M S NAIDU is Professor in the Department of High Voltage Engineering, Indian Institute of Science, Bangalore. This chapter discusses some of the general principles used in high voltage technology.
ELECTRIC FIELD STRESSES
An understanding of insulation failure will be possible through the study of the possible mechanisms by which failure may occur. The most common cause of insulation failure is the presence of discharges in the cavities of the insulation or across the surface of the insulation.
GAS/VACUUM AS INSULATOR
As mentioned earlier, the most important material used in high-voltage apparatus is insulation. It can also be defined as the tft voltage at which the current begins to rise to very high values unless controlled by an external circuit impedance.
LIQUID BREAKDOWN
Under high vacuum conditions, where the pressure is below 10"4 torr*, the decomposition cannot occur due to collisional processes as in gases, and therefore the decomposition strength is quite high.
SOLID BREAKDOWN
Electric Field
A brief overview of the concepts of electric fields is presented, as it is essential for high voltage engineers to have a knowledge of the field intensities in various media under electric voltage. The field intensity E at any location in an electrostatic field is the ratio of the force on an infinitesimally small charge at that location to the charge itself as the charge decreases to zero.
Electric Field in a Single Dielectric Medium
In simple cases electric field problems are solved using the Laplace or Poisson equation for the potential
Electric Field In Mixed Dielectrics
Nevertheless, it provides information on stress concentration factors — the ratios of the largest local stress gradients to the mean value in adjacent areas of relatively uniform stress. Various aspects of insulation coordination in high-voltage power systems are discussed in Chapter 8.
GASES AS INSULATING MEDIA
IONIZATION PROCESSES
- Ionization by Collision
- Photo-ionization
- Secondary Ionisation Processes
- Electron Attachment Process
The higher the ionization energy, the shorter the wavelength of the radiation capable of causing ionization. Electron attachment process depends on the energy of the electron and the nature of the gas and is a very important process from an engineering point of view.
TOWNSE^D 1 S CURRENT GROWTH EQUATION
In the attached or insulating gases, the atoms or molecules have vacancies in their outermost shells and therefore have an affinity for electrons. The adhesion process plays a very important role in removing free electrons from an ionized gas when arcing occurs in gas-insulated switchgear.
CURRENT GROWTH IN THE PRESENCE OF SECONDARY PROCESSESPROCESSES
The effect of adhesion on the decomposition of gases is discussed in sec. ii) The excited atoms or molecules in avalanches can emit photons, and this will lead to the emission of electrons due to photoemission. The secondary ionization coefficient y is defined similarly to a, where the net number of secondary electrons produced per incident positive ion, photon, excited particle, or metastable particle, and the total value of y is the sum of the individual coefficients resulting of the three different processes, i.e. y = YI + Y2 + Ys- Y is called Townsend's secondary ionization coefficient and is a function of the gas pressure p and E/p.
TOWNSEND'S CRITERION FOR BREAKDOWN
EXPERIMENTAL DETERMINATION OF COEFFICIENTS a AND Ya AND Y
Knowing a, y can be found from Eq. 2.12) using points on the upward portion of the graphs. This condition sets a limit for E/p below which no breakdown is possible regardless of the value of d, and the limit value is called the critical E/p.
TIME LAGS FOR BREAKDOWN
STREAMER THEORY OF BREAKDOWN IN GASES
By the time the electrons reach the anode, the positive ions are practically in their original positions and form a positive space charge at the anode. The three successive stages in the development of the streamer are diagrammatically shown in Fig.
PASCHEN'S LAW
This equation shows a relationship between V and pd and implies that the breakdown voltage varies as the product pd varies. It can be noted from the above formula that the breakdown voltage at constant pressure and temperature is not constant.
BREAKDOWN IN NON-UNIFORM FIELDS AND CORONA DISCHARGESDISCHARGES
Corona Discharges
Corona The starting and breakdown voltages for the spherical arrangement are shown in fig. The actual degradation characteristics of the spherical plane gap in air are shown in fig.
Breakdown in Non-uniform Fields
In the Townsend discharge (see Figure 2.21), the current increases gradually as a function of the applied voltage. The color of the glow discharge depends on the cathode material and the gas used.
PRACTICAL CONSIDERATIONS IN USING GASES FOR INSULATION PURPOSES
Of the above properties, dielectric strength is the most important property of a gaseous dielectric for practical use. The relative dielectric strengths and the chemical and physical properties of some of the commercially important gases are shown in Table 2.2.
