HIGH VOLTAGE INSULATION AND ELECTRICAL DISCHARGE

Transcription

1 HIGH VOLTAGE INSULATION AND ELECTRICAL DISCHARGE 1.0 INTRODUCTION High voltages used for a wide variety of applications covering the power systems, industry and research laboratories. High voltage apparatus necessitate careful design of its insulation and the electrostatic field profiles. The principle media of insulation used are gases, vacuum, solid and liquid, or a combination of these. 1.1 ELECTRIC FIELD SRESSES In high voltage applications, the dielectric strength of insulating materials and the electric field stresses developed in them when subjected to high voltages are the important factors in high voltage systems. The electric stress to which an insulating material is subjected to is numerically equal to the voltage gradient, and is equal to the electric field intensity. E = - ϕ E : electric field intensity ϕ : applied voltage a x + a y + a x y z z Most important material used in a high voltage apparatus is the insulation. The dielectric strength of an insulating material can be defined as the maximum dielectric stress which the material can withstand. 1

2 The electric breakdown strength of insulating materials depends on pressure, temperature, humidity, field configurations, nature of applied voltage, imperfections in dielectric materials, material of electrodes, surface condition of electrodes, etc. Most common cause of insulation failure is the presence of discharges. 1.2 GAS/VACUUM INSULATION Air at atmospheric pressure is the most common gaseous insulation. Breakdown occurs in gases due to the process of collisional ionization. Practical insulating gases used are carbon dioxide (CO 2 ), dichlorodifluoromethane (CCl 2 F 2 ) (freon), sulphur hexafluoride (SF 6 ) and nitrogen (N 2 ). SF 6 has been found to maintain its insulation superiority compared to any other gases. Ideally, vacuum is the best insulator with field strengths up to 10 7 V/cm. Under high vacuum conditions, where the pressures are below 10-4 torr, the breakdown cannot occur due to collisional processes like in gases, and hence the breakdown strength is quite high. 1.3 LIQUID INSULATION Liquids are used in high voltage equipment to serve the dual purpose of insulation and heat conduction. Highly purified liquids have dielectric strength as high as 1 MV/cm. The breakdown strength can reduce due to the presence of impurities. Petroleum oils are the commonest insulating liquids. However, askarels, fluorocarbons, silicones and organic esters including castor oil are used in significant quantities. 2

3 The important electrical properties of the liquid are dielectric strength, conductivity, flash point, gas content, viscosity, dielectric constant, dissipation factor, stability, etc. Askarels and silicones are particularly useful in transformers and capacitors and can be used at temperatures of 200 C and higher. Castor oil is a good dielectric for high voltage energy storage capacitors because of its high corona resistance, high dielectric constant, non-toxicity and high flash point In practical applications, liquids are normally used at voltage stresses of about kv/cm when the equipment is continuously operated. 1.4 SOLID BREAKDOWN The breakdown stress in solid will be as high as 10 MV/cm if the solid insulating material is truly homogeneous and free from imperfections. The breakdown occurs due to many mechanisms such as intrinsic or ionic breakdown, electromechanical breakdown, treeing and tracking, thermal breakdown, electrochemical breakdown and breakdown due to internal discharges. These mechanisms of breakdown vary depending on the time of application of voltages. In general, the breakdown occurs over the surface than in the solid itself, and the surface insulation failure is the most frequent cause of trouble in practice. The failure of solid insulation by discharges, which may occur in the internal voids and cavities of the dielectric, is called partial discharges. The effect of the partial discharges can be minimized by vacuum impregnation of the insulation. High voltage switchgear, bushings, cables and transformers are typically devices for which partial discharge effects should be considered in design. 3

4 1.5 ESTIMATION AND CONTROL OF ELECTRIC STRESS The electric field distribution is usually governed by the Poisson s eqn, i.e.; 2 ϕ = ρ ε 0 ϕ : potential, ρ : space charge density, ε o : vacuum permittivity For space charges are not present, potential distribution is governed by the Laplace s eqn, i.e.; 2 ϕ = 0 Most commonly methods for determining the potential distribution are, a) the electrolytic tank method b) the numerical method Figure 1.0 shows the methods of stress control at an electrode edge. In the design of hv apparatus, the electric field intensities have to be controlled, otherwise higher stresses will trigger or accelerate the aging of the insulation. Numerical methods are employed for the calculation of electric fields,i.e., a) Finite Difference Method (FDM) b) Finite Element Method (FEM) c) Charge Simulation Method (CSM) d) Surface Charge Simulation Method (SSM) or Boundary Element Method (BEM) 4

