DEPARTMENT OF ELECTRICAL ENGINEERING DIT UNIVERSITY HIGH VOLTAGE ENGINEERING

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1 UNIT 1: BREAKDOWN IN LIQUIDS 1.) Introduction: The liquid dielectrics due to their inherent properties appear as though they would be more useful as insulating materials than either solids or gases. The liquids like gases fill the complete volume to be insulated and simultaneously will dissipate heat by convection. Oil is about 10 times more efficient than air or nitrogen in its heat transfer capability when used in transformers. The liquid dielectrics are used mainly as impregnates in high voltage cables and capacitors, and for filling up of transformers, circuit breakers etc. Liquid dielectrics also act as heat transfer agents in transformers and as arc quenching media in circuit breakers. Petroleum oils (transformer oil) are the most commonly used liquid dielectrics. Synthetic hydrocarbons and halogenated hydrocarbons are also used for certain applications. For very high temperature application, silicone oils and fluorinated hydrocarbons are used. However some of the isomers of poly chlorinated diphenyls (generally called askerels) have been found to be very toxic and poisonous and hence their use has been almost stopped. Liquid dielectrics normally are mixtures of hydrocarbons and are weakly polarised. The most important factor that affects the electrical strength of insulating oil is the presence of water in the form of fine droplets suspended in the oil. The presence of even 0.01% water in transformer oil reduces its electrical strength to 20% of the dry oil value. The dielectric strength of oil reduces more sharply if it contains fibrous impurities in addition to water. The table 1 shows dielectric properties of some liquid dielectrics. Property Transformer Oil Cable Oil Capacitor PETEP Silicone Oil Oil Oils Breakdown strength at 20 0 C on > kv/mm 30 kv/mm 20 kv/mm 2.5 mm standard sphere gap kv/mm kv/mm Relative permittivity (50 Hz) Tan δ (50 Hz) (1 khz) x x x x Resistivity (ohm cm) > x Maximum permissible water content ( in ppm) < 30 Viscosity at 20 o C Specific gravity at 20 o C The transformer oil is the cheapest and the most commonly used. 2.) Classification of Liquid Dielectrics: In recent years, a substitute to mineral oils, other polyester oils have been developed which are extensively used in transformers in Europe and other countries. One such oil is the halogen free Penta Etythrite Tetra Fatty Acid Polyester oil (PETFP oil) which has very good electrical, physical and thermal properties. It is also biodegradable i.e. when decomposed has almost negligible toxicity and does not contribute to pollution. (a) Transformer Oil (Mineral Oil) It is the most commonly used liquid dielectric in power apparatus. It is almost colourless liquid consisting of a mixture of hydrocarbons which include paraffins, iso paraffins, naphthalenes and aromatics. In service the transformer oil undergoes a gradual ageing process as it is being subjected to prolonged heating at high temperatures of about 95 0 C resulting in the oil becoming darker due to formation of acid and resins or sludge in the liquid. (b) Synthetic Hydrocarbons Among synthetic liquid dielectrics, polyolefins are the dielectrics of choice for application in power cables. Over 55% of synthetic hydrocarbons produced today are 1

