overvoltages in electrical power system

Overvoltages which appear on electrical power system can be broadly described as internal and external.

Internal overvoltages:

These type of over voltages originate in the system itself and may be transient, dynamic and stationary type.

Transient Over Voltages: Over voltages of transient nature will have frequency un related to the normal system frequency and will persist for few cycles only. They can be caused by the operation of the circuit breakers when switching inductive and capacitive loads.

Dynamic Over Voltages: This over voltages occur at normal frequency and persist for only few seconds. This may be caused by the disconnection of the generator with overspeeds, or when large part of the load is suddenly removed.

Stationary Over Voltages: This occurs at system frequency but persists for some time ranges from seconds to hours. Condition such as earth fault on one line is sustained when the neutral is connected to the earth through the arc suppression coil then the voltage levels on the other two healthy phases rises.

The over voltages of above types rarely exceed three to five times the normal phase to neutral peak voltage o the system. Apparatus are to be designed in a manner to provide adequate insulation to withstand these over voltages

External over voltages:

These over voltages are produced by the atmospheric discharges such as static charges and lightning strikes. These have the magnitude to cause the considerable stress on the insulation of the equipment ( motors, generators, transformers,etc..). To avoid the break down, the insulation strength of the equipment to be protected ( transformers, motors, generators) should be greater than the over voltage protection equipments such as surge arresters and surge divertors. In the present day practice high voltage switchgear is designed to with stand the impulse voltage which is generated due to lightning strikes and other factors.

However it should be considered that while designing the insulation for the EHV lines (greater than 400 kV) switching impulse caused during the switching of the circuits is considered as it is more severe than the lightning impulse at very higher voltages.