VACUUM INSULATION
Introduction
One of the qualitative effects of mixing SF* with air is to reduce the overall cost of the gas, and at the same time achieve relatively high dielectric strength or simply prevent the appearance of corona at the desired operating voltages. Apart from the recent use of SF* gas, everyone knows the essential quality of air as an insulating medium for overhead power transmission lines and in air blast circuit breakers.
What Is Vacuum?
The table also shows that the addition of 30% SF* to air (by volume) increases the dielectric strength of air by 100%.
Vacuum Breakdown
Thus, the onset of breakdown depends on the conditions and the characteristics of the cathode surface. The quantity W is the product of gap voltage (V) and the charge density on the lump.
LIQUIDS AS INSULATORS
Transformer Oil
Over time, the oil darkens due to the formation of acids and resins, or sludge in the fluid. Some acids are corrosive to the solid insulation materials and metal parts in the transformer.
Electrical Properties
However, in practical conditions the dielectric strength depends on the material of the electrodes, temperature, type of applied voltage, gas content in the liquid, etc., which change the dielectric strength by changing the molecular properties of the liquid. The above factors that control the breakdown strength and lead to electrical breakdown of liquid dielectrics are discussed in the following sections.
PURE LIQUIDS AND COMMERCIAL LIQUIDS
Purification
Dielectric strength is the most important parameter in selecting a particular liquid dielectric for a particular application.
Breakdown Tests
CONDUCTION AND BREAKDOWN IN PURE LIQUIDS
- Suspended Particle Theory
- Cavitation and the Bubble Theory
- Thermal Mechanism of Breakdown
- Stressed Oil Volume Theory
According to this mechanism, the breakdown strength depends on the pressure and the molecular structure of the liquid. The change in breakdown voltage with the loaded volume of oil is shown in fig.
INTRODUCTION
INTRINSICBREAKDOWN
Electronic Breakdown
Avalanche or Streamer Breakdown
ELECTROMECHANICAL BREAKDOWNBREAKDOWN
THERMAL BREAKDOWN
Equilibrium is reached when the heat generated (W^.c.or ^Vc.) becomes equal to the heat dissipated (Wj-). For a given loss angle and applied stress, the heat generated is proportional to frequency and therefore thermal breakdown is more serious at high frequencies.
BREAKDOWN OF SOLID DIELECTRICS IN PRACTICE
Breakdown Due to Internal Discharges
When a voltage Vis is applied, V\ reaches the breakdown strength of the medium in the cavity (Vj) and breakdown occurs. The lifetime of insulation with internal discharges depends on the applied voltage and the number of discharges.
BREAKDOWN IN COMPOSITE DIELECTRICS .1 Introduction.1 Introduction
Properties of Composite Dielectrics
The use of layered construction is very important in the case of insulating paper, since the paper thickness itself varies from point to point and, as a result, the dielectric strength over its surface is not homogeneous. thickness gives the paper a rough surface which can produce an electric field voltage comparable to that of the discharge channel. Discharges usually occur at the interfaces, and the magnitude of the discharge depends on the associated surface resistance and capacitance.
Mechanisms of Breakdown in Composite Dielectrics
It is shown that K1- is highly dependent on dielectric permittivity and cavity thickness g. It is generally observed that the discharge characteristics change with the life of the insulation.
SOLID DIELECTRICS USED IN PRACTICE
Paper
The relative dielectric constant of impregnated paper depends on the permittivity of the pulp from which the paper is made, the permittivity of the impregnation and the density of the paper.
LO 1.2 Trichlorodiphenyl
- Fibres
- Mica and Its Products
- Glass
- Ceramics
- Rubber
- Epoxy Resins
Tables 4.6 and 4.7 give the different dielectric properties of some ceramics commonly used for electrical insulation purposes. The thickness of the insulation is 3.1 mm (see Tables 4.8 and 4.9 for the properties of the material).
INTRODUCTION
Class H: 18O0C: As class B with silicone resin binder, silicone rubber, aromatic polyamide (nomex paper and fiber), polyimide film (enamel, varnish and film) and esterimide enamel. First we deal with insulation in power apparatus under which insulation in rotating electrical machines, transformers and switchgear is discussed followed by the insulation in capacitors and cables.
APPLICATIONS IN POWER TRANSFORMERS
Transformer oil provides the required dielectric strength and insulation and also cools the transformer by circulating itself through the core and coil structure. These products cause chemical damage to the paper insulation and metal parts of the transformer.
APPLICATIONS IN ROTATING MACHINES
Being heavy sludge, it reduces the heat transfer capabilities of the oil, and also forms a heat insulating layer on the structure of the coil, core and tank walls. The solvent in the spray must not attack any of the insulating materials used and the resin must have long-term compatibility with these materials.