2 polyolefins. The most commonly used olefins are poly butylene and alkylaromatic hydrocarbons. Their general characteristics are very similar to those of mineral oils. (c) Chlorinated Hydrocarbons Two aromatic hydrocarbons, benzene and diphenyl, are chlorinated to produce chlorinated aromatic compounds called askerels. They possess high fire point and excellent electrical properties. In recent years their use has been banned throughout the world because they pose serious health hazards. (d) Silicone Oils They are expensive in comparison to polychlorinated biphenyl oils. They have high long term thermal stability even at a temperature of C. Silicone oils are resistant to most chemicals, and are oxidation resistant even at higher temperatures. They can be used at higher temperatures than mineral oils. However they have slightly inferior non-flammable properties in comparison to polychlorinated biphenyl. (e) Esters Natural esters like castor oil have been used as a capacitor impregnant for many years but currently two types of synthetic esters are being used i.e. organic esters and phosphate esters. Organic esters have high boiling points in relation to their viscosity and therefore have high fire points. They have a good viscosity temperature relationship and are extensively used in capacitors. The phosphate esters have high boiling point and low flammability and therefore are used in transformers that are to be installed in hazardous areas. (f) Latest Development Some new oils have been introduced in recent years. They are being marketed under different commercial names such as high temp. hydrocarbon oil, tetrachloroehtylene and perfluoropolyether. High Temp. Hydrocarbon (HTH) oils have good electrical insulating and adequate heat transfer properties. They are chemically similar to regular mineral transformer oils but possess higher boiling points and higher fire points. However they have higher viscosity which reduces heat transfer capability. Tetrachloroethylene is a non-flammable insulating fluid and is used in mixtures with mineral oil. It has very low viscosity and therefore gives excellent heat transfer properties. 3.) Characteristics of Liquid Dielectrics: The liquid dielectrics should possess the following characteristics: (a) Electrical Properties The following electrical properties are essential in determining the dielectric performance of a liquid dielectric: (i) Its capacitance per unit volume or its relative permittivity. (ii) Its resistivity. (iii) Its loss tangent (tan δ) or its power factor which is an indication of power loss under a.c. voltage application. (iv) Its ability to withstand high electric stresses. Permittivity of most of the petroleum oils vary from 2.0 to 2.6 while those of silicone oils from 2.0 to 73. In case of non-polar liquids the permittivity is independent of frequency but in case of polar liquids such as water it changes with frequency. Resistivity of insulating liquids used for high voltage applications should be more than ohm metre and most of the liquids in their pure state exhibit this property. Power factor of a liquid dielectric under a.c. voltage will determine its performance under load conditions. Power factor is a measure of power loss and is an important parameter in cable and capacitor systems. In the case of transformers, the dielectric loss in the oil is negligible when compared to copper and iron losses. Pure and dry transformer oil will have a very low power factor varying between 10-4 at 20 0 C and 10-3 at 90 0 C at a frequency of 50 Hz. Dielectric strength is the most important parameter in the choice of a given liquid dielectric for a given application. The dielectric strength depends on the atomic and molecular properties of the liquid itself but under practical conditions it depends on the material of the electrodes, temperature, type of 2

3 applied voltage, gas content in the liquid etc. which change the dielectric strength by changing the molecular properties of the liquid. (b) Heat Transfer Characteristics In case of equipments filled with liquid dielectric (transformer, cable, circuit breaker etc) heat is transferred mainly by convection. Under natural atmospheric cooling conditions convection (N) is given by N = f [K 3 AC/v] n Where K = thermal conductivity, A = coefficient of expansion, C = specific heat per unit volume, v = kinematic viscosity and n = The main factors that control the heat transfer are thermal conductivity (K) and viscosity (v). A higher value of K is preferable for apparatus likely to operate continuously at a high temperature. However a low value of K and high viscosity can lead to localized overheating or even dielectric burnout. Silicone oils do not exhibit these properties and therefore pose severe overheating problems in equipment that use these dielectric oils. (c) Chemical Stability The insulating liquids are subjected to thermal and electrical stresses in the presence of materials like O2, water, fibres and decomposition products of solid insulation. These either singly or in combination cause degradation of the liquid with the result that soluble solid and gaseous products are found which can result in corrosion, deterioration of electrical properties, increased dielectric losses, discharges and arcing. In the absence of any action to prevent degradation this cycle continues resulting in decreasing purity and quality of liquid and affecting equipment condition. The main consideration in the selection of a liquid dielectric is its chemical stability. The other considerations are the cost, the saving in space, susceptibility to environmental influences etc. The use of liquid dielectric has brought down the size of equipment tremendously. 4.) Pure Liquids and Commercial Liquids: Pure liquids are those which are chemically pure, structurally simple and do not contain any other impurity even in traces of 1 in Examples of such simple pure liquids are n hexane, n heptane and other paraffin hydrocarbons. By using simple and pure liquids it is easier to separate out the various factors that influence conduction and breakdown in them. On the other hand, the commercial liquids are not chemically pure, normally consists of mixtures of complex organic molecules which cannot be easily specified or reproduced in series of experiments. 5.) Conduction and Breakdown in Pure Liquids: In case of pure liquids when low electric fields less than 1 kv/cm are applied, then conductivities of to mho/cm are obtained due to impurities remaining even after purification. When a curve is drawn between conduction current and electric field as shown in fig. 1 then it shows three distinct regions. At very low fields the current is due to the disassociation of ions whereas with intermediate fields the current reaches a saturation value and at high fields the current generated because of the electron emission from the cathode gets multiplied in the liquid medium by a Townsend type of mechanism. The current multiplication also occurs from the electrons generated at the interface of liquid and impurities. The increase in current by these processes continues till breakdown occurs. Conduction current Ionic Saturation Field aided Electric field (MV/cm) Fig. 1 Conduction current electric field characteristic in a hydrocarbon liquid 3

