When the voltage goes higher than the rated voltage is called Over Voltage. It is a very common issue in Power systems. System voltage is to be maintained as per the designed voltage for the stability of the power system. All the equipment and insulators used in substations and transmission lines are designed for a rated voltage, due to over voltage, it can be damaged.
The causes of overvoltage in power systems are the following two types.
Over voltage due to external causes
External overvoltage happens mainly for lighting and the atmosphere. Over voltage due to external causes is not predictable as it depends on the external source. However, the over-voltage of the equipment is designed in a way that can withstand the external overvoltage. Following external over voltage are observed in power system transmission line.
- Lightning: Lightning over voltage is a very serious issue in a power system. The height magnitude of surge over voltage was found due to the lightning. It can damage the connected equipment in the substation. To protect the substation equipment, lightning surge arresters are used in substations and transmission lines to protect the system.
- Atmosphere: Electrostatically induced voltage can generate due to the presence of charged cloud around the long transmission line. Due to changes in weather conditions, the capacitance with the ground can increase and it will create over-voltage during the switch of the transmission line. Over voltage due to the atmosphere may not be as high as the lightning over voltage.
Over voltage due to internal causes
Internal over voltage generates due to the internal operation mood of the system and fault conditions in the power system. Inter over voltage can generate at power frequency, resonance frequency and at high frequency as transient over voltage.
Followings are the causes of internal overvoltage.
- Switching over voltage: switching over voltage is commonly observed in long-length transmission line. Due to high ground capacitance in long length transmission line, over-voltage was observed in the transmission line. A line reactor installs in the transmission line to control the switching over voltage.
- Resonances Effects: When inductive reactance and capacitive reactance are equal in the power system, the resonance occurs. Du to the resonance of the circuit, the over voltage found in the system.
- Restriking or Transient Over Voltage: During the opening of the circuit breaker in at load or fault condition, transient over voltage is found across the CB contacts. It will heavily affect the circuit breaker and due to restriking voltage, the circuit breaker can damage.