Overvoltages can be caused by both external and internal factors in power systems. Switching surges are now the dominant design factor for EHV and UHV systems, while lightning surges are less important. Switching surges are generated by events like energizing lines, load rejection, and fault clearing. They take the form of traveling waves on the lines. Temporary overvoltages can last from cycles to seconds and are caused by events like load rejection, the Ferranti effect, and ground faults. Overvoltages can be controlled by phase-controlled switching, use of resistors, reactors, and draining trapped charges. Surge arresters like zinc oxide varistors protect equipment by conducting current during an overvoltage and limiting

1. Overvoltages

2. Overvoltage
Overvoltage
types
external internal
g
generated by changes
y g
generated by in the operating
atmospheric conditions
disturbances(lightning)
di t b (li ht i )
of the network

3. Internal
overvoltages
l
Switching
g Temporary
p y
o.v. o.v.

4. Switching overvoltages
Switching overvoltages
switching 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. • 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. 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. 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. Energization of unloaded transmission line
of unloaded transmission line
e

11. Temporary overvoltages
Temporary overvoltages
they 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

12. Load rejection
Load rejection

13. Ferranti effect
Ferranti effect

14. Ground Faults
Ground Faults
A 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

15. Travelling wave
Travelling wave

16. For lossless line:

17. 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.

19. Velocity of wave propagation
Velocity of wave propagation
• For the T.L.:
=3x108 m/sec

20. • F th
For the cable:
bl
3x108 m/sec

21. Reflection and refraction of travelling wave
Reflection and refraction of travelling wave

23. Lattice diagram
Lattice diagram

25. Overvoltage protection
Overvoltage protection
The adverse effects of overvoltages on power
Th d ff t f lt
networks 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)

26. Control of switching surges
Control of switching surges
• Resistor switching
• Phase Controlled Closure
Phase‐Controlled Closure
• Use of Shunt Reactors
• Drainage of Trapped Charges

27. 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)

28. 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 )

29. 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

30. 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

31. 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

32. Control of temporary overvoltages
Control 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

33. Overvoltage protection using surge
arreters
Surge 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.

34. 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.

35. Horn gap arresters
Horn gap arresters
• The arc can be easily interrupted

36. 2‐Metal‐oxide surge arresters
• N li
Non linear resistor of the relation:
it f th l ti

37. Adavatages
• very simple construction.
• Rapid operation
Rapid operation
• No arc
• No follow current after surge absence.

38. 3‐ Zinc Oxide Varistors
• (ZnO) varistors are semiconducting ceramics having
highly nonohmic current‐voltage characteristics

39. 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

40. Surge arrester selection
Surge arrester selection
• Protective Level Ratio( Np)
• Earthing Cofficient(EC)

42. • Discharge Current: which the arrester
Discharge Current: which the arrester
material has to discharge without damage to
itself.
itself
• Protective Level.