5 Fig 1.0 Control of stress at an electrode edge. 5

6 2. CONDUCTION AND BREAKDOWN IN GASES 2.0 INTRODUCTION The most commonly gases are Nitrogen (N 2 ), Carbon dioxide (CO 2 ), Freon (CCl 2 F 2 ) and sulphur hexafluoride (SF 6 ). Various phenomena occur in gaseous dielectric when a voltage is applied. When the applied voltage is low, small currents flow between the electrodes and the insulation retains its electrical properties. If the applied voltages are large, the current flowing through the insulation increases very sharply, and an electrical breakdown occurs. The electrical discharges in gases are of two types, i.e. i) non-sustaining discharges ii) self-sustaining discharge The breakdown in a gas, called spark breakdown is the transition of a non-sustaining discharge into a self-sustaining discharge. The build-up of high currents in a breakdown is due to the process known as ionization in which electrons and ions are created from neutral atoms or molecules, and their migration to the anode and cathode respectively leads to high currents. There are two types of breakdown mechanism under different conditions, viz. i) Townsend theory ii) Streamer theory The various physical conditions of gases, namely, pressure, temperature, electrode field configuration, nature of electrode surfaces and the availability of initial conducting particles are known to govern the ionization processes. 6

7 2.1 IONIZATION PROCESSES When a high voltage is applied between the two electrodes immersed in a gaseous medium, the gas becomes a conductor and an electrical breakdown occurs. The processes that are primarily responsible for the breakdown of a gas are ionization by collision, photo-ionization and the secondary ionization processes. In insulating gases (also called electron-attaching gases) the process of attachment also plays an important role Ionization by Collision Ionization - The process of liberating an electron from a gas molecule with the simultaneous production of a positive ion. In the process of ionization by collision, a free electron collides with a neutral gas molecule and gives rise to a new electron and a positive ion. When electric field E is applied across two plane parallel electrodes (as shown in Figure 2.0) then, any electron starting at the cathode will be accelerated more and more between collisions with other gas molecules during its travel towards the anode. The process can be represented as; ε > V i e - + A e - + A + + e - where A is the atom, A + is the positive ion and e - is the electron. ε : energy gained V i : ionization potential 7

8 Fig 2.0 Arrangement for study of a Townsend discharge Photo-ionization The phenomena associated with ionization by radiation, or photoionization, involves the interaction of radiation with matter. Photoionization occurs when the amount of radiation energy absorbed by an atom or molecule exceeds its ionization potential. The processes by which radiation can be absorbed by atoms or molecules are; i) excitation of the atom to a higher energy state. ii) continuous absorption by direct excitation of the atom or dissociation of diatomic molecule or direct ionization etc. Ionization occurs when λ c. h : Planck s constant = x Js c : velocity of light λ : wavelength of the incident radiation V i : ionization energy of the atom (ev), 1 ev = x J. h V i Radiation having a wavelength of 1250 Å is capable of causing photoionization of almost all gases. 8

9 2.1.3 Secondary Ionization Processes Secondary ionization processes by which secondary electrons are produced are the one which sustain a discharge after it is established due to ionization by collision and photo-ionization. a) Electron Emission due to Positive Ion Impact Positive ions are formed due to ionization process and travel towards the cathode. These positive ions can cause emission of electrons from the cathode by giving up its kinectic energy on impact. The probability of the process is measured as γ i which is called the Townsend s secondary ionization coefficient due to positive ions. γ i increases with ion velocity and depends on the kind of gas and electrode material used. b) Electron Emission due to Photons To cause an electron to escape from a metal, the enough energy is given to overcome the surface potential barrier. The energy is in the form of a photon of ultraviolet light of suitable frequency. The frequency (ν) is given by the relationship; ν ϕ = h, is known as the threshold frequency. ϕ is the work function (ev) of the metallic electrode. c) Electron Emission due to Metastable and Neutral Atoms A metastable atom or molecule is an excited particle whose lifetime is very large (10-3 s) compared to the lifetime of an ordinary particle (10-8 s). 9

