Overvoltages 1


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Overvoltages 1

  1. 1. Overvoltages
  2. 2. Overvoltage  Overvoltage types external internal g generated by changes  y g generated by  in the operating  atmospheric  conditionsdisturbances(lightning)di t b (li ht i ) of the network
  3. 3. Internal  overvoltages lSwitching  g Temporary  p y o.v. o.v.
  4. 4. Switching overvoltages Switching overvoltagesswitching surges have become the governing factor in the design of insulation for EHV and UHV systems In the meantime lightning overvoltages come as a and UHV systems. In the meantime, lightning overvoltages come as a secondary factor in these networks for two reasons:• Overvoltages produced on transmission lines by produced on transmission lines by  lightning strokes are only slightly dependent on  the power system voltages. As a result, their  the power system voltages. As a result, their magnitudes relative to the system peak voltage  decrease as the latter is increased decrease as the latter is increased• external insulation has its lowest breakdown  strength under surges whose fronts fall in the  strength under surges whose fronts fall in the range 50‐500 µS, which is typical for switching  surges
  5. 5. • According to the IEC recommendations, all equipment designed for operating voltages above 300 k should b tested under switching impulse. kV h ld be d d h l
  6. 6. Origin of switching overvoltages Origin of switching overvoltages• Energization of transmission lines and cables.  Specially: – Energization of a line that is open circuited at the  far end – Energization of a line that is terminated by an  unloaded transformer – Energization of a line through the low‐voltage side  of a transformer• Re‐energization of a line. Specially when high‐ speed reclosures are used. speed reclosures are used.
  7. 7. Origin of switching overvoltages cont. Origin of switching overvoltages cont• Load rejection.• Fault initiation and clearing. Fault initiation and clearing.• witching on and off of equipment. Particularly: – Switching of high‐voltage reactors – Switching of transformers that are loaded by a  reactor on their tertiary winding. – Switching of a transformer at no load Switching of a transformer at no load
  8. 8. Energization of unloaded transmission line of unloaded transmission line e
  9. 9. Temporary overvoltages Temporary overvoltagesthey last for long durations, typically from a few cycles to a few seconds. They take the form of undamped or slightly damped oscillations at a frequency equal or close to the power frequency. Some of the most important origins are:• Load rejection• Ferranti effect• Ground faults Ground faults
  10. 10. Load rejectionLoad rejection
  11. 11. Ferranti effectFerranti effect
  12. 12. Ground Faults Ground FaultsA single li t i l line‐to‐ground f lt will cause th voltages d fault ill the lt to ground of the healthy phases to rise. In the case of a line‐to‐ground fault systems with neutrals line to ground fault, isolated or grounded through a high impedance may develop overvoltages on healthy phases y p g y p higher than normal line‐to‐line voltages. Solidly g rounded systems, on the other hand, will only permit phase‐to‐ground overvoltages well b l it h t d lt ll below the line‐to‐line value. An earth fault factor is defined as the ratio of the higher of the two sound phase voltages to the line‐to‐neutral voltage at the same point in the system with the fault removed. p y
  13. 13. Travelling waveTravelling wave
  14. 14. For lossless line:
  15. 15. Surge impedance(Z Surge impedance(Z0)• The surge impedance is clearly independent of The surge impedance is clearly independent of  the line length. In practice, it is about 300‐400  ohm for overhead transmission lines and  h f h d i i li d about 30‐80 ohm for underground cables.
