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Hve Unite-2

Home > PowerPoint Slides > Hve Unite-2
Published in: Electrical
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A subject of BTech -3rd semester of electrical engineering

Keerthi / Hyderabad

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Qualification: Diploma in Electrical Engineering

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  1. HIGH VOLTAGE ENGINEERING UNIT-I" Breakdown in Liquid Dielectics
  2. Breakdown in Liquid Dielectics Liquid dielectrics : • a) 1000 times high dielectric strength ( 10 times than air) i.e 107 v/cm b) good heat conduction • c) Cheapest & Commonly used [Note : Most commonly used oil is transformer oil it is a mixture of hydrocarbons includes (T= 950 C) ] Paraffin's , • Iso —paraffin's, Naphthalene etc. Liquid dielectrics electrical properties: • a) High permittivity b) High Resistivity
  3. Types of Liquid dielectrics e Liquid dielectrics Pure liquids The liquids which are chemically pure and no impurity is called pure liquids ex: n- hexane(C6H14) n- heptane(C7HI 6) Parafins Hydracarbons Commertial liquids •The liquids which are not chemically pure and having impurity called commertial liquids ex : It is a mixure of complex organ molecules
  4. Purification process of commercial liquids cooling tower C? vaccume gauge To vaccume pump Distilation column Filter To vaccume pump Test cell with electrodes To vaccume pump Resorvoir Fig 3.1 Purifaction proces of commericial oil to pure oil
  5. Breakdown in Pure Liquid Dielectics Pure Liquid dielectrics : At 1KV/cm field the conductivity is 10-18 to 10-20 mho/cm a) l- Ionic region : At low fields the current is low due to dissociation of ions only. b) ll- Saturation region: If the field increased current reaches to saturation according to Townsend's b. d criterion. c) Field aided region: If the field increased more than 100Kv/cm then the field aided and electrons get multiplies due to secondary ionization process and causes breakdown in liquids. o 2 Il-saturation I-Ionic 0.4 0.2 Ill - field aided 1.0 E(MV/cm) fig 3.2 Breakdown curve in pure liquids
  6. Breakdown in Commercial Liquid Dielectics Break down in commercial liquids are three types based on type of impurities are present in oil, 1. Suspended particle theory ( Solid impurities) 2. Cavitations & Bubble Theory ( Gas bubbles) 3. Stressed oil Volume Theory (Weak Links)
  7. Breakdown in Solid Dielectics • Electrical Properties: High resistivity 1. 2. Low Dielectric loss ( Tan(delta)) 3. High mechanical strength 4. Free from moisture contents 5. No chemical reaction High thermal stability 6. Free from gaseous inclusions 7.
  8. Breakdown methods in solid dielectrics l._lntrinsic (or) Ionic Breakdown 2. Electro-Mechanical Breakdown 3. Failure due to Treeing & Tracking 4. Thermal Breakdown 5. Electro-Chemical Breakdown Intrinsic bd electro-rnechanica] bd Streemer bd Thermal bd 3 6. Breakdown due to Internal Discharges fig3.3 02 0.4 Breakdown curve Electro-chernical bd Treeing & Tricking bd 'Iternal ischarges 1.0 Duration (Time) sold dielectrics
  9. • • • Intrinsic (or) Ionic Breakdown Intrinsic (or) Ionic Breakdown To reach upper limit of dielectric strength of solid dielectrics when applied the voltages with in the short duration(10-8sec) called Intrinsic breakdown" --15MV/cm at -1960C The max. electric strength is recorded For PVC In general max. electric strength is recorded — 5MV/cm Intrinsic breakdown depends on pressure of free electron which are capable of mitigation through the lattice of dielectric Intrinsic breakdown Electronic break down Streemer bd
  10. Electro-Mechanical Breakdown 'Principle: When solid dielectrics are subjected to high electric field fields stresses, failure occurs when electro-static compressive forces which are exceeds the electromechanical compressive strength" Electrostatic compressive forces = EO er (V2/2d2 ) = Y In(do/d) Electro- mechanical compressive forces Equilibrium condition EO Er y In(do/d) d2 [2y /EO Er] In (do/d) Mechanical instability occurs, d/ do = 0.6 (or) = do / d = 1.67 Max. stress before breakdown, Emax = V/do = 0.6 [y / EO Er11/2
  11. Thermal Breakdown "Principle: When solid dielectrics are subjected to high electric field fields stresses, failure occurs when it will under goes high temperature due to heat generated init " Heat generated in specimen - Heat dissipated Heat generated (Wdc) (Wac) Heat dissipated WT - oE2 w/m3 - E2 fer tan 5/ 1.8x 1012 W/m3 = cv dT/dt + div[K. grad(T)] cv- Specific heat T — Temperature K — Thermal Conductivity 5 — loss angle
  12. Treeing & Tracking Breakdown • Treeing: "A conducting channels are formed on the surface in irrespective branches of a tree called Treeing' It can be reduced by - clean and dry undamaged surface & clean environment Tracking: A mechanism by leakage current passes through the conducting channel leads to the formation of spark called Tracking. It can be reduced by - providing resistant to tracking like greasing, increasing creepage distance
  13. • B.b. in Composite dielectrics Composite dielectrics : 'The combination of of dielectrics (different permittivity) connected in series or parallel to increase the breakdown strength is called composite dielectrics". Properties: - 1. Effect of multi layer 2. Effect of layer thickness 3. Effect of interfaces Composite dielectrics B.d. Short time b.d Long term b.d Ageing & bd due to PD Aging & bd due to accumulation of charges
  14. Solid Dielectrics used in practice Solids dielectrics used inpractice el Organic Material 'Vegetable / animal matter • Low b.d strength •Ex: Cotton Rubbor ape r Wax Wood Pressboard . 02. Inorganic Material •Medium b.d strength o • 250 c 'Ex: Ceramics Glass Mica Asbestas . 03. Synthetic Polymers •Thermoseting plastic Thermoplastic High b.d strength 100C -208c Ex: Polyehylene per p ex Poly propylene Poly vinyle chloride poly carborates.. Ex: -Bakelite Epoxy Resin Elastomers

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