Over voltages or surges in the power system are the abrupt rise in the voltage level in the system. There could be several reasons for over voltage. The normal operating voltage of the system do not stress the insulation severely. But the voltage stresses due to over voltages can be so high that they may become dangerous to both the cables and the connected equipment and may cause damage, unless some protective measure against over voltages are taken. Over voltages occurrence in the system can be categorized by reasons:
1. External over voltages:
These over voltage originate from the atmospheric disturbances, mainly due to lightning. These over voltages take the form of a unidirectional impulse whose maximum possible amplitude has no direct relation with the operating voltage of the system. They may be due to any one of the following causes.
a. Direct lightning stroke
b. Electromagnetically induced voltages due to lightning discharge near the line
c. Voltage induced due to the changing atmospheric condition along the transmission line
d. Electrostatic ally induced over voltages due to presence of the charge clouds
e. Due to friction of the charged particles like dust, snow in the atmosphere or due to change in the altitude of the line.
2. Internal over voltages:
Caused due to changes in operating conditions of the network, further classifieds into two groups.
a. Switching or transient over voltages
The over voltages are generally of oscillatory nature caused by transient phenomena which appears when the state of network is changed by switching operations or fault condition.
The frequency of oscillation is governed by the inherent inductance and capacitance of the system and may very from few hundres Hz to few kHz
b. Steady state or temporary over voltages
These are over voltages developed due to the disconnection of loads at power frequency.