UNIT 1: BREAKDOWN IN SOLIDS 1.) Introduction: The solid dielectric materials are used in all kinds of electrical apparatus and devices to insulate current carrying part from another when they operate at different voltages. The study of breakdown of solid insulating materials is of great importance. The breakdown of a solid insulator damages it permanently and then it needs to be replaced. The solid insulators in case of some equipment besides providing insulation to the live parts of the equipment also provide mechanical support to it. The dielectric strength of solid insulators is higher than that of liquids and gases. There are generally two types of solid insulators organic materials such as paper, wood, rubber etc. and inorganic materials such as PVC, epoxy resins, Perspex etc..) Breakdown Mechanism of Solids: The breakdown mechanism of solids is a complex phenomenon which varies depending on the time of application of voltage as shown in fig.1 The following are the various breakdown mechanisms in case of solids: (a) Intrinsic or ionic breakdown (b) Electromechanical breakdown (c) Failure due to treeing and tracking (d) Thermal breakdown (e) Electrochemical breakdown (f) Breakdown due to internal discharges Intrinsic, electromechanical Breakdown strength Streamer Thermal Erosion and electrochemical Log time Fig.1 Variation of Breakdown strength with time after application of voltage 3.) Intrinsic Breakdown: When voltages are applied only for a short duration of the order of 10 8 s then the dielectric strength of solid dielectric increases very rapidly to an upper limit called intrinsic electric strength. Intrinsic breakdown depends upon the presence of free electrons which are capable of migration through the lattice of the dielectric. Usually a small number of conduction electrons are present in a dielectric alongwith some structural imperfections and small amounts of impurities. Two types of intrinsic mechanisms have been proposed (a) Electronic breakdown and (b) Avalanche or streamer breakdown (a) Electronic Breakdown In case of a pure homogeneous dielectric material the conduction and valence bands are separated by a large forbidden energy gap. At room temperature, the electrons in the absence of electric field cannot acquire sufficient energy to make transition 1
from valence to conduction band. When an adequate electric field is required, the electrons gain energy from the electric field and cross the forbidden energy gap to move from valence to conduction band. When this process is repeated, more and more electrons become available in the conduction band finally leading to the breakdown of the solid dielectric. (b) Avalanche or Streamer Breakdown This is similar to breakdown in gases due to cumulative ionization. The conduction electrons on gaining sufficient energy above a certain critical electric field cause liberation of electrons from the lattice atoms by collisions. If the electrodes are embedded in the specimen then breakdown occurs under uniform field when an electron avalanche bridges the electrode gap. An electron in the conduction band will move from the cathode to the anode under the Influence of electric field and during this motion will gain energy from the field which is then lost in collisions. When the energy gained by the electron exceeds the lattice ionization potential, an additional electron will be liberated due to collision of the first electron. This process repeats itself resulting in the formation of an electron avalanche. Practically the breakdown does not results due to a single avalanche but occurs as a result of many avalanches formed within the dielectric and extending step by step through the entire thickness of the material as shown in fig. Fig. Breakdown channels 4.) Electromechanical Breakdown: In case of solid dielectrics when high electric fields are applied then they are subjected to electrostatic compressive forces. When these electrostatic compressive forces exceed the mechanical compressive strength of the solid dielectric then it results in its failure. If the thickness of specimen is do and it is compressed to a thickness d on application of voltage V across it then the electrically developed compressive stress is in equilibrium if V do ε oεr = Y ln d d ---- (1) where Y is Young s modulus
Y do Or V = d ln ---- () εoεr d Usually mechanical instability occurs when d/do = 0.6 or do/d = 1.67 Substituting this value in equation () the highest apparent electric stress before breakdown is given by V Y Emax = = 0. 6 d o εoεr The above equation is only approximate as Y depends on the mechanical stress. Also when the material is subjected to high stresses the theory of elasticity does not hold good and plastic deformation is also to be considered. This theory has been recently modified on the basis of the concept of fracture mechanics. According the new mechanism, filamentary shaped cracks propogate through the dielectric material releasing both the electrostatic energy and the electromechanical strain energy stored in the material due to the applied electric field. 