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HIGH VOLTAGE ENGINEERING
Faculty Name :S.B.Kumbhar
Department : EEE
UNIT - I
OVER VOLTAGES IN ELECTRICAL
POWER SYSTEMS
3
LIGHTING
 Causes of over voltage due to Lightning
phenomenon
 Charge formation of Lightning
 Rate of Charging of thunder cloud
 Mechanism of lightning strokes
 Characteristics of Lightning strokes
4
LIGHTING
 Factors contributing to good line design
 Protection afforded by ground wires.
 Interaction between lightning and
power system
5
Causes of Lightning
 Lightning phenomenon
- peak discharge in which charge
accumulated in the cloud into
neighbouring cloud or to the ground
 Electrode separation – cloud to cloud or
cloud to ground is about 10 km or more
6
CHARGE FORMATION OF CLOUD
 Positive and negative charges become
separated by heavy air current with ice
crystals in the upper part and rain in the
lower region.
 Charge separation depends on height of
cloud (200 – 10,000m).
 Charge centers at a distance about 300 –
2km
7
CHARGE FORMATION OF CLOUD
 Charge inside the cloud – 1 to 100 C
 Cloud potential – 107 to 108 V
 Gradient within a cloud – 100 V/cm
 Gradient at initial discharge point –
10kV/cm
 Energy at discharge – 250 kWhr
8
CHARGE FORMATION OF CLOUD
9
MECHANISM OF LIGHTNING FLASH
 Pilot streamer and Stepped leader
 Ground streamer and return stroke
 Subsequent strokes
10
PILOT STREAMER AND STEPPED
LEADER
11
GROUND STREAMER AND RETURN
STROKE
12
CHARACTERISTICS OF LIGHTNING
STROKES
 Current-time characteristics
 Time to peak or Rate of rise
 Probability distribution of current and
time
 Wave shapes of lightning voltage and
current
13
LIGHTNING CURRENT
 Short front time - 10µs
 Tail time – several ms.
14
RATE OF RISE
 50% lightning stroke current – greater
than 7.5kA/µs.
 10% lightning strokes current – exceeds
25 kA/µs.
 Stroke current above half value – more
than 30µs.
15
SURGE VOLTAGE
 Maximum surge voltage in transmission
line – 5MV
 Most of the surge voltage is less than
1000 kV on line.
 Front time – 2 to 10 µs
 Tail time – 20 to 100 µs
 Rate of rise of voltage – 1MV/ µs
16
LIGHTNING STROKES
 Direct stroke directly discharges on to
transmission line or line wires
 Induced stroke cloud generates negative
charge at its base, the earth object
develop induced positive charge
17
OVER VOLTAGE DUE TO SWITCHING
SURGES
INTRODUCTION
 In switching, the over voltage thus generated
last for longer durations and therefore are
severe and more dangerous to the system
 The switching over voltages depends on the
normal voltage of the system and hence
increase with increased system voltage
18
ORIGIN OF SWITCHING
SURGES
 Making and breaking of electric circuits
with switchgear may results in abnormal
over voltages in power systems having
large inductances and capacitances.
 over voltages may go as high as 6 times
the normal power frequency voltage.
19
ORIGIN OF SWITCHING
SURGES
 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.
 Switching surges may include high natural
frequencies of the system, a damped normal
frequency voltage component, or restriking and
recovery voltage of the system with successive
reflected waves from terminations.
20
CHARACTERISTICS OF
SWITCHING SURGES
 De-energizing of transmission lines, cables,
shunt capacitor, banks, etc.
 Disconnection of unloaded transformers,
reactors, etc.
 Energization or reclosing of lines and reactive
loads.
 Sudden switching off of loads.
 Short circuit and fault clearances.
 Resonance phenomenon like ferro-resonance,
arcing grounds, etc.
21
CONTROL OF OVERVOLTAGES
DUE TO SWITCHING
 Energization of transmission lines in one or
more steps by inserting resistances and
withdrawing them afterwards.
 Phase controlled closing of circuit breakers.
 Drainage of trapped charges before reclosing
 Use of shunt reactors.
 Limiting switching surges by suitable surge
diverters.
22
PROTECTION AGAINST
OVERVOLTAGS
 Minimizing the lightning overvoltages
are done by suitable line designs,
 Providing guard and ground wires,
 Using surge diverters.