  16. 16. Velocity of wave propagation Velocity of wave propagation• For the T.L.: =3x108  m/sec
  17. 17. • F th For the cable: bl 3x108  m/sec
  18. 18. Reflection and refraction of travelling waveReflection and refraction of travelling wave
  19. 19. Lattice diagramLattice diagram
  20. 20. Overvoltage protection Overvoltage protectionThe adverse effects of overvoltages on power Th d ff t f ltnetworks can be reduced in two ways:• by using protective device(surge arresters)• Reducing their magnitudes wherever the  surge originates(overvoltage control) surge originates(overvoltage control)
  21. 21. Control of switching surges Control of switching surges• Resistor switching• Phase Controlled Closure Phase‐Controlled Closure• Use of Shunt Reactors• Drainage of Trapped Charges
  22. 22. Resistor switching• At the time of energization, the main breaker  is open while the auxiliary breaker closes. The  i hil th ili b k l Th voltage impressed at the line entrance is thus  Ve =e(t).Z0/(R+Z0)
  23. 23. The value of resistance R in general depends on a large number of factors as follows:• The value of R is selected to achieve optimum  f y results for the system.• The surge impedance of connected lines when  there is a single line or multiple lines. there is a single line or multiple lines• The insertion time of the resistance controls  the overvoltage.(normally ½ cycle).• The value of resistance is slightly higher than The value of resistance is slightly higher than  the surge impedance of a single line which is  switched.(normally 400 ohm) it h d ( ll 400 h )
  24. 24. Phase controlled closure Phase controlled closure• By properly timing of the closing of the circuit breaker poles, the resulting switching p g g overvoltage can be greatly reduced. Phase‐ controlled switching should be carried out successively for the three poles to accomplish a reduction in the initial voltages on all three phases. This is extremely difficult with conventional circuit breakers but is quite p possible with solid‐state circuit breakers
  25. 25. Use of Shunt Reactors Use of Shunt Reactors• Shunt reactors are used on many high‐voltage high voltage transmission lines as a means of shunt compensation to improve the performance of the line, which would otherwise draw large capacitive currents from the supply. They have the additional advantage of reducing g g energization surge magnitudes. This is accomplished mainly by the reduction in temporary overvoltage
  26. 26. Drainage of Trapped Charges Drainage of Trapped Charges• Charges are trapped on the capacitance to Charges are trapped on the capacitance to  ground of transmission lines after their  sudden reenergization. If the line is  dd i i If h li i reenergized soon after, usually by means of  g , y y automatic reclosures, these charges may  cause an increase in the resulting surge. In  i i th lti I practice, trapped charges may be partially  drained through the switching resistors  incorporated in circuit breakers incorporated in circuit breakers
  27. 27. Control of temporary overvoltagesControl of temporary overvoltages = • As seen in the above equation, the voltage can be  reduced by increasing capacitive reactance. a  y g p shunt reactor of reactance Xr is added to the  transmission line, the equivalent input reactance  , q p of that line will be increased from Xc to
  28. 28. Overvoltage protection using surge  arretersSurge Protective Devices should: •Remain inactive while the volage is normal •Activate rapidly when the surge is detected •Activate rapidly when the surge is detected •Be able to withstand the associated current •Derivate current to the earth termination •Reduce the surge to a non‐hazardous level R t t i ti it th di •Return to inactivity once the surge disappears.
  29. 29. 1‐spark gap arresters Drawbacks• the time lag that occurs before the gap sparks over the time lag that occurs before the gap sparks over• the variation of the sparkover voltage with the polarity  and surrounding condition and surrounding condition• The current continues even after the overvoltage has  disappeared, causing a line‐to‐ground short circuit on  disappeared causing a line to ground short circuit on the network.
  30. 30. Horn gap arresters Horn gap arresters• The arc can be easily interrupted
  31. 31. 2‐Metal‐oxide surge arresters• N li Non linear resistor of the relation: it f th l ti
  32. 32. Adavatages• very simple construction.• Rapid operation Rapid operation• No arc• No follow current after surge absence.
  33. 33. 3‐ Zinc Oxide Varistors• (ZnO) varistors are semiconducting ceramics having  highly nonohmic current‐voltage characteristics
  34. 34. Properties p• The resistivity of a ZnO varistor is very high  (more than 1010 ohm.cm) below a certain  ( ) threshold voltage (Vtb), whereas it is very low  ( (less than several ohm.cm) above the threshold  ) voltage.• below the threshold voltage ZnO varistors are below the threshold voltage, ZnO are  highly capacitive. The dielectric constant of ZnO is 8.5, whereas an apparent dielectric constant  is 8 5 whereas an apparent dielectric constant of a ZnO varistor is typically 1000.• T i l values of ZnO varistors are from 30 to  Typical α l fZ O i t f 30 t 100
  35. 35. Surge arrester selection Surge arrester selection• Protective Level Ratio( Np) • Earthing Cofficient(EC)
  36. 36. • Discharge Current: which the arrester Discharge Current: which the arrester  material has to discharge without damage to  itself. itself• Protective Level.
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