawi 1

Prof. Dr. Magdi M. El-Saadawi

www.saadawi1.net

E-mail : saadawi1@gmail.com

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 Materials29/23/2017

Chapter 2

Generation of High Voltages and Currents

2.1. Introduction

2.2. Generation of High D.C. Voltages2.2.1 Half-Wave Rectifier Circuit

2.2.2 Cascade circuits

2.2.3 Electrostatic Generators

2.3. Generation of High A.C. Voltages2.3.1 Cascaded Transformers

2.3.2 Series Resonant Circuit

2.4. Generation of Impulse Voltages and Currents2.4.1 Impulse Generator Circuits

2.4.2 Multistage Impulse Generator Circuit

2.4.3 Components of a Multistage Impulse Generator

2.5. Solved Examples39/23/2017 Prof. Dr. Magdi El-Saadawi

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High voltages (d.c., a.c., and impulse) are required for

several applications.

Electrostatic precipitators المرسبات الكهروستاتيكية , particle

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. Prof. Dr. Magdi El-Saadawi

2.1. Introduction

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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

,or the short circuit testing of switchgearموجهات الصواعق

high current testing with several hundreds of amperes is of

importance. It may reach to several kiloamperes. Prof. Dr. Magdi El-Saadawi

2.1. Introduction

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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

Prof. Dr. Magdi El-Saadawi

2.2. Generation of High D.C. Voltages

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The simplest circuit for generation of high direct voltage is

the half wave rectifier shown in Fig. 2.1

Here RL 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 Vmax during conduction of

the diode D.Prof. Dr. Magdi El-Saadawi

2.2.1 Half-Wave Rectifier Circuits

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2.2.1 Half-Wave Rectifier Circuits

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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

Prof. Dr. Magdi El-Saadawi

Voltage Multiplying Circuits

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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 2Vmax and is

needed for the Cascade circuit.

Vilard Circuit

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Suppose B is more positive with respect to A and the diode

D1 conducts thus charging the capacitor C1 to Vmax with

polarity as shown in Fig. 2.2.

During the next half cycle terminal, A of the capacitor C1

rises to Vmax and hence terminal M attains a potential of

2Vmax.

Greinacher voltage doubler circuit

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Thus, the capacitor C2 is charged to 2 Vmax through D2.

Normally the voltage across the load will be less than 2Vmax

depending upon the time constant of the circuit C2RL.

Greinacher voltage doubler circuit

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Cockroft-Walton Voltage Multiplier Circuit

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• The portion ABM’MA is exactly

identical to Greinacher voltage

doubler circuit and the voltage

across C becomes 2Vmax when M at

voltage 2Vmax.

• 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 C2 is

charged through D2. Next half cycle

A becomes more positive and

potential of M and N rise thus

charging C2 through D2.

• Finally, all the capacitors C’1, C’2,

C’3, C1, C2, and C3 are charged.

Cockroft-Walton at No Load Operation:

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• The voltage across the column of

capacitors consisting of C1, C2, C3,

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

Vmax, 4 Vmax and 6 Vmax. Therefore,

voltage across all the capacitors is

2Vmax except for C1 where it is Vmax

only.

Cockroft-Walton at No Load Operation:

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• The total output voltage is:

2nVmax

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.

Cockroft-Walton at No Load Operation:

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Cockroft-Walton Generator Loaded:

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Cockroft-Walton Generator Loaded:

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Fig. 2.5

A Cockroft–Walton

d.c. generator for

voltages up to

900 kV/10 mA

with fast polarity

reversal at ETH

Zurich

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Solved Examples p. 48

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Solved Examples p. 48

Video Link

https://www.youtube.com/watch?v=DI8Yt1AQrH8

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