23
PROTECTION AGAINST
OVERVOLTAGS
 Shielding the overhead lines by using
ground wires above the phase wires,
 Using ground rods and counter-poise
wires,
 Including protective devices like
explosion gaps, protector tubes on the
lines, and surge diverters at the line
terminations and substations

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High Voltage Engineering- OVER VOLTAGES IN ELECTRICAL POWER SYSTEMS

  • 1. HIGH VOLTAGE ENGINEERING Faculty Name :S.B.Kumbhar Department : EEE
  • 2. UNIT - I OVER VOLTAGES IN ELECTRICAL POWER SYSTEMS
  • 3. 3 LIGHTING  Causes of over voltage due to Lightning phenomenon  Charge formation of Lightning  Rate of Charging of thunder cloud  Mechanism of lightning strokes  Characteristics of Lightning strokes
  • 4. 4 LIGHTING  Factors contributing to good line design  Protection afforded by ground wires.  Interaction between lightning and power system
  • 5. 5 Causes of Lightning  Lightning phenomenon - peak discharge in which charge accumulated in the cloud into neighbouring cloud or to the ground  Electrode separation – cloud to cloud or cloud to ground is about 10 km or more
  • 6. 6 CHARGE FORMATION OF CLOUD  Positive and negative charges become separated by heavy air current with ice crystals in the upper part and rain in the lower region.  Charge separation depends on height of cloud (200 – 10,000m).  Charge centers at a distance about 300 – 2km
  • 7. 7 CHARGE FORMATION OF CLOUD  Charge inside the cloud – 1 to 100 C  Cloud potential – 107 to 108 V  Gradient within a cloud – 100 V/cm  Gradient at initial discharge point – 10kV/cm  Energy at discharge – 250 kWhr
  • 9. 9 MECHANISM OF LIGHTNING FLASH  Pilot streamer and Stepped leader  Ground streamer and return stroke  Subsequent strokes
  • 10. 10 PILOT STREAMER AND STEPPED LEADER
  • 11. 11 GROUND STREAMER AND RETURN STROKE
  • 12. 12 CHARACTERISTICS OF LIGHTNING STROKES  Current-time characteristics  Time to peak or Rate of rise  Probability distribution of current and time  Wave shapes of lightning voltage and current
  • 13. 13 LIGHTNING CURRENT  Short front time - 10µs  Tail time – several ms.
  • 14. 14 RATE OF RISE  50% lightning stroke current – greater than 7.5kA/µs.  10% lightning strokes current – exceeds 25 kA/µs.  Stroke current above half value – more than 30µs.
  • 15. 15 SURGE VOLTAGE  Maximum surge voltage in transmission line – 5MV  Most of the surge voltage is less than 1000 kV on line.  Front time – 2 to 10 µs  Tail time – 20 to 100 µs  Rate of rise of voltage – 1MV/ µs
  • 16. 16 LIGHTNING STROKES  Direct stroke directly discharges on to transmission line or line wires  Induced stroke cloud generates negative charge at its base, the earth object develop induced positive charge
  • 17. 17 OVER VOLTAGE DUE TO SWITCHING SURGES INTRODUCTION  In switching, the over voltage thus generated last for longer durations and therefore are severe and more dangerous to the system  The switching over voltages depends on the normal voltage of the system and hence increase with increased system voltage
  • 18. 18 ORIGIN OF SWITCHING SURGES  Making and breaking of electric circuits with switchgear may results in abnormal over voltages in power systems having large inductances and capacitances.  over voltages may go as high as 6 times the normal power frequency voltage.
  • 19. 19 ORIGIN OF SWITCHING SURGES  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.  Switching surges may include high natural frequencies of the system, a damped normal frequency voltage component, or restriking and recovery voltage of the system with successive reflected waves from terminations.
  • 20. 20 CHARACTERISTICS OF SWITCHING SURGES  De-energizing of transmission lines, cables, shunt capacitor, banks, etc.  Disconnection of unloaded transformers, reactors, etc.  Energization or reclosing of lines and reactive loads.  Sudden switching off of loads.  Short circuit and fault clearances.  Resonance phenomenon like ferro-resonance, arcing grounds, etc.
  • 21. 21 CONTROL OF OVERVOLTAGES DUE TO SWITCHING  Energization of transmission lines in one or more steps by inserting resistances and withdrawing them afterwards.  Phase controlled closing of circuit breakers.  Drainage of trapped charges before reclosing  Use of shunt reactors.  Limiting switching surges by suitable surge diverters.
  • 22. 22 PROTECTION AGAINST OVERVOLTAGS  Minimizing the lightning overvoltages are done by suitable line designs,  Providing guard and ground wires,  Using surge diverters.
  • 23. 23 PROTECTION AGAINST OVERVOLTAGS  Shielding the overhead lines by using ground wires above the phase wires,  Using ground rods and counter-poise wires,  Including protective devices like explosion gaps, protector tubes on the lines, and surge diverters at the line terminations and substations