Overvoltage - Causes and Protection

Overvoltages occur in a system when the system voltage rises over 110% of the nominal rated voltage.  Overvoltage can be caused by a number of reasons, sudden reduction in loads, switching of transient loads, lightning strikes, failure of control equipment such as voltage regulators, neutral displacement,.  Overvoltage can cause damage to components connected to the power supply and lead to insulation failure, damage to electronic components, heating, flashovers, etc.

Overvoltage relays can be used to identify overvoltages and isolate equipment.  These relays operate when the measured voltage exceeds a predetermined set-point.  The voltage is usually measured using a Potential Transformers.  The details of the ratio of the potential transformer are also entered into the relay.  These relays are usually provided with a time delay.  The time delay can be either instantaneous, fixed time or for IDMT (inverse definite minimum time) curves.

Generally, overvoltage relays are provided with sufficient time delay in order to avoid unwanted trippings due to transients (See article on Transients).

These relays can be used to isolate feeders and other equipment connected to the network.  In the case of generators, these relay also switch off the excitation system to the generators thereby preventing voltage build-up. 

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