There are different causes of overvoltage in power systems. These causes are mainly classified as follows.
Internal causes
1. Switching surges
2. Arcing ground
3. Insulation failure
4. Resonance
External causes
1. Lightning strokes
The rise in the voltage due to internal causes is very less as compared to the external causes.
The rise in voltage due to internal causes hardly increases its magnitude to twice of the system voltage.
However, the lightning surges (external cause) are very severe since the system voltage may increase to several times that of the normal voltage. Hence the care against the internal causes is taken by providing the proper insulation level.
Protection against the lightning stroke is provided by connecting the surge arrester/ diverter in parallel with the electrically connected equipment in the power system.
Internal Causes :
If the circuit condition is suddenly changed then the oscillations are set up. These oscillations cause an increase in the system voltage about twice that of the normal voltage.
Circuit condition is suddenly changed due to the switching operation of a circuit breaker, insulation failure, arcing grounds, or due to resonance in the circuit.
(1) Switching Operations :
(a) Switching surges :
The overvoltage produced on the power system due to switching operations is known as switching surges.
The switching operation is mainly performed by using circuit breakers under normal as well as abnormal conditions.
When the unloaded line (open), is charged the traveling waves are set up which produces the overvoltage on the line.
When the unloaded line is charged (i.e. connected to supply), a voltage wave is set up. This voltage wave travels along the line when it reaches the terminal PP', it is reflected back to the supply.
The reflected wave has the same polarity as that of the supply voltage. Therefore the voltage doubling effect occurs on the line. If Erms is the supply voltage, then the instantaneous voltage that line will have to withstand will be 2√ 2E.
This overvoltage is of a temporary nature. It is because the line losses attenuate the wave and in a very short time, the line settles down to its normal supply voltage E.
Similarly, if an unloaded line is switched off the line will attain a voltage of 2√ 2E for a moment before settling down to the normal value.
(b) Switching of loaded line :
When a loaded line is suddenly interrupted, then it causes overvoltage across the circuit breaker contacts. The overvoltage may be of magnitude 2Zni.
Where
i= instantaneous value of current.
Zn= Natural impedance of the line.
e.g if Zn 800 ohm
I = 140 A (rms)
and the circuit is interrupted when the current is maximum.
Therefore voltage across the breaker,
= 2 √ 2 x 140 x 800
= 316.8 kV. 1
If Vm is the peak value of voltage in kV the maximum voltage to which the line may be subjected is =(Vm + 316.8 kV).
(c) Current chopping :
It is the phenomenon of current interruption before the natural current zero is reached.
Current chopping mainly occurs in air-blast circuit breakers because they retain the same extinguishing power irrespective of the magnitude of the current to be interrupted.
When breaking low current with such breakers the powerful deionizing effect of air blast causes the current to fall abruptly to zero will before the natural current zero is reached.
The phenomenon is known as current chopping and results in the production of high voltage transient across the contacts of the circuit breaker as discussed below :
Suppose the arc current is i when it is chopped down the zero value as shown by point an in Fig.
As the chop occurs at current i, therefore the energy stored in inductance is Li2/2. This energy will be transferred to the capacitance C charging the latter to a prospective voltage e given by,
Li2/2 = Le2/2
e = i√(L/C) Volts.
For example if L and C are 4 mH and 0.001microFarade respectively a current chop of magnitude 50 A would induce a voltage of
e = i√(L/C) = 50
√(4
x 10-3/0.001 x 10-6) = 100 x 103 volts = 100 kV.
(2) Insulation failure :
The most common cause of insulation failure in a power system is the fault in which earth connection is involved and may cause overvoltage in the system
Suppose a line a potential E is earthed at point. The earthing of the line causes two equal voltages of - E to travel along XQ and XP containing current-E/Zn and+E/Zn, respectively.
Both these currents pass through X to earth so that current to earth is 2E /Zn.
(3) Arcing ground:
The phenomenon of arcing grounds is commonly experienced with ungrounded systems. A temporary fault caused by falling on a branch, lightning surge, etc creates an are between an overhead line and
ground.
The arc extinguishing and restrikes in a repeated, regular manner. the phenomenon is called "arcing ground".
Each line has an inherent distributed capacitance with respect to earth. Consider an earth fault on line B.
The distributed capacitance discharges through the fault when the gap between f and ground breaks down.
The capacitance again gets charged and again discharged such repeated charging and discharging of line to ground capacitance resulting in repeated arcs between line and ground is called arcing ground.
Arcing grounds produce severe voltage oscillations reaching three to four times normal voltage, Secondly, a temporary fault grows into a permanent fault due to arcing grounds.
Resonance :
When the inductive reactance is equal to the capacitive reactance of the circuit then the condition is known as the resonance condition.
The circuit becomes purely resistive under resonance conditions; also the power factor becomes unity.
This resonance causes overvoltage in the system.
The chances of occurrence of resonance conditions in the power system are less since the capacitance offered by the transmission line is less.
(4) Lightening :
An electric discharge between cloud and earth, between the charge centers of the same cloud, is known as lightning.
When clouds are charged to a high potential (either + ve or – ve) with respect to the earth or neighboring cloud and if the voltage gradient set up by these clouds is more than that of dielectric strength of the media between these, then a huge spark occurs between them and is called as lightning stroke.
The lightning phenomenon can be explained by various theories, however, the most accepted one is explained here.
During the uprush of warm moist air from the earth, the friction between the air and the tiny particles of water takes place.
This causes the building up of charges, larger drops become positively charged and the smaller drop becomes negatively charged.
When the drops of water are accumulated they form clouds and hence clouds may possess either a positive or a negative charge, depending upon the charge of drops of water they contain.
The charge on a cloud may become so great that it may discharge to another cloud or to earth and we call this discharge as lightning.
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