Electrical Insulation in Power Systems

N.H. Malik, A. A. Al-Arainy, M.I. Qureshi

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781351453523
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

Covers the design, operations, diagnostics and testing of electrical insulation in high-voltage power networks. The book presents the fundamental properties of dielectrics essential for the optimum design of power systems. It provides a survey of advanced digital and electro-optic techniques used in both the field and research.

1

Introduction to Electrical Insulation in Power Systems

1.1 INTRODUCTION

The economic development and social welfare of any modern society depends upon the availability of a cheap and reliable supply of electrical energy. Extensive networks of electrical power installations have been built in industrialized countries and are being constructed in developing countries at an ever-increasing rate. The major function of such power systems is to generate, transport and distribute electrical energy over large geographical areas in an economical manner while ensuring a high degree of reliability and quality of supply.

The transmission of large amounts of electrical power over long distances is best accomplished by using high voltage (HV), extra high voltage (EHV) or ultra high voltage (UHV) power lines (see Table 1.1 for voltage classification). Thus, high voltage equipment is the backbone of modern power systems. Besides generation, transmission and distribution of electrical energy, high voltages are also extensively used for many industrial, scientific and engineering applications such as:

1. Electrostatic precipitators for the removal of dust from flue gases

2. Atomization of liquids, paint spraying and pesticide spraying

3. Ozone generation for water and sewage treatment

4. X-ray generators and particle accelerators

5. High-power lasers and ion beams

6. Plasma sources for semiconductor manufacture

7. Superconducting magnet coils

Table 1.1 Voltage Classifications (V = Rms, Line to Line Voltage)

Voltage class	Voltage range
Low voltage (LV)	V ≤ 1kV
Medium high voltage (MHV)	1 kV < V ≤ 70 kV
High voltage (HV)	110 kV ≤ V ≤ 230 kV
Extra high voltage (EHV)	275 kV ≤ V ≤ 800 kV
Ultra high voltage (UHV)	1000 kV ≤ V

In all such applications, the insulation of the high voltage conductor is of primary importance. For proper design and safe and reliable operation of the insulation system, knowledge of the physical and chemical phenomena which determine the dielectric properties of the insulating material is very important. In addition, the basic processes which lead to degradation and failure of such materials and appropriate diagnostic techniques are of prime importance since any such failure can cause temporary or permanent damage to the system, thereby influencing its reliability and cost. Considering the high cost and comparatively long life span (20–40 years) of high voltage equipment, every effort should be made to select the most appropriate materials as well as the design, installation and operational parameters for such apparatus. This book attempts to provide an overview of different aspects of electrical insulation as practiced mainly in high voltage power systems.

This chapter outlines some basic definitions and fundamental concepts which are essential for proper understanding of the electrical insulation behavior. It further provides a brief overview of general categories of insulating materials, a short description of their applications and some desirable properties of various classes of dielectric materials. Subsequent chapters provide the detailed properties, applications, failure modes and diagnostic techniques used for evaluating and testing different insulating materials.

1.2 PROPERTIES OF DIELECTRICS

There are several properties of a dielectric which are of practical importance for an engineer. The most important of these properties are briefly defined here.

1.2.1 DC Conductivity

DC conductivity, σ, is defined as σ = J/E where J is current density (in A/m²) resulting from the application of a direct electric stress E (in V/m). It is related to the bulk resistivity ρ of the dielectric by σ = 1/ρ and is calculated from measured values of the insulation resistance. Alternatively, if ρ and geometry are known, insulation resistance can be calculated. Insulation resistance is used as an indication of conduction behavior of insulating materials in many practical applications, such as in hi-pot testing. For most insulating materials, σ depends upon the material purity, its temperature T and electric stress E. It generally increases as the ionic impurities in the insulation system increase. Similarly σ also tends to increase with T and E in most cases following a relationship of the type

$\sigma \left(\text{T}\right)=\text{A}{\text{e}}^{-\text{E}/\text{k}\text{T}}$

(1.1)

where k is the Boltzman constant, A is a constant, and σ(T) is the value of σ at temperature T. In addition, due to polarization effects, σ also depends upon time of application of the stress. It influences the power losses in a dielectric and controls the electric stress distribution under direct voltage applications.

1.2.2 Dielectric Permittivity

Dielectric permittivity, relative permittivity or dielectric constant, ε_r, of an insulating material is defined as ε_r = C/C_o, where C is the capacitance between two parallel plates having the space between them filled with the insulating material under discussion, and C_o is the capacitance for the same parallel plates when these are separated by vacuum. Generally ε_r is not a fixed parameter but depends upon temperature, frequency and molecular structure of the insulating material.

1.2.3 Complex Permittivity, Loss Angle and Dissipation Factor

In order to determine the response of dielectrics to alternating voltages, it is traditional to model the dielectric by a parallel RC network. Such a network along with its phasor diagram is shown in Figure 1.1. Here R represents the lossy part of the dielectric taking account of losses resulting from electronic and ionic conductivity, dipole orientation and space charge polarization, etc., and C is the capacitance in the presence of the dielectric as defined earlier. If an AC voltage, v = √2 V sin ωt, is applied then the capacitive component of current is I_C = jωCV while the resistive component of current is I_R = −jI tan δ. Since loss angle δ is usually very small, I_C ≈ I and I_R = −jI_C tan δ. Hence, total current I = I_R + I_C can be expressed as:

Figure 1.1 (a) Parallel equivalent circuit of a dielectric material and (b) corresponding phasor diagram.

$\text{I}=\text{j}\omega {\text{C}}_{\text{o}}\text{V}({\epsilon }_{\text{r}}-\text{j}{\epsilon }_{\text{r}}\tan \delta )=\text{j}\omega {\text{C}}_{\text{o}}\text{V}\epsilon *$

(1.2)

where ε* = complex relative permittivity having a real part...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Series Introduction
• Preface
• 1 Introduction to Electrical Insulation in Power Systems
• 2 Gas Dielectrics
• 3 Air Insulation
• 4 SF6 Insulation
• 5 Liquid Dielectrics
• 6 Solid Dielectrics
• 7 Vacuum Dielectrics
• 8 Composite Dielectrics
• 9 High Voltage Cables
• 10 Generation and Measurement of Testing Voltages
• 11 New Measurement and Diagnostic Technologies
• 12 Insulation Testing
• Index