10 Electron can be ejected from the metal surface by the impact of excited (metastable) atoms, provided that their total energy is sufficient to overcome the work function. Neutral atoms in the ground state also give rise to secondary electron emission if their kinetic energy is high ( 1000 ev). 2.2 TOWNSEND S CURRENT GROWTH EQUATION n o : electrons emitted from the cathode. α : average number of ionizing collisions made by an electron per cm travel in the direction of the field. α depends on gas pressure p and E/p, and is called the Townsend s first ionization coefficient. n x : number of electrons at any distance x from the cathode. at x = 0, n x = n o also dn dx x = α n x n x = n o exp(αx) (2.1) Then, number of electrons reaching the anode (x = d) is n d = n o exp(αd) The number of new electrons created on the average by each electron is, exp( d ) - 1 = n - n d o α (2.2) n o Average current in the gap = the number of electrons travelling per second I = I o exp(αd) (2.3) 10

11 I o = initial current at the cathode. 2.3 CURRENT GROWTH IN THE PRESENCE OF SECONDARY PROCESS Since the amplification of electrons [exp(αd)] is occurring in the field, the probability of additional new electrons being liberated in the gap by other mechanisms increases, ie; i) The positive ions liberated may have sufficient energy to cause liberation of electrons from the cathode when they impinge on it. ii) The excited atoms or molecules in avalanches may emit photons, and this will lead to the emission of electrons due to photo-emission. iii) The metastable particles may diffuse back causing electron emission. The electrons produced by these processes are called secondary electrons, and the secondary ionization coefficient γ is defined in the same way as α. γ is called the Townsend s secondary ionization coefficient and is a function of the gas pressure p and E/p. Assume n o = number of secondary electrons produced due to secondary processes. n o = total number of electrons leaving the cathode. Then n o = n o + n o Total number of electrons n reaching the anode becomes, n = n o exp(αd) = (n o + n o ) exp(αd), and n o = γ [n - (n o + n o )] 11

12 n = or I = n o exp ( αd ) 1 - γ[exp( αd) - 1] I o exp ( αd ) 1 - γ[exp( αd) - 1] (2.4) (2.5) 2.4 TOWNSEND S CRITERION FOR BREAKDOWN From equation (2.5); γ [ exp(αd) - 1 ] = 1 Townsend s breakdown criterion (2.6) Normally exp(αd) >> 1, then equation (2.6) reduces to; γ exp(αd) = 1 (2.7) For a given gap spacing and at a give pressure, the value of the voltage which gives the values of α and γ satisfying the breakdown criterion is called the spark breakdown voltage V s and the corresponding distance d s is called sparking distance. 12

13 2.5 EXPERIMENTAL DETERMINATION OF COEFFICIENTS α AND γ Experimental arrangement is shown in Figure 2.1. Fig 2.1 Experimental arrangement to measure ionization coefficients The electrode system is placed in an ionization chamber. The chamber is evacuated to a very high vacuum of the order of 10-4 to 10-6 torr. Then it is filled with desired gas. Cathode is irradiated using an ultra-violet lamp in order to produce initiatory electrons (n o ). Typical current growth curve in a Townsend discharge is shown in Figure 2.2. In the regions T 1 and T 2 the current increases steadily due to the Townsend mechanism. 13

14 Fig 2.2 Typical current growth curve in a Townsend discharge For determining the α and γ, the V-I characteristics for different gap settings are obtained. A log I/I o versus gap distance plot is obtained under constant field (E) conditions as shown in Figure 2.3. The slope of initial portion of the curves gives the value of α. Then by using equation (2.5), γ can be found using points on the upcurving portion of the graph. Fig 2.3 Townsend type log (I/I o ) vs d plot 14