5.) Thermal Breakdown: In case of solid dielectrics, it is generally observed that the breakdown voltage generally increases with the increase in thickness. However this is true only upto a certain thickness beyond which it does not holds. The breakdown of the solid dielectric depends upon the heat generated due to the conduction of current inside the dielectric. When an electric field is applied across a dielectric it results in the flow of current no matter how small its magnitude is resulting in heating up of the dielectric. The heat generated is transferred to the surrounding medium by conduction through the solid dielectric and by radiation from its outer surface. When the amount of heat generated becomes equal to the heat used to raise the temperature of dielectric plus the heat radiated out then equilibrium is established. The heat generated under d.c. stress E is given by 3 W d. c. = E σw /cm where σ is the d.c. conductivity of the specimen. When a.c. field is applied across a solid dielectric then the heat generated is given by E fεr tanδ 3 Wa. c. = W / cm where f = frequency in Hz 1 1.8 10 δ = loss angle of the dielectric material E = rms value The heat dissipated (WT) is given by dt = C div KgradT where CV = specific heat of specimen WT V + ( ) dt T = temperature of the specimen K = thermal conductivity of the specimen t = time over which the heat is dissipateds Equilibrium condition is achieved when the heat generated (Wa.c. or Wd.c.) becomes equal to the heat dissipated (WT). Practically there is always some heat that is radiated out. When the heat generated becomes more than the heat dissipated then it results in breakdown. The fig.3 shows the thermal instability condition where the heat lost is shown by a straight line while heat generated at fields E1 and E are shown by separate curves. At field E breakdown occurs both at temperatures TA and TB. In the temperature region of TA and TB heat generated is less than the heat lost for the field E and hence the breakdown will not occur. 1 3
Heat generated Heat generated or heat lost E 1 E Heat lost 4 T 0 T A T B Temperature Fig.3 Thermal Stability in solid dielectrics 6.) Electrochemical Breakdown: In case of some solid dielectrics chemical changes occur when they come in presence of air and other gases under a continuous electrical stress. The following are some of the following important chemical reactions which occur: a) Oxidation: In the presence of air or oxygen, materials such as rubber and polyethylene undergo oxidation giving rise to surface cracks. b) Hydrolysis: When moisture or water vapour is present on the surface of a solid dielectric, hydrolysis occurs and the materials lose their electrical and mechanical properties. Electrical properties of materials such as paper, cotton tape and other cellulose materials deteriorate very rapidly due to hydrolysis. c) Chemical action: In case of insulating materials a progressive chemical degradation occurs even in the absence of electric fields due to variety of processes such as chemical instability at high temperatures, oxidation and cracking in the presence of air and ozone, hydrolysis due to moisture and heat. Since different insulating materials come into contact with each other in many practical apparatus, chemical reactions occur between these various materials leading to reduction in electrical and mechanical strength leading to failure. 7.) Breakdown due to Treeing and Tracking: When a solid dielectric subjected to electrical stresses for a long time fails since their life gets severely reduced by the degradation processes when subjected to high voltage systems. There are two types of visible markings are observed on the dielectric materials the presence of a conducting path across the surface of insulation and a mechanism where the leakage current passes through the conducting path finally leading to the formation of a spark. The insulation deterioration occurs as a result of these sparks. Tracking: The formation of continuous conducting paths across the surface of the insulation mainly due to surface erosion under voltage application is called tracking. During use the insulator progressively gets coated with moisture that causes increased conduction leading to the formation of surface tracks. Usually tracking occurs even at very low voltages of the order of about 100 V. Let us consider a system of a solid dielectric having a conducting film and two electrodes on its surface. Practically a conducting film is formed on the surface of solid insulator due to moisture. Now when a voltage is applied across it then the film starts conducting resulting in generation of heat which makes the surface dry. The conducting film becomes separate due to drying resulting in the formation of sparks at its surface. In case of organic insulating materials
the dielectric carbonizes at the region of sparking and the carbonized regions act as permanent conducting channels resulting in increased stress over the rest of the region. This is a cumulative process and the insulation failure occurs when these carbonized tracks bridge the distance between the contacts. This phenomenon is called tracking and is quite common between layers of Bakelite, paper and similar dielectrics built of laminates. Treeing: The spreading of spark channels during tracking in the form of the branches of a tree is called treeing. Under a.c. voltages treeing can occur in a few minutes or several hours. It occurs under high voltage conditions. When a dielectric is placed between two electrodes then there is always a possibility of air coming in between the electrodes and dielectric. As the permittivity of air is less than the dielectric it comes under greater electrical stress when a voltage is applied across the electrodes. Due to this a sparking may occur in the air gap which results in accumulation of charge on the surface of the insulation and sometimes the spark also erodes the surface of the insulation. With the passage of time the breakdown channels spread through the insulation in an irregular tree like fashion leading to the formation of conducting channels. This kind of channeling is called treeing. It can be prevented by having clean, dry and undamaged surfaces. This phenomenon is observed in capacitors and cables. 