4 The exact mechanism of current growth is not known; however it appears that the electrons are generated from the cathode by field emission of electrons. The electrons so liberated get multiplied by a process similar to Townsend s primary and secondary ionization in gases. As the breakdown field is approached, the current increases rapidly due to a process similar to the primary ionization process and also the positive ions reaching the cathode generate secondary electrons leading to breakdown. The breakdown voltage depends on the field, gap separation, cathode work function and the temperature of the cathode. It has been observed that the increase in breakdown strength is more, if the dissolved gases are electronegative in character. Similarly an increase in hydrostatic pressure increases the breakdown strength. 6.) Conduction and Breakdown in Commercial Liquids: In case of commercial liquids there are various impurities like gas bubbles, suspended particles etc. which reduce the breakdown strength of these liquids considerably. The breakdown mechanism in commercial liquids is dependent on several factors such as the nature and condition of the electrodes, the physical properties of the liquid and the impurities present in liquid. Several theories have been proposed to explain the breakdown in liquids which are classified as follows: (a) Suspended particle mechanism. (b) Cavitation and bubble mechanism. (c) Stressed oil volume mechanism. 7.) Suspended Particle Theory: In the case of commercial liquids there are generally always solid impurities present either as fibres or as dispersed solid particles. The permittivity of these particles will be different from the permittivity of the liquid. Let us assume that these impurities are spherical in shaped having radius r, a field E is applied then the particle experiences a force F which is given by This force will be directed towards the areas of maximum stress if the permittivity of the particle is greater than the permittivity of the liquid. On the other hand the force will be directed towards the areas of lower stress if the permittivity of the particle is less than the permittivity of the liquid. If the voltage is continuously applied (d.c.) or the duration of the voltage is long (a.c.) then this force drives the particles towards the areas of maximum stress. If the number of particles present is large, they become aligned due to these forces and thus form a stable chain bridging the electrode gap causing a breakdown between the electrodes. If there is only a single conducting particle between the electrodes, it gives rise to local field enhancement depending on its shape and if this field exceeds the breakdown strength of the liquid then local breakdown will occur near the particle resulting in the formation of gas bubbles which may lead to the breakdown of the liquid. The value of the breakdown strength of liquids containing solid impurities was found to be much less than the values for pure liquids. The large larger the size of the particles the lower is the breakdown strength. 8.) Cavitation and Bubble Theory: It has been observed experimentally that the dielectric strength of liquid depends upon the hydrostatic pressure above the gap length. The higher the hydrostatic pressure, the higher the electric strength which leads to the conclusion that a change in phase of the liquid in involved in the breakdown process. This means a kind of vapour bubble formed is responsible for the breakdown. The following processes might lead to formation of bubbles in the liquids: (i) Gas pockets on the surface of electrodes (ii) Due to irregular surface of electrodes, point charge concentration may lead to corona discharge thus vapourizing the liquid (iii)changes in temperature and pressure 4

