2. Transient over –voltage
Lightning over voltage
Due to lightning surges
Switching over voltage
Sudden change in circuit
condition
Switching
Resonance
Arcing grounds
3. Causes of Over - Voltages
Phenomena causes effect
lightning Discharge of clouds and
breakdown of air
Line insulators
flashover or puncture
switching Breaking inductive circuit
rises the potential across
capacitor
Switching on capacitive line
Circuit breaker
voltage stressed .
Restrike is possible
Transmission line
insulation is stressed
Arcing ground The capacitance line to
ground gets discharged
through earth fault
Temporary fault grows
into permanent fault
Resonance The fault causing resonance
between capacitance and
inductance in part of the
circuit
Very high surge
voltage occcur
5. What is lightning?
The large spark accompanied by light
produced by an abrupt,
discontinuous discharge of electricity
through the air from the cloud under
turbulent condition
of atmosphere is called lightning
6.
7. Some figures
Voltage : 2x108 volts, (200 MV)
Current 4x44 A
Duration : 10-5 seconds
Power : 8x10 9 kW
Energy : 22 kWh
Lightning cannot be prevented, but it can with some
success be intercepted and its current conducted to
grounding system without side flashes
10. Direct flash (A stroke): The current can be
considered as an ideal current source. The
resulting Over voltage therefore depends on
the impedance seen by the current source. The
impedance is the surge impedance of the line
(200-400 ohm). Then a very high voltage
appears
Near flash (B stroke) : Due to change in electro-
magnetic field a voltage is induced the line. Ex:
in high voltage line with 10 m high for lightning
current of 30 kA, the induced voltage is in order
of 100 kV for a flash at 100 m distance
11. How does it effects?
The lightning discharge causes
Thermal
Mechanical
Electrical
Effects to the system
Line insulators flashover or puncture. The travelling wave
reaches the substations and generating stations. The
insulation of equipment and machines stressed
12. Cont.
Travelling wave: A lightning stroke terminating on a
power system initiates wave that propagate within the
system. To determine the resulting surge voltage and
current a travelling wave analysis is required
Surge voltage: In general surge voltage and surge
current should be considered. The current exists only
microseconds. The surge voltage exceeds the insulation
strength a flashover occurs
13. Current and Voltage waves
A very steep front and rapid damping are
the characteristic of lightning
phenomena
14. Raising of earthing potential
When lightning current flows through the ground it affects to the
Earthing poential of electrical installtions
16. Raise of eathing potential
V=0.2 Iρ/D
Where :
I: lightning current
ρ : soil resitivity
D : distance
If I=20 kA, ρ=1000 ohm/m
For D=100 m V=40 kV
D=50 m V=80 kV
19. Rod gap
Limitations
a. After the surge is over short circuit may occur
b. The rods may melt or get damaged due to excessive heat produced
by the arc.
c. The climatic conditions (e.g. rain, humidity, temperature etc.)
affect the performance of rod gap arrester.
d. The polarity of the f the surge also affects the performance of this
arrester.
e. Due to the above limitations, the rod gap arrester is only used as a
back-up protection in case of main arresters.
20. Horn gap
The gap between horns is less at the bottom and largest
at the top. The arc is produced at the bottom during
high voltage. It moves up due to electromagnetic field
action and heat.
21. Arresters
Lightning arrester is connected between line and earth.
Main elements are non-linear resistor unit and gap unit.
At power frequency the resistor unit offers a very high
resistance and no current flows through the ground
For discharge current the resistance is low and
breakdown occurs the gap.
22.
23.
24. Shielding
Shielding masts are commonly used in substations, and
overhead ground wires are used in both substations and
on transmission lines
25. Use of earth wire
The earth wire is at the top of the OH line and at every
and each tower the ground wire is earthed. The
negatively charged strokes are attracted by this wire.
Without ground wire the stroke hits the live conductors
which bring the disaster.
It does not provide 100% protection against lightning
Shielding angle of 35 % can be considered as satisfactory
26. Insulation co-ordination
The correlation of the insulation of the equipment and
circuit with the characteristic of protective devices so
that the insulation is protected from the over voltages
Normal voltage: r.m.s line to line voltage by which the
system is designed
Highest voltage: highest r.m.s. line to line voltage which
can be sustained under normal operating conditions
27. Cont.
Insulation level : combination of voltage values both
power frequency and impulse which characterize the
insulation of that equipment with regard to capability
of withstanding dielectric stress.
28. Coordinating the
characteristic
Lightning arrester should have the lowest spark voltage.
The residual voltage should be less than transformer
insulation strength
The voltage time curve of individual components must
be taken into account
The insulation co-ordination should be based on worst
atmospheric conditions