15 2.6 BREAKDOWN IN ELECTRONEGATIVE GASES The process that give high breakdown strength to a gas is the electron attachment. Free electrons get attached to neutral atoms or molecules to form negative ions. Electron attachment represents an effective ways of removing electrons which otherwise would have led to current growth and breakdown at low voltage. The gases in which attachment plays an active role are called electronegative gases. The most common attachment processes are; a) Direct attachment; AB + e AB - + hν b) Dissociative attachment; AB + e A + B - + e The gases that the attachment process occured are SF 6, O 2, freon, CO 2 and fluorocarbon. Townsend current growth equation is modified to include ionization and attachment. An attachment coefficient (η) is defined as the number of attaching collisions made by one electron drifting one cm in the direction of the field. Under these conditions, the current reaching the anode can be written as; I = I o α η exp( α -η)d - ( α -η) ( α -η) α 1- γ [ exp( α -η)d -1] ( α -η) (2.8) 15

16 The Townsend breakdown criterion for attaching gases; γ α ( α - η) exp ( α - η)d - 1 = 1 γ α ( α - η) exp ( α - η)d = 1 (2.9) Normally γ is very small ( 10-4 ). When α = η, this condition puts a limit for E/p below which no breakdown is possible irrespective of the value of d, and the limit value is called the critical E/p. Critical E/p for SF 6 is 117 V cm -1 torr -1, and for CCl 2 F 2 it is 121 V cm -1 torr -1 (both at 20 C). 2.7 TIME LAGS FOR BREAKDOWN Time lag is a time difference between the application of a voltage sufficient to cause breakdown and the occurrence of breakdown itself The time which lapses between the application of the voltage sufficient to cause breakdown and the appearance of the initiating electron is called statistical time lag, t s. The time required for the ionization process to develop fully to cause the breakdown is called formative time lag, t f. Total time lag, t = t s + t f, as shown in Figure 2.4. Statistical time lag depends upon the amount of pre-ionization present. Formative time lag depend mostly on the mechanism of the avalanche grow. Formative time lag is usually much shorter than the statistical time lag. 16

17 Fig STREAMER THEORY OF BREAKDOWN IN GASES Townsend mechanism when applied to breakdown at atmospheric pressure was found to have certain drawbacks, i.e. i) Current growth occurs as a result of ionization processes only. But in practice breakdown voltages were found to depend on the gas pressure and the geometry of the gap. ii) The mechanism predicts time lags of the order of 10-5 s, while in actual practice breakdown was observed to occur at very short times of the order of 10-8 s. iii) Townsend mechanism predicts a very diffused form of discharge, but in actual practice, discharges were found to be filamentary and irregular. The Townsend mechanism failed to explain all these observed phenomena and as a result, around 1940, Raether, Meek and Loeb independently proposed the streamer theory. 17

18 The streamer theories predict the development of a spark discharge directly from a single avalanche in which the space charge developed by the avalanche itself is said to transform the avalanche into a plasma streamer. Consider Figure 2.5; Fig 2.5 Field distortion in a gap due to space charge * A single electron starting at the cathode by ionization builds up an avalanche that crosses the gap. * Electrons in the avalanche move very fast compared with the positive ions. This enhances the field, and the secondary avalanches are formed due to photo-ionization in the space charge region. This occurs first near the anode when the space charge is maximum. This results in a further increase in the space charge. * The process is very fast and the positive space charge extends to the cathode very rapidly resulting in the formation of streamer. 18

19 * As soon as the streamer tip approaches the cathode, a cathode spot is formed and a stream of electrons rush from the cathode to neutralize the positive space charge in the streamer, the result is a spark and breakdown has occured. The field E r produced by the space charge at the radius r is given by; -7 α exp( αx) E r = 5.27 x 10 1/ 2 x p V/cm α : Townsend s first ionization coef. p : gas pressure in torr. x : distance to which the streamer has extended in the gap. Generally, for pd values below 1000 torr-cm and gas pressure varying from 0.01 to 300 torr, the Townsend mechanism operates, while at higher pressures and pd values, the streamer mechanism plays the dominant role in explaining the breakdown phenomena. 2.9 PASCHEN S LAW The breakdown criterion in gases is given as; γ [ exp(αd) - 1 ] = 1 (2.10) α and γ are functions of E/p. α E = f ; p p 1 γ = f 2 E p Also, E = V/d From equation (2.9), by letting η = 0, the equation can be rewrite as; V V exp pd f -1 =1 2 pd pd f 1 (2.11) 19