5 (iv) Dissociation of products by electron collisions giving rise to gaseous products. Once a bubble is formed it will elongate in the direction of the electric field under the influence of electrostatic forces. The volume of the bubble remains constant during elongation. The breakdown occurs when the voltage drop along the length of the bubble becomes equal to the minimum value on the Paschen s curve for the gas in the bubble. The breakdown field is given by _ V b E O r reo Where σ is the surface tension of the liquid, ε1 is the permittivity of the liquid, ε2 is the permittivity of the gas bubble, r is the initial radius of the bubble assumed as a sphere and Vb is the voltage drop in the bubble (corresponding to minimum on the Paschen s curve). Thus the breakdown strength depends on the initial size of the bubble which in turn is influenced by the hydrostatic pressure and temperature of the liquid as seen from the above equation. However this theory does not takes into account the production of the initial bubble and hence the results given by this theory do not agree well with the experimental results as shown in fig Breakdown Stress (MV/cm) Hydrostatic pressure (lb/cm 2 ) Fig.2 Theoretical and experimental breakdown stresses in n hexane 9.) Thermal Mechanism of Breakdown: According to this mechanism high current pulses are believed to originate from the tips of the microscopic projections on the cathode surface with densities of the order of 1 A/cm 3. These high density current pulses give rise to localised heating of the oil which may lead to the formation of vapour bubbles which are formed when the energy exceeds 10 7 J/cm 2. The bubble breakdowns either because of its elongation to a critical size or when it completely bridges the gap between the electrodes resulting in the formation of a spark. According to this mechanism, the breakdown strength depends on the pressure and the molecular structure of the liquid. However this theory is only applicable at very small lengths (< 100 µm) and does not explain the reduction in breakdown strength with increased gap lengths. 5

6 10.) Stressed Oil Volume Theory: In case of commercial liquids, minute traces of impurities are present which greatly affects the breakdown strength of the liquids. Due to the presence of these impurities the electrical stress is not uniformly distributed. On the basis of statistical data obtained it was proposed that the electrical breakdown strength of the oil is defined by the weakest region in the oil, namely, the region which is stressed to the maximum and by the volume of oil included in that region. For non uniform fields the stressed oil volume is taken as the volume which is contained between the maximum stress (Emax) contour and 0.9 Emax contour. According to this theory the breakdown strength is inversely proportional to the stressed oil volume. In case of oil the breakdown voltage is highly influenced by the gas content in it, its viscosity and the presence of other impurities. The variation of the breakdown voltage stress with the stressed oil volume is shown in fig With steady voltage rise One min. withstand voltage Breakdown Stress (kv/cm) Va Stressed Oil Volume (CC) Fig.3 Variation of breakdown voltage stress with stressed oil volume 11.) Conclusion: In case of commercial liquids, minute traces of impurities are present which greatly affects the breakdown strength of the liquids. However all the theories so far studied which attempt to explain the breakdown phenomenon in liquids, no single theory can explain all the experimental observations satisfactorily. Therefore, recently fast optical techniques have been developed to study the processes of liquid dielectric breakdown. None of the theories so far studied consider the dependence of breakdown strength on the gap length. According to all theories so far considered we try to obtain the maximum obtainable breakdown strength only. However the experimental evidences showed that the breakdown strength of a liquid depends on the gap length given by the following expression Vb = Ad n Where d = gap length, A = constant and n = constant always less than 1 The breakdown voltage also depends on the nature of the voltage, the mode in which the voltage is applied and the time of application. The above relationship is of practical importance and the electrical 6

7 stress of a given oil used in design is obtained from this. The actual mechanism of breakdown in oil is not a simple phenomenon and the breakdown voltages are determined by experimental investigation only. The table 2 gives the typical breakdown strengths for highly purified liquids. Dielectric Liquid Used in Equipment Design Field Strength (MV/m) Breakdown Strength (MV/m) Transformer Oil Transformers n hexane Cables Polybutane (Synthetic hydrocarbon) Capacitors