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Chapter 3 (cont) Solid and Liquid Insulating Materials. Solid and Liquid Insulating Materials. Liquids and solids have higher molecular densities than gases – in terms of Paschen’s Law a better insulating performance is expected
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Solid and Liquid Insulating Materials • Liquids and solids have higher molecular densities than gases – in terms of Paschen’s Law a better insulating performance is expected • Practical factors, such as impurities and non- homogeneity cause a worse than expected performance.
Dielectric constant/Polarization • Dielectrics have the property of polarization, i.e. the molecules are dipoles. • If the applied field is zero, the dipoles are randomly arranged. • When a field isestablished, thedipoles are alignedin the direction ofthe field.
Losses in dielectrics: tan • Dielectric losses. • AC: molecules try to follow field • Polarization of dielectric • Inter-molecular friction due to applied ac field • Heat generation • Proportional to tan , V2: • P = VIR = VICtan = V(V 2f C) tan = 2 f C V2 tan
Solid Insulating MaterialsThermal breakdown • Thermal runaway • When heat generation > Heatloss- thus thermal instability • Two cases • overvoltage and • increase in thermalresistance due to drying outof surrounding soil –overloadingof cables. Overvoltage condition: Soil drying out:
Solid Insulating Materials FailureInternal Partial Discharges • Electric Field higher in gas-filled voids (2.2.4). (1E1=2E2) • Discharges in the voids causedamage to the solid material: • electron bombardment • UV • ozone
Solid Insulating Materials FailureTracking and Erosion • Surface discharges caused by poor design (3.3.2c) • Conducting carbonaceous tracks form (tracking) or • Loss of material(erosion) • Aggravated by dustand pollution.
Insulating Liquids • Mineral oil used in transformers for cooling and insulation, together with paper. • Molecules more closely packed - higherflashover voltage expected. • Causes of failure: • Water (50 ppm: 50 => 23 kV/mm) • Fibre bridges • Gas bubbles • Standard oil test: 2.5 mm gap withstands 60 kV
InsulatingLiquids (cont.) • Flashover mechanisms: • Fibre bridge formation: cellulose fibre dipole (+absorbed water) are arranged head-on-tail (dipoles move to the highest field) • Air bubbles, Water drops(partial discharges) • Oil test: 60 kV across a2.5 mm gap.
InsulatingLiquids (cont.) • Practical aspects: • Conservator (expansion tank) + silica gel breathers. • Barriers (hard paper insulation) between windings prevent fibre bridge formation. • Regular oil purification toremove fibres andmoisture. • Chemical oil tests andgas analysis.
Gas Analysis of Oil • Oil analysis (gas in oil) nowadays plays an important part in transformer diagnostics. • Transformer insulation basically consists of oil and cellulose (paper). • The main degradation processes of these materials are: • Corona • Pyrolysis (decomposition due to heating in the absence of oxygen) • Arcing.
Gas Analysis of Oil (cont.) • Typical gases that are formed are: • methane (CH4) • ethane (C2H6) • ethylene (C2H4) • acetylene (C2H2) • hydrogen (H2) • Carbon monoxide (CO) • and carbondioxide (CO2).
Combinations of insulation systemsParallel systems • Electric field in both parts the same: • E=V/d • Example: • The two electrodes are d = 5 cm apart. Material A is a solid material with a relativepermittivity of 6 and a dielectric strength of600 kV/cm, while materialB is an insulating oil witha 2.3 permittivity and adielectric strength of120 kV/cm. At whatvoltage will the systemfail? Ignore end effects.
Combinations of insulation systemsSeries systems • Example (1E1=2E2) • In Fig. (a), a uniform air gap of 10 cm is shown with a voltage of V = 200 kV across the gap. It follows that E = 200/ 10 = 20 kV/cm.The gap will not flash over at this voltage. • In order to "improve" the reliability, a 9 cm thick sheet of epoxy with a relativepermittivity r = 3 wasintroduced in the gap asshown in Fig.(b). Did itwork?
Combinations of insulation systemsSeries systems (cont.) • Void in dielectric(1E1=2E2)
Practical Design: Insulator • Note that the field is higher in the air sections of the series path between the electrodes. • Further the field strength is high near the electrodes.
Surface Discharges • Field lines cross boundarybetween solid and air. • Field strength in air highest(1E1=2E2) • Air has a lower breakdownstrength than glass andbreaks down first. • Air on the surface of theglass breaks down – theglass remains intact.
InsulatorPollution • A salty layer forms on the insulator surface. • Near the coast or in industrial areas. • Moisture (fog, humidity)is absorbed into the layerand forms a conductinglayer on the insulator.
Insulator Pollution (cont.) • The layer dries out at thenarrowest parts. • Highestcurrent density/powerdissipation. • Arcs develop over the drybands – until completeflashover occurs.
Polymeric Materials • EPDM Rubber (Ethylene Polythene Diene Rubber): • Not hydrophobic – has been shown to age in severely polluted regions. • Silicone rubber: • Hydrophobic, even when polluted.
Summary • In this chapter the properties of gaseous, liquid and solid insulating materials and combinations thereof were explored. • In the next chapter laboratory methods to be used to evaluate the strength of insulation systems will be discussed.
