GENERATOR OVERVOLTAGE PROTECTION

One type of overvoltage in a generator is that due to transient surges caused by lightning or switching surges. These transients are protected by surge protective devices that are designed for this purpose. Power frequency over-voltages are possible if the generator controls are defective or have inadequate transient response. A defective voltage regulator, for example, can cause the exciter to ramp to its ceiling voltage. If the voltage control is performed manually, a sudden change in load will result in an increase in voltage. The loss of load may cause high voltage on units that are remotely located in the system. This is particularly true of remote hydro units since it may not be possible for the governor to close the wicket gates of large hydro units fast enough to prevent an overvoltage due to loss of load. The result is over-speed, which is associated with overvoltage. This type of overvoltage is not likely on a steam unit, since they have tighter control against