20 Equation (2.11) shows relationship between V and pd. V = f (pd) (2.12) Equation (2.12) is known as Paschen s law. Fig 2.6 shows the relationship between breakdown voltage and pd. Fig 2.6 Breakdown voltage-pd curve (Paschen s law) The Paschen s curve is shown in Figure 2.7 for three gases CO 2, air and H 2. Fig

21 The cathode materials also affect the breakdown values. This is shown in Figure 2.8. Fig 2.8 Dependence of breakdown voltage on the cathode materials For the effect of temperature, the Paschen s law is generally stated as V = f(nd), where N is a density of the gas molecules. The pressure of the gas changes with temperature according to the gas law pν = NRT, where ν is a volume of the gas, T is the temperature and R is a constant. Based on the experimental results, the breakdown potential of air is expressed as; 293 pd 293 pd V = T 760 T 1/ 2 (2.13) 21

22 2.10 BREAKDOWN IN NON-UNIFORM FIELDS AND CORONA DISCHARGES Corona Discharges If the field is non-uniform, an increase in voltage will first cause a discharge in the gas to appear at points with highest electric field intensity. This form of discharge is called a corona discharge and can be observed as a bluish luminescence. The corona discharge is accompanied by a hissing noise, and the air surrounding the corona region becomes converted into ozone. Corona is responsible for considerable loss of power from high voltage transmission lines, deterioration of insulation and rise its radio inteference. Voltage gradient required to produce visual a.c. corona in air at a conductor surface is called the corona inception field. There is a distinct difference in the visual appearance of the corona under positive and negative polarities of the applied voltage. - When the voltage is positive - corona appears as a uniform bluish white sheath over the entire surface of the conductor. - When voltage is negative - like reddish glowing spot distributed along the length of wire. Corona inception and breakdown voltages of the sphere-plane arrangement are shown in Figure 2.9. a) Region I (small spacing) - the field is uniform. Breakdown voltage depends on the spacing. b) Region II (fairly large spacing) - field is non-uniform. Breakdown voltage depends both on the sphere diameter and the spacing. c) Region III (large spacing) - the field is non-uniform. Breakdown is preceded by corona. The corona inception voltage mainly depends on the sphere diameter. 22

23 Fig 2.9 Breakdown and corona inception characteristics The study of corona and non-uniform field breakdown is very complicated and investigations are still under progress Breakdown in non-uniform fields d αd in Townsend s criterion is rewritten by replacing αd by α dx and 0 becomes; d γ exp dx -1 =1 α 0 (2.14) When applied the non-uniform field breakdown process to streamer theory, the field produced by space charge is modified as; E r = 5.27 x 10 x -7 x exp( α dx) 0 1/2 V/cm (2.15) x p α x : value of α at the head of avalanche. 23

24 When space charge field, E r = applied field at the head of avalanche - formation of streamer is reached. From the practical engineering point of view, rod-rod gap and spheresphere gap are of great importance, as they are used for the protection of electrical apparatus and for the measurement of high voltage POST-BREAKDOWN PHENOMENA The phenomena that occur in the region CG (as shown in Figure 2.10) are the post-breakdown phenomena (glow discharge, CE and arc discharge, EG) Fig 2.10 d.c. voltage-current characteristics of an electrical discharge with electrodes having no sharp points or edges 2.12 PRACTICAL CONSIDERATION IN USING GASES FOR INSULATION PURPOSES Generally, the preferred properties of a gaseous dielectric for high voltage application are; a) high dielectric strength b) thermal stability and chemical inactivity towards material of construction. c) non-flammability and physiological inertness. 24

25 d) low temperature of condensation. e) good heat transfer. f) ready availability at moderate cost. SF 6 - possess most of the above requirement. has higher dielectric strength and low liquification temperature - can be used in wide range. has excellent arc-quenching properties. Additional of 30% SF 6 to air - increases the dielectric strength of air by 100%. One of qualitative effect of mixing SF 6 to air is to reduce the overall cost of the gas, and attaining relatively high dielectric strength or simply preventing the onset of corona. Figure 2.11 shows the dielectric strength of gases, comparable with solid and liquid dielectrics. Fig 2.11 d.c. breakdown strength of different insulations in uniform fields 25

