Over voltages are caused on power systems due to external and internal influencing factors. The voltage stress caused by over voltage can damage the lines and equipment’s connected to the system. Over voltages arising on a system can be generally classified into two main categories as below:
External Over voltages
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:
a) Direct lightning stroke
b) Electromagnetically induced over voltages due to lightning discharge taking place near the line, called 'side stroke'.
c) Voltages induced due to atmospheric changes along the length of the line.
d) Electrostatically induced voltages due to presence of charged clouds nearby.
e) Electrostatically induced over voltages due to the frictional effects of small particles like dust or dry snow in the atmosphere or due to change in the altitude of the line.
Internal Over voltages
These over voltages are caused by changes in the operating conditions of the power system. These can be divided into two groups as below:
1. Switching over voltages or Transient over operation voltages of high frequency:
This is caused 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. Similarly when the primary side of the transformers or reactors is switched on, over voltage of transient nature occurs.
2. Temporary over voltages:
These are caused when some major load gets disconnected from the long line under normal or steady state condition.
EFFECTS OF OVER VOLTAGES ON POWER SYSTEMS
Over voltage tends to stress the insulation of the electrical equipment’s and likely to cause damage to them when it frequently occurs. Over voltage caused by surges can result in spark over and flash over between phase and ground at the weakest point in the network, breakdown of gaseous/solid/ liquid insulation, failure of transformers and rotating machines.