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Overvoltage phenomenon

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Overvoltage phenomenon

  1. 1. HIGH VOLTAGE ENGINEERING Prof.P.Swaminathan Asst.Prof.[SG]/EEE KARUNYA UNIVERSITY Overvoltage Phenomenon and Insulation Coordination
  3. 3. Charge Formation in the Clouds
  4. 4. Cioud model according to Simpson's theory
  5. 5. Mechanism of Lightning Strokes
  6. 6. Travelling Waves on Transmission Lines
  7. 7. Classification of Transmission Lines  Transmission lines are usually classified as  (a) lines with no loss or ideal lines,  (b) lines without distortion or distortion less lines,  (c) lines with small losses, and  (d) lines with infinite and finite length defined by all the four parameters.
  8. 8. Reflection and Transmission of Waves at Transition Points
  9. 9. Successive reflections and lattice diagrams  (/) all waves travel downhill, i.e. into the positive time  (M) the position of the wave at any instant is given by means of the time scale at the left of the lattice diagram  (//O the total potential at any instant of time is the superposition of all the waves which arrive at that point until that instant of time, displaced in position from each other by time intervals equal to the time differences of their arrival  (/v) attenuation is included so that the amount by which a wave is reduced is taken care of and  (v) the previous history of the wave, if desired can be easily traced. If the computation is to be carried out at a point where the operations cannot be directly placed on the lattice diagram, the arms can be numbered and the quantity can be tabulated and computed.
  10. 10. Reflection lattice of a travelling wave
  11. 11. Behaviour of Rectangular Travelling Wave [Unit Step Function] at Transition Points—Typical Cases  Case (i): Open ended transmission line of surge impedance Z:  Case (Ii): Short circuited line:  Case (Hi): Line terminated with a resistance equal to the surge impedance of the line  Case (Iv): Line terminated with a capacitor:  Case (v): Transmission terminated by an inductance L:  Case (yi): Line having a series inductor:  Case (ViI): Line terminated with a transformer  (taken as an L-C parallel combination):
  12. 12. OVERVOLTAGE DUE TO SWITCHING SURGES, SYSTEM FAULTS AND OTHER ABNORMAL CONDITIONS  Origin of Switching Surges  The making and breaking of electric circuits with switchgear may result in abnormal overvoltages in power systems having large inductances and capacitances.  The over voltages may go as high as six times the normal power frequency voltage. In circuit breaking operation, switching surges with a high rate of rise of voltage may cause repeated restriking of the arc between the contacts of a circuit breaker, thereby causing destruction of the circuit breaker contacts.  The switching surges may include high natural frequencies of the system, a damped normal frequency voltage component, or the restriking and recovery voltage of the system with successive reflected waves from
  13. 13. Characteristics of Switching Surges  (i) De-energizing of transmission lines, cables, shunt capacitor, banks, etc.  (ii) Disconnection of unloaded transformers, reactors, etc.  (Uf) Energization or reclosing of lines and reactive loads,  (i v) Sudden switching off of loads.  (v) Short circuits and fault clearances.  (w) Resonance phenomenon like ferro- resonance, arcing grounds, etc
  14. 14. Switching Overvoltages In EHV and UHV Systems  Interruption of low inductive currents (current chopping) by high speed circuit breakers. This occurs when the transformers or reactors are switched off  Interruption of small capacitive currents, such as switching off of unloaded lines etc.  ferro-resonance condition  This may occur when poles of a circuit breaker do not close simultaneously  Energization of long EHV or UHV lines.
  15. 15. Energization of long EHV or UHV lines  (a) single pole closing of circuit breaker  (b) interruption of fault current when the L-G or L- L fault is cleared  (c) resistance switching used in circuit breakers  (d) switching lines terminated by transformers  (e) series capacitor compensated lines  (O sparking of the surge diverter located at the receiving end of the line to limit the lightning over voltages
  16. 16. Power Frequency Over voltages in Power Systems  The power frequency over voltages occur in large power systems and they are of much concern in EHV systems, i.e. systems of 400 kV and above. The main causes for power frequency and its harmonic over voltages are  (a) sudden loss of loads,  (b) disconnection of inductive loads or connection of capacitive loads,  (c) Ferranti effect, unsymmetrical faults, and  (d) saturation in transformers, etc.
  17. 17. Control of Over voltages Due to Switching  The overvoltages due to switching and power frequency may be controlled by  (d) energization of transmission lines in one or more steps by inserting resistances  and withdrawing them afterwards,  (b) phase controlled closing of circuit breakers,  (c) drainage of trapped charges before reclosing,
  18. 18. Protection of Transmission Lines against Over voltages  Protection against Lightning Overvoltages and Switching Surges of short Duration  Overvoltages due to lightning strokes can be avoided or minimized in practice by  (d) shielding the overhead lines by using ground wires above the phase wires,  (b) using ground rods and counter-poise wires, and  (c) including protective devices like expulsion gaps, protector tubes on the lines, and surge diverters at the line terminations and substations.
  19. 19. Lightning Protection Using Shielded Wires or Ground Wires
  20. 20. Protective Devices  (i) Expulsion gaps  (H) Protector tubes  (Hi) Rod gaps  (iv) Surge diverters or lightning arresters
  21. 21. Expulsion gaps 1. External series gap 2. Upper electrode 3. Ground electrode 4. Fibre tube 5. Hollow space
  22. 22. Protector tube mounting  1 Line conductor on string insulator  2. Series gap  3. Protector tube  4. Ground connection  5. Cross arm  6. Tower body
  23. 23. Surge diverters or lightning arresters
  24. 24. Valve Type Lightning Arrestor
  26. 26. Basic Impulse Insulation Levels
  27. 27. Surge Protection of Rotating Machine