8.) Breakdown due to Internal Discharges: Solid insulating materials contain voids or cavities within the medium or at the boundaries between the dielectric and the electrodes. These voids are generally filled with a medium of lower dielectric strength and the dielectric constant of the medium in the voids is lower than that of the insulation. Thus the electric field in the voids is higher than that across the dielectric which even under normal working voltages the field in the voids may exceed their breakdown value resulting in breakdown. Let us consider a dielectric between two conductors as shown in fig. 4 (a). Let us divide the insulation into three parts forming an electrical network of C1, C and C3 as shown in fig. 4 (b) where C1 represents the capacitance of the void or cavity, C is the capacitance of the dielectric which is in series with the void and C3 is the capacitance of the rest of the dielectric. t C1 C1 C C3 d C C3 When a voltage V is applied then the voltage V1 across the void is given by Vd1 V1 = ε 0 d1+ d ε1 where d1 and d are the thickness of the void and the dielectric respectively having permittivities ε0 and ε1. Usually d1 << d and if we assume that the cavity is filled with a gas then 5
d1 V = 1 Vε r d Where ε r is the relative permittivity of the dielectric. When a voltage V is applied, V1 reaches the breakdown strength of the medium in the cavity (Vi) and breakdown occurs where Vi is called the discharge inception voltage. When the applied voltage is a.c. then breakdown occurs in both the half cycles and the number of discharges depend on the applied voltage. These internal discharges (also called partial discharges) will have the same effect as treeing on the insulation. The breakdown occurring in the voids result in the formation of electrons and positive ions which on reaching the void surfaces may break the chemical bonds. The heat dissipated in the cavities with each discharge carbonizes the surface of the voids and will cause erosion of the material. All these effects will result in gradual erosion of the material and consequent reduction in the thickness leading to breakdown. The life of the insulation with internal discharges depends upon the applied voltage and the number of discharges. However breakdown by this process may occur in a few days or may take a few years. 9.) Breakdown in composite dielectrics: In case of an electrical equipment it is very difficult to consider that it will consist of only one type of insulation. If we consider an insulation system as a whole then it will be found that more than one insulating material is used. These different materials can be in parallel with each other, such as air or SF6 gas in parallel with solid insulation or in series with one another. Such insulation systems are called composite dielectrics. Composite insulating materials are generally composed of different chemical substances or they come into contact with materials of different compositions. When a voltage is applied to them then chemical reactions occur. If the applied voltage is continuous and temperature is high then rate of these reactions increases which causes the composite to undergo chemical deterioration leading to reduction in both the electrical and mechanical strengths. A composite dielectric generally consists of a large number of layers arranged one over the other. This is called layer construction and is widely used in cables, capacitors and transformers. The following are the properties of composite dielectrics which are important to their performances: (a) Effect of multiple layers The simplest composite dielectric consists of two layers of the same material. Two thin sheets have a higher dielectric strength than a single sheet of the same total thickness. (b) Effect of layer thickness Normally increase in layer thickness gives increased breakdown voltage. In case of a layered construction breakdown channels occur at the interfaces only and not directly through another layer. Various investigations on the composite dielectrics have shown that (i) The discharge inception voltage depends on the thickness of the solid dielectric as (ii) well as on the dielectric constant of both the liquid and solid dielectric and The difference in the dielectric constants between the liquid and solid dielectrics does not significantly affect the rate of change of electric field at the electrode edge with the change in the dielectric thickness. (c) Effect of interfaces The interface between two dielectric surfaces in a composite dielectric system plays and important role in determining it pre breakdown and breakdown strengths. Discharges usually occur at the interface and the magnitude of the discharge depends on the associated surface resistance and capacitance. 6
10.) Mechanism of breakdown in Composite Dielectrics In case of composite there are several factors besides dielectric losses which can cause short and long time breakdown as follows: (a) Short Term Breakdown If the electric field stresses are very high failure may occur in seconds or even faster without any substantial damage to the insulating surface prior to breakdown. There exists a critical stress in the volume of the dielectric at which discharges of given magnitude can enter the insulation from the surface and propagate rapidly into its volume to cause breakdown. Experimentally it has been observed that breakdown occurs more rapidly when the electric field in the insulation is such that it assists the charged particles in the discharge. If the bombarding particles are electrons rather than positive ions then breakdown occurs more readily. (b) Long Term Breakdown The long term breakdown is also called the ageing of insulation. The main effects responsible for the ageing of the insulation which eventually leads to breakdown arises from the thermal processes and partial discharges. Partial discharges normally occur within the volume of the composite insulation system. In addition the charge accumulation and conduction on the surface of the insulation also contributes significantly towards the ageing and failure of insulation. 7