1. OVER VOLTAGE
The voltage waves having magnitude more than its normal value.
In other words, sudden rise to an excessive or abnormal value
It remains for a very short duration of time.

2. CAUSES OF OVER VOLTAGES
The over voltages occur due to
Lightning
Switching
Faults
Travelling Waves

3. TYPES OF OVER VOLTAGES
Over voltage is classified into two types.They are
External Over voltages
Internal Over voltages

4. EXTERNAL OVER VOLTAGES
External Over voltages are caused due to
Direct lightning strokes
Indirect lightning strokes

5. INTERNAL OVER VOLTAGES
Internal Over voltages are caused due to
Switching operation
Fault condition
Sudden release of load in the network.

6. DIRECT LIGHTNING STROKES
Here the discharge or the current path is directly from the cloud to the overhead line.
The voltage will be in millions which can cause flash over or puncture of insulators.
This wave may reach to the substation and damage the equipments because of excessive stress produced.

7. INDIRECT LIGHTNING STROKES
These strokes are due to electro statically induced charges on the conductors due to the presence of charged clouds.
Maximum over voltages in the transmission lines are caused by indirect lightning strokes.

8. SWITCHING OPERATION
The switching over voltage occurs during opening and closing EHV AC lines, breaking inductive loads, capacitive loads which damage the electrical equipments.
During resonance, substantial over voltages might develop.

9. FAULT CONDITION
Interruption of the fault current causes over voltages.
The magnitude of voltage depends on the amount of fault current and the speed with which the fault is cleared.
This will damage transformers, cables and motors supplied from the lines.

10. SUDDEN RELEASE OF LOAD
Switching off of the capacitor banks produce over voltages.
Adjustable Speed Drives and Solid State Motor Controllers are quite sensitive to voltage rises.
ASD might shut down the motor due to over voltage beyond maximum tolerance.

11. SURGE DIVERTER AND SURGE ABSORBER

12. SURGE DIVERTER Ideal characterstics:
Should not carry current at normal voltage (breakdown voltage > normal or abnormal fundamental frequency voltage)
At transient voltage the peak value > spark overvoltage
After breakdown , it should be capable to carry discharge current
Diverter interrupt as quickly as possible when transient voltage reduces below breakdown value

13. SURGE DIVERTER OPERATION

14. CLASSIFICATION Station type -can carry 10kA
-voltage range 3.3kV to 123kV
- large power stations and substations
Line type arrester -can carry 5kA
-voltage range 3.3kV to 245kV
- transformers, power stations and substations
Distribution type and secondary type surge arresters:
-can carry 2.5kA and 1.5kA
-voltage range 3.3kV
- distribution transformers

15. TYPES OF ARRESTER Horn gap arrester Rod gap arrester Multigap arrester
Expulsion type arrester
Valve type lightning arrester
Metal oxide lightning arrester

16. HORN GAP ARRESTER

17. ADVANTAGE Doesnot cause SC as surge is over
The follow current is maintain to small value with the help of series resistance

18. DISADVANTAGE Setting of Horn gap should be changed
Time in this type of arrester is slow
Harmful to employ
Due to external factors it not work properly

19. ROD GAP ARRESTER

20. ROD GAP SYSTEM VOLTAGE(kV) ROD GAP(cm) 33 66 132 275 22.66 35.56 66.04
124.46

21. MULTIGAP ARRESTER

22. EXPULSION TYPE ARRESTOR

23. ADVANTAGE Cheaper and install easily
After surge is over the current is interrupt immmediately

24. DISADVANTAGE Cannot protect expensive station equipments
Perform only limited number of operation

25. VALVE TYPE ARRESTER

26. VOLT – AMPERE CHARACTERSTICS

27. TEST ON LIGHTNING ARRESTER
1/50 impulse spark over test
Wave front impulse spark over test
Impulse current withstand test
Switching impulse voltage test
Discharge capability of durability

28. SURGE ABSORBER

29. SURGE ABSORBER & TRAVELLING WAVES

30. SURGE ABSORBER : A Protective device. Main function :
Can reduce the steepness of wavefront of a surge.
Absorb the energy in travelling wave.

31. ELIMINATION OF SURGES :
SURGE DIVERTER :
It divert the surge to the earth.
SURGE ABSORBER :
It absorbs energy contained in the surge.

32. TYPES : Surge absorber using capacitor : using only capacitor.
using resistor and capacitor.
another type :
using parellel combination of inductor and resistor.
Ferranti Surge absorber :
using inductive coil.