APPLICATIONS IN CIRCUIT BREAKERS
In the condenser type bushings, paper is wound on the electrode and metal foils are wrapped around it at intervals throughout the diameter so that the capacitance between successive foils is constant. The different types of insulating materials used in the construction of high voltage switchgear are classified in Table 5.2.
APPLICATIONS IN CABLES
It cannot be operated continuously at higher voltages, although it can be used up to 850C continuously at low voltages. The best material for high voltage and high temperature operation is teflon (P.T.F.E.) which can be used up to 25O0C.
V.C Polyethylene
APPLICATIONS IN POWER CAPACITORS
Properties required for the insulating paper for capacitor applications are high dielectric strength, low dielectric loss, high dielectric constant, uniform thickness and minimal conductive particles. The impregnating agent for power capacitors must have high dielectric strength, a dielectric constant equal to that of paper, high permeability to paper and sufficient viscosity to penetrate the voids in the paper.
APPLICATIONS IN ELECTRONIC EQUIPMENT
- Materials for Low Frequency Applications (Polar Materials) Large quantities of wires and cables are used for connecting various components in
- Materials for High Frequency Applications
- Materials for Resistors
- Materials of Electronic Capacitors
Polystyrene film capacitors are extremely stable in capacitance value and can be used up to several megacycles. Therefore, it cannot be used for low voltage capacitors, but can be used in high voltage capacitors.
GENERATION OF HIGH d.C. VOLTAGES
- Half and Full Wave Rectifier Circuits
- Voltage Doubler Circuits
- Electrostatic Machines: Basic Principle
Thus, a voltage is produced across the coil L equal to Vmax = / VL/C/>, where Cp is the parasitic capacitance across the coil of inductance L. A power supply of approximately 500 V to the pulse generator is sufficient to generate a high-voltage DC. The voltage drop under load is about 5% for load currents of about 150 p, A. The voltage drops quickly at high load currents. So if an insulated band of charge density 6 moves in an electric field uE(x)" between two electrodes of separation V, then i) the charge on the band strip at a distance dx is dq = 8 b.dx where b is the width of the belt, and.
GENERATION OF HIGH ALTERNATING VOLTAGES
- Cascade Transformers
- Resonant Transformers
- Generation of High Frequency a.c. High Voltages
The high voltage winding of the first unit is connected to the tank of the second unit. The magnitude of the voltage across the capacitance C of the test object will be
GENERATION OF IMPULSE VOLTAGES .1 Standard Impulse Waveshapes.1 Standard Impulse Waveshapes
- Theoretical Representation of impulse Waves
- Circuits for Producing Impulse Waves
- Multistage Impulse Generators—Marx Circuit
- Components of a Multistage Impulse Generator
- Generation of Switching Surges
Limitations of the ratio of generator to load capacitances, C1XCa on circuit performance. The effect of inductance is to cause oscillations in the wave front and in the wave tail.
GENERATION OF IMPULSE CURRENTS
- Definition of Impulse Current Waveforms
- Circuit for Producing Impulse Current Waves
- Generation of High Impulse Currents
- Generation of Rectangular Current Pulses
It can be shown that the maximum value of im is normally independent of the values of V and C for a given energy W = - CV2, and the effective inductance L. Ui) an additional air core inductor with a high current value. jv) good shunts and oscillograph for measuring purposes, and. v) a trigger unit and spark gap for starting the power generator.
TRIPPING AND CONTROL OF IMPULSE GENERATORS
- Resistance Potential Dividers for d.c. Voltages
- Other Methods—Oscillating Spheroid
- Measurement of Ripple Voltage In d.c. Systems
1OxIO3VOrIOkV Primary voltage or supply voltage, E\. Magnetizing current and transformer core losses are neglected.). What is the maximum output voltage of the generator, if the charging voltage is 120 kV.
MEASUREMENT OF HIGH a.c. AND IMPULSE VOLTAGES
- Capacitance Potential Dividers and Capacitance Voltage Transformers
- Potential Transformers (Magnetic Type)
- Electrostatic Voltmeters
- Peak Reading a.c. Voltmeters
- Potential Dividers for Impulse Voltage Measurements
The voltage V2 (meter voltage) will be in phase with the input voltage V\. The phasor diagram of the CVT under resonant conditions is shown in Fig. Since the distance 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 disks F and M. The guard rings H are kept at a constant potential in the space by a capacitance divider that ensures a uniform special potential distribution.