Over Voltage In Electrical Power System
POWER SYSTEMS ENGINEERING AND ANALYSIS
K .M.NARMADA RANAWEERA
MSc(Spain) BSc. Eng.(Hons) (University Of Ruhuna) AMIESL
Over Voltage In Electrical Power System
 An overvoltage is a voltage that exceeds the maximum
value of operating voltage in an electric circuit.
 Increase in voltage for a very short time in power system
is called as the over voltage. it is also known as the voltage
surge or voltage transients. The voltage stress caused by
over voltage can damage the lines and equipment’s
connected to the system
Over Voltage In Electrical Power System
Causes of over voltage in Power System
 There are two types of causes of over voltage in power system.
1. Over voltage due to external causes
2. Over voltage due to internal causes
 Transient over voltages can be generated at high frequency (load switching and
lightning), medium frequency (capacitor energizing), or low frequency.
Over voltage due to external causes
This type of over voltages originates from atmospheric
disturbances, mainly due to lightning. This takes the form of a
surge and has no direct relationship with the operating voltage of
the line.
It may be due to any of the following causes:
• Direct lightning stroke
• Electromagnetically induced over voltages due to lightning
discharge taking place near the line, called 'side stroke'.
• Voltages induced due to atmospheric changes along the length of
the line.
• Electrostatically induced voltages due to presence of charged
clouds nearby
• Electrostatically induced over voltages due to the frictional effects
of small particles like dust or dry snow in the atmosphere
Over voltage due to internal causes
These over voltages are caused by changes in the operating conditions of the power
system. These can be divided into two groups as below:
 Switching over voltages or Transient over operation voltages of high frequency
• when switching operation is carried out under normal conditions or when fault occurs in the
network.
• when an unloaded long line is charged, due to Ferranti Effect the receiving end voltage is
increased considerably resulting in over voltage in the system.
• when the primary side of the transformers or reactors is switched on, over voltage of
transient nature occurs.
 Temporary over voltages
These are caused when some major load gets disconnected from the long line under normal or
steady state condition.
The waveform of a typical switching surge
Overvoltage Protection
Methods of Protection Against Lightning
There are mainly three methods generally used for protection against
lightning. They are,
• Earthing screen.
• Overhead earth wire.
• Lightning arrester or surge dividers
Overvoltage Protection
 Earthing Screen
Earthing screen is generally used over electrical substation. In
this arrangement a net of GI wire is mounted over the substation. The GI wires, used for earthing screen are properly
grounded through different sub-station structures. This,
provides very low resistance path to the ground for lightning
strokes.
This method of high voltage protection is very simple and
economic, but the main drawback is, it can not protect the
system from travelling wave which may reach to the substation via different feeders.
Overvoltage Protection
 Overhead Earth Wire
Overhead earth wire is placed over electrical
transmission network. One or two stranded GI wires of
suitable cross-section are placed over the transmission
conductors. These GI wires are properly grounded at
each transmission tower.
These overhead ground wires or earth wire divert all the
lightning strokes to the ground instead of allowing them
to strike directly on the transmission conductors.
Overvoltage Protection
 Lightning Arrester
The lightning arrester is a devices which provides
very low impedance path to the ground for high
voltage travelling waves.
This device behaves like a nonlinear electrical
resistance. The resistance decreases as voltage
increases and vice-versa, after a certain level of
voltage.
Insulation Coordination
 The maximum amplitude of transient over voltages reach the components, can be limited by
using protecting device like lightning arrestors in the system. If we maintain the insulation level
of all the power system component above the protection level of protective device, then ideally
there will be no chance of breakdown of insulation of any component.
 Generally, the insulation level is established at 15 to 25 % above the protective level voltage of
protective devices.
Insulation Coordination
 Nominal System Voltage
Nominal System Voltage is the phase to phase voltage of the system for which the system is
normally designed. Such as 11 KV, 33 KV, 132 KV, 220 KV, 400 KV systems.