33. SURGE ABSORBER USING CAPACITOR:
LINE
CAPACITOR
GROUND

34. USING RESISTOR &CAPACITOR :
LINE
CAPACITOR
RESISTOR
GROUND

35. PARELLEL COMBINATION :
RESISTOR
LINE
INDUCTOR

36. FERRANTI SURGE ABSORBER :
Inductive coil magnetically coupled.
Current induced in the metal sheild.
Absorber is enclosed in a boiler plate tank, and the vaccum is impregnated.
the energy transferred through mutual induction.
It is dissipated in the form of heat in the dissipator.

37. Figure 1:
METAL SHIELD
LINE

38. Figure 2 :
METAL SHIELD
LINE

39. FIELD OF APPLICATIONS:
Placed near the rotating machines.
Across series reactors.

40. TRAVELLING WAVES : WHAT IS IT MEANT BY?
Fault on the power system cause transients that propagate along the transmission lines as waves.
They continue to travel along the power system until they dimnished due to impedance.

41. WAVE FORM 1:

42. WAVE FORM 2 :

43. EFFECTS : Attenuation Distortion Wave shape modification
Discontinuity of operation.

44. IS IT NECESSARY TO AVOID THIS :
Yes it is necessary to nullify it,
So as to preserve the wave shape
Transmit the signal with strength
Fidelity
Without interferance.

45. TRW DETECTION SYSTEM : Capacitive coupling voltage transformer(CCVT)
Fault transient interface unit(FTIU)
GPS reciever.

46. OVERVOLTAGE PROTECTIVE DEVICES
&
ITS SELECTION

47. Protective Devices:
Mitigate surge over voltage and over current of limited duration.
Activated in case of exceeding nominal voltage/current.
Used to detect abnormal conditions & to take corrective actions.
Single component or complex design.
Increasing in need

48. Characteristics ..... Protective devices are usually divided into
over voltage elements.
&
over current elements.

49. Over voltage components:
Two types of over voltage components are
voltage switching components
&
voltage limiting components

50. Types of over voltage protective devices:
Air gap protectors
Surge Protector
Gas discharge tubes
Semi-conductor protective devices
Fuses
Heat coils
Self restraining current limiting devices
Fusible links

51. Air gap protectors Connected between each wire of a line & earth.
Limits the voltage
Inexpensive
Frequent replacement

52. Surge Protector :
A surge protector is designed to protect electrical devices from voltage spikes.
It diverts excess voltage/current to grounding wire.
Limit the voltage supplied to an electric device.

53. Installed In… power distribution panels, process control systems,
communications systems,
And
other heavy-duty industrial systems.

54. Gas discharge tubes: Connected between each wire of a line & earth.
Limiting purpose.
Compact
No need of Frequent replacement

55. Semi-conductor protective devices:
Primary protective devices
&
Secondary protective devices

56. Primary protective devices:
Carbon electrodes
Benefits: elimination of circuit noise
Disadvantage: higher costs
low current handling capability
Gas discharge tubes
Benefits: elimination of spark discharge
Disadvantage: low current handling capability

57. Secondary protective devices:
Depending on location it may designed to operate in level as low as 1V.
Fast-acting
Easily damaged unless co-ordinated correctly

58. Fuses: Connected in series with each wire of a line
Disconnect when excessive current flows
No protection against lightining surges
Manual replacement.

59. Heat coils: Connected in series with each wire of a line
Have some fusible components
Does not give adequate protection
Manual replacement.

60. Self restraining current limiting devices:
Connected in series with each wire of a line
Increase their resistance when over voltage occurs
Operates automatically

61. INSULATION CO-ORDINATION

62. DEFINITION:
The term insulation co-ordination is the correlation of the insulation of electrical equipment and the lines with the characteristics of protective devices such that the insulation of the whole power system is protected from excessive over voltage.

63. In power system various equipments like transformers, circuit breakers ,etc. have a different breakdown voltage and volt-time characteristics.
The insulation for the various protective devices must be properly coordinated.
Mainly used to select the suitable values for different components in a power system.

65. Curve A- volt-time curve for the protective devices
Curve B- volt-time curve for the equipment to be protected
Any insulation having a withstand voltage strength in excess of the insulation strength.

66. VOLT-TIME CURVE:
It shows the relation between the crest flashover voltage and time to flashover for a series of impulse applications.
In this waves are in same shape but different peak values are applied to the insulation.

67. INSULATION LEVEL:
In order to protect the equipment from overvoltage, its necessary to fix the insulation level for the system.
BASIC IMPULSE INSULATION LEVEL(B.I.L) is in terms of withstanding voltages of apparatus and lines.
It is expressed in impulse crest voltage with a standard wave not longer than 1.2/50microsec wave.

68. PROBLEMS IN INSULATION LEVEL:
Selection of a suitable insulation level.
Assurance that the breakdown strength of all insulation in the system will be equal or exceeds the selected level.
Selection of protective device will give the good protection for apparatus.

70. Thank you