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Transcript
ENGINEERING PHYSICS II DIELECTRICS 4.8 DIELECTRIC LOSS If a dielectric is subjected to an electric field, the electrical energy is absorbed by the dielectric and certain quantity of electrical energy is dissipated in the form of heat energy. This is known as dielectric loss. The dielectric loss can occur both in direct and alternating voltages. The dielectric loss is less in direct voltage than that of alternating voltages. Loss in Purified gas If an alternating voltage is applied across the capacitor having vaccum (or) purified gas then the resulting current leads the applied voltage by 90 degree as shown in fig If I lead V exactly by 90 degree we can say no electrical energy is lost. Explanation We know power loss PL = VI cos θ When θ=90˚ ; PL=0 Loss in commercial dielectric Now, when a practical dielectric is present the current leads the voltage by (90- δ ), then it shows that there is some loss in electrical energy, and is called loss angle as shown 75 ENGINEERING PHYSICS II DIELECTRICS Explanation In this case the power loss PL = VI cos θ Since θ=90-δ, we have PL = VI cos(90-δ) ………..(1) ∴ PL = VI sinδ We know V=IR, therefore I = V R If the capacitive reactance is Xc then we can write, I= V Xc ……….(2) Substituting eqn (2) in (1) we get PL = V 2 δ/Xc ………(3) Power loss We know frequency 1 2𝜋𝑅𝐶 1 = 2𝜋Xc 𝐶 𝑓= ∴ Xc = 1 2𝜋f𝐶 ………(4) Substituting eqn (4) in (3) we get If δ is very small, then sinδ = tanδ 𝑃𝐿 = 2𝜋f𝐶𝑉 2 sinδ ∴ Power loss PL = 2πfCV 2 tanδ Here tanδ is called the power factor of the dielectric. If f, C, V are constants then 𝑃𝐿 ∝ 𝑡𝑎𝑛δ Naturally the power loss varies with frequency. The power loss at various frequency ranges is shown in figure. 76 ENGINEERING PHYSICS II DIELECTRICS In the electrical frequency regions the power loss is high, due to diffusion of ions from one equilibrium position to another. In the optical region the power loss is less because here the dielectric loss is associated with the electrons. 4.9 DIELECTRIC BREAKDOWN When a dielectric is placed in an electric field and if the electric field is increased, when the field exceeds the critical field, the dielectric loses its insulating property and becomes conducting i.e., large amount of current flows through it. This phenomenon is called dielectric breakdown. The electric field strength at which the dielectric breakdown occurs is known as dielectric strength. The dielectric strength =Dielectric voltage/Thickness of dielectric There are different mechanisms by which the dielectric break down takes place. Some types of dielectric breakdowns are Intrinsic (or) avalanche breakdown Thermal breakdown Chemical and electrochemical breakdown Discharge breakdown Defect breakdown Intrinsic Breakdown When a dielectric is subjected to electric field then the electrons in the valence band acquire sufficient energy and go to conduction band by crossing the energy gap and hence become conducting electrons. Therefore large current flows and is called intrinsic breakdown (or) Zener breakdown. Impurities in the dielectric material create additional energy levels in the energy gap and so they help the intrinsic breakdown to occur at low applied voltages. Avalanche Breakdown This conduction electron on further application of field, collide with the valence electrons in the co-valent bond and remove more electrons hence transferring them as conduction electrons. These Secondary conduction electrons again dislodge some other bound electrons in the valence band and this process continues as a chain reaction. Therefore very large current flows through the dielectrics and hence called as avalanche breakdown. Characteristics 77 ENGINEERING PHYSICS II DIELECTRICS It can occur even at lower temperatures. It requires relatively large electric fields. This kind of breakdown occurs in thin sample. It does not depend on the configuration of electrodes and shape of the material. It occurs within a short span of time (milli seconds) Thermal Breakdown In general, when a dielectric is subjected to an electric field, heat is generated. This generated heat is dissipated by the dielectric. In some cases the heat generated will be very high compared to the heat dissipated. Under such conditions the temperature inside the dielectric increases and heat may produce breakdown. This type of breakdown is known as thermal breakdown. Characteristics It occurs at higher temperature. It requires moderate electric fields. It depends on the size and shape of the dielectric material. Since, the dielectric loss is proportional to frequency, the breakdown occurs at relatively lower field strength for a.c. fields than that of d.c. fields. It occurs in the order of milliseconds. Chemical and Electrochemical Breakdown This type of breakdown is almost similar to the thermal breakdown. If the temperature is increased mobility of the ions will increase and hence the electrochemical reaction may be induced to take place. Therefore when mobility of ions are increased, insulation resistance decreases and hence dielectrics become conducting. This type of breakdown is called as chemical and electro chemical breakdown. Characteristics It occurs only at low temperatures. It occurs even in the absence of electric field. In rubber, due to oxides produced in air, they gradually lose their dielectric property. It depends on concentration of ions, magnitude of leakage current. Discharge Breakdown 78 ENGINEERING PHYSICS II DIELECTRICS In some dielectric occluded gas bubbles may be present. When these type of dielectrics are subjected to electric field, the gas present in the material will easily ionize and hence produce large ionization current and is known as discharge breakdown. Characteristics It occurs at low voltages. It occurs due to the presence of occluded gas bubbles. It depends upon the frequency of the applied voltage. Defect Breakdown Some dielectrics have defects such as cracks, pores, blow holes etc. These vacant position may have moisture (or) impurities which leads to breakdown called as defect breakdown. 4.10 REMEDIES FOR BREAKDOWN MECHANISMS To avoid breakdown, the dielectric material should have the following properties. It should have high resistivity. It must posses high dielectric strength. It should have sufficient mechanical strength. Dielectric loss should be low. Thermal expansion should be small. It should be fire proof. It should be resistive to oils, liquids and gases etc. It must have less density. There should not be any defects. It must be in pure form. 79