 Maximum System Voltage
Maximum System Voltage is the maximum allowable power frequency voltage which can
occurs may be for long time during no load or low load condition of the power system. It is
also measured in phase to phase manner.
 Protection Level Voltage of Protective Device
The protection level of over voltage protective device is the highest peak voltage value
which should not be exceeded at the terminals of over voltage protective device when
switching impulses and lightening impulses are applied
Transmission Line Transient Over voltages
(Travelling waves of a power system)
Open-Circuited Line
Let a source of constant voltage E be switched suddenly
on a line open-circuited at the far end.
Neglecting the effect of line resistance and possible
conductance to earth, a rectangular voltage wave of
amplitude E and its associated current wave of amplitude
I = E/Zc will travel with velocity v towards the open end.
Travelling waves of a power system
Open-Circuited Line
At the open end, the current must fall to zero, and
consequently the energy stored in the magnetic field must
be dissipated in some way. This energy can only be used in
the production of an equal amount of electrostatic field
(resistance and conductance have been neglected).
Therefore, the voltage at the point will be increased
by an amount E such that the energy lost by the
electromagnetic field (0.5 LI2) is equal to the
energy gained by the electrostatic field (0.5CV2)
Hence, the total voltage at the open end becomes 2E.
Travelling waves of a power system
Open-Circuited Line
At sending end, the voltage is held by the source to the value E,
it follows that there must be a reflected voltage of –E and
associated with it there will be a current wave of –I.
When these reach the open end the conditions along the line
will be voltage E and current –I.
The reflected waves due to these will be –E and +I (Energy
conservation). When these have travelled to the sending end
they will have wiped out both voltage and current
distributions, leaving the line for an instant in its original state.
The above cycle is then repeated.
Travelling waves of a power system
Short-Circuited Line
The voltage at the far end of the line must be zero, so that as
each element of the voltage wave arrives at the end there is a
conversion of electrostatic energy into electromagnetic energy.
Hence, the voltage is reflected with reversal sign while the
current is reflected without any change of sign.
Theoretically, the current will eventually become infinite as is
to be expected in the case of a lossless line. In practice, the
resistance of the line produces attenuation so that the
amplitude of each wave-front gradually diminishes as it travels
along the line and the ultimate effect of an infinite number of
reflections is to give the steady Ohm’s law of current E/R.
Travelling waves of a power system
Junction of Lines of Different Characteristic Impedance
Travelling waves of a power system
Since the reflection is accompanied by a change in sign of either voltage or current but not both:
The voltage entering the second line at any instant will be the algebraic sum of the incident and reflected
voltages in the first line.
The difference between the incident current I and the current I’’ transmitted into the second circuit is the
reflected current I’ or
Travelling waves of a power system
The Bewley Lattice Diagram
This is a diagram which shows at a glance the position and direction of motion of every incident, reflected
and transmitted wave on the system at every instant of time.
Example 1:
consider the case of an open-circuited line having the following parameters:
𝑅 = 0.5Ω 𝑝𝑒𝑟 𝑘𝑚, 𝐺 = 10 × 10−7 𝑆 𝑝𝑒𝑟 𝑘𝑚, 𝑙 = 400𝑘𝑚
if a wave of amplitude A at any point of the line, the amplitude Ax at some point distant x from the original point
is,
𝐴𝑥 = 𝐴𝑒 −α𝑥
For the distortionless line, the attenuation constant is given by
α = 𝑅𝐺
α = 𝑅𝐺 =
0.5 × 10 × 10−7 = 0.000707
When 𝑥 = 𝑙 = 400𝑘𝑚, 𝑒 −0.2828 = 0.7536
The Bewley Lattice Diagram
 At the sending end, reflection co-efficient Г,
 At the receiving end, the line is open-circuited and Г = +1
The Bewley Lattice Diagram
• The voltage at the receiving end at time t =
0.7536+0.7536
• The voltage at the receiving end at time 3t =
0.7536+0.7536-0.428-0.428
• The final voltage at the receiving end is the sum to
infinity of all such increments
2(0.7536-0.428+….)