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Prof. Mohamed Adel Abdallah

High Voltage Engineering. Prof. Mohamed Adel Abdallah. Chapter 1 High Voltage Insulation . * Need for Electrical Insulation . Prevent leakage current from HV parts to nearby grounded parts . Safety for Workers . * Area of Insulation Application. Phase to earth. Phase to Phase.

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Prof. Mohamed Adel Abdallah

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  1. High Voltage Engineering Prof. Mohamed Adel Abdallah

  2. Chapter 1High Voltage Insulation

  3. * Need for Electrical Insulation Prevent leakage current from HV parts to nearby grounded parts Safety for Workers

  4. * Area of Insulation Application Phase to earth Phase to Phase Inter-turn (In a coil) Inter-Coil (between coils)

  5. * Function of Electrical Insulation Materials Mechanical Support Arc Quenching Heat transfer

  6. * Basic Electrical Properties of Dielectric Materials 1- DC Resistivity : Ω.m 2- DC Conductivity: 3- Dielectric Permittivity: =

  7. 4- Electric Polarization: Decrease of the electric field due to the dielectric material with the material tends to neutralize some charges

  8. 5- Loss angle and dissipation factor: Parallel equivalent ( Ic ) V Rp Cp V ()

  9. = = =

  10. Parallel equivalent I I Rs V Cs Loss factor (dissipation factor)=

  11. 6- Power Loss in a dielectric: = 7- Breakdown Voltage: 8- Dielectric Strength: 9- Partial discharge and Corona (PD):

  12. * Non Electrical Properties of Dielectrics Maximum Operating Temp. • Hardness Elasticity Coefficient of expansion. Thermal conductivity Specific gravity Tensile strength Compressive strength Density

  13. * Classification of Insulating Materials Gases Vacuum Liquids Solids

  14. Chapter 2 Generation of Testing Voltage

  15. * Direct voltages 1- A.C. to D.C. conversion Single-phase half-wave rectifier

  16. full-wave rectifier circuit

  17. Cascade circuits

  18. Cascade circuit

  19. * AC Voltages cascaded transformers. (1) Primary windings. (2) Secondary h.t. windings. (3) Tertiary exciting windings

  20. Series resonant circuits

  21. * Impulse Voltage Generation

  22. Single-stage generator

  23. For R2 >> R1 and C1>> C2 )= 3 R1 = 0.7 (R1 +R2)(C1+C2)

  24. Multistage generator

  25. Chapter 3 High Voltage Testing

  26. High Voltage tests are applied to determine the ability of the insulation to meet its design requirements Destructive Non destructive For solid insulators To measure the maximum Dielectric strength For all types of insulators To insure the quality of Insulation

  27. * HV Test Classifications 1- AC - DC - Impulse ( switching - Lightning) 2- Type tests - Routine Tests - Special Tests 3- Destructive Tests - Nondestructive Tests 4- Acceptance - Design and Research

  28. Test Voltages (I) Direct voltages • Mainly used for:- • Pure scientific research work • For testing equipment related to HVDC transmission systems. • For HVAC power cables of long length, as the large capacitance of those cables would take a high current if tested with a.c. voltages .

  29. AC Impulse BIL for lightning ( 1.2/50 µs) BSL for switching

  30. Rp Cp Breakdown Tests For self restoring insulation ( liquid and gas insulations) Dielectric Loss Tests Ic tan δ = δ Vc

  31. The Schering bridge

  32. Purposes of tan δ tests 1- to assess the level of degradation suffered by the dielectric 2- to assess the quality of the insulation 3- to assess the level of contamination (liquid dielectric) 4- to assess the presence of cavities ( solid dielectrics)

  33. Partial Discharge Tests Partial discharges are in general a consequence of local electrical stress concentrations in the insulation or on the surface of the insulation. The term ‘partial discharge’ includes a wide group of discharge phenomena: (i) internal discharges occurring in voids or cavities within solid or liquid dielectrics; (ii) surface discharges appearing at the boundary of different insulation materials; (iii) corona discharges occurring in gaseous dielectrics in the presence of inhomogeneous fields; (iv) continuous impact of discharges in solid dielectrics forming discharge channels (treeing).

  34. The detection and measurement of discharges is based on the exchange of energy taking place during the discharge. These exchanges are manifested as: (i) Electrical pulse currents (with some exceptions, i.e. some types of glow discharges); (ii) Dielectric losses; (iii) Radiation (light); (iv) Sound (noise); (v) Increased gas pressure; (vi) Chemical reactions.

  35. The basic PD test circuit Va Ca = C`a +C``a

  36. Let us now assume that the sample was charged to the voltage Va but the terminals A, B are no longer connected to a voltage source. If the switch S is closed and Cc becomes completely discharged, the current ic(t) releases a charge δqc = Cc δVc from Cc, a charge which is lost in the whole system as assumed for simulation. By comparing the charges within the system before and after this discharge, we receive the voltage drop across the terminal δVa as

  37. = = =

  38. Surge Over Voltage Due to Traveling Wave on Transmission Lines Chapter 4

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