Overvoltage is defined as: “Voltage between one phase and ground or between two phases, having a crest value exceeding the corresponding crest of maximum system voltage.” 

Under conditions of load rejection, the supply voltage will increase in magnitude. This situation would normally be rectified by voltage regulating equipment such as AVR’s or on-load tap changers. However, failure of this equipment to bring the system voltage back within prescribed limits leaves the system with an overvoltage condition which must be cleared in order to preserve the life of the system insulation. Hence, overvoltage protection which is suitably time delayed allowing for normal regulator action, may be applied.

During earth fault conditions on a power system there may be an increase in the healthy phase voltages. Ideally, the system should be designed to withstand such Overvoltages for a defined period of time. Normally, there will be a primary protection element employed to detect the earth fault condition and to issue a trip command if the fault is uncleared after a nominal time. However, it would be possible to use an overvoltage element as a back-up protection in this instance. A single stage of protection would be sufficient, having a definite time delay.   

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.

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

This type of Overvoltages 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: 

o   Direct lightning stroke

o   Voltages induced due to atmospheric changes along the length of the line.

o   Electrostatically induced voltages due to presence of charged clouds nearby.

o   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.

These over voltages are caused by changes in the operating conditions of the power system. These can be divided   into two groups as below:

o   Switching over 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. 

o   Temporary over voltages:

These are caused when some major load gets disconnected from the long line under normal or steady state condition.