26 2.13 VACUUM INSULATION Introduction In the absence of any particles, as in the case of perfect vacuum, there should be no conduction. However in practice, the presence of metallic electrodes and insulating surfaces within the vacuum, a sufficiently high voltage will cause a breakdown. In vacuum systems, the pressure is always measured in terms of mm mercury (Hg). 1 mm Hg = 1 Torr Standard atmospheric pressure = 760 mm Hg at 0 C. Vacuum may be classified as; a) high vacuum : 1 x 10-3 to 1 x 10-6 Torr b) very high vacuum : 1 x 10-6 to 1 x 10-8 Torr c) ultra high vacuum : 1 x 10-9 and below For electrical insulation purposes, the range of vacuum generally used in the high vacuum Vacuum Breakdown In a high vacuum, an electron crosses the gap without encountering any collisions. Therefore the current growth prior to breakdown cannot be due to the formation of electron avalanches. However if a gas is liberated in the vacuum gap, then the breakdown can occur by the Townsend process. Three categories of the mechanisms for breakdown in vacuum. a) Particle exchange mechanism b) Field emission mechanism c) Clump theory 26

27 a) Particle exchange mechanism A charge particle would be emitted from one electrode under the action of the high electric field, and when it impinges on the other electrode, it liberates oppositely charged particles. The particles are accelerated by the applied voltage back to the first electrode where they release more of the original type of particles. When this process becomes cumulative, a chain reaction occurs which leads to the breakdown of the gap. The particle-exchange mechanism involves electrons, positive ions, photons and the absorbed gases at the electrode surfaces. Figure 2.12 shows the particle-exchange mechanism. The breakdown will occur if the coefficients of production of secondary electrons exceeds unity; (AB + CD) > 1 (2.16) where : A : released positive ions from the impact of charged particle (electron) at anode. B : liberated electrons from the impact of each positive ion (A). C : photons - from the impact of charged particle (electrons) at anode. D : liberated electrons from the impact of each photon (C). Fig 2.12 Particle exchange mechanism of vacuum breakdown 27

28 Trump and Van de Graff showed that the coefficients in equation (2.16) were too small for the process of breakdown to take place. Then the theory was modified to allow for the presence of negative ions, and the criterion for breakdown becomes; (AB + EF) > 1 (2.17) E and F represent the coefficients for the negative and positive ion liberation by positive and negative ions. b) Field emmision theory i) Anode heating mechanism Fig Electron beam anode heating mechanism of vacuum breakdown Electrons produced at small micro-projections on the cathode due to field emission bombard the anode causing a local rise in temperature and release gases and vapours into the vacuum gap. These electrons ionize the atoms of the gas and produce positive ions. These positive ions arrive at the cathode, increase the primary electron emission due to space charge formation and produce secondary electrons by bombarding the surface. The process continues until a sufficient number of electrons are produced to give rise to breakdown. 28

29 ii) Cathode heating mechanism Fig Breakdown in vacuum caused by the heating of a microprojection on the cathode Sharp points on the cathode surface are responsible for the existence of the pre-breakdown current. These current causes resistive heating at the tip of a point and when a critical current density is reached, the tip melts and explodes, thus initiating vacuum discharge. Experimental evidence shows that breakdown takes place by this process when the effective cathode electric field is of the order of 10 6 to 10 7 V/cm. 29

30 iii) Clump mechanism Fig Clump mechanism of vacuum breakdown Basically this theory has been developed on the following assumptions; i) A loosely bound particle (clump) exists on one of the electrode surfaces. ii) This particle get charged when high voltage is applied, and get detached from the mother electrode and is accelerated across the gap. iii) The breakdown occurs due to a discharge in the vapour or gas released by the impact at the target electrode. 30

31 Although there has been a large amount of work done on vacuum breakdown phenomena, so far, no single theory has been able to explain all the available experimental measurements and observations. The most significant experimental factors which influence the breakdown mechanisms are; gap length, geometry and material of the electrodes, surface uniformity and treatment of the surface, presence of extraneous particles and residual gas pressure in the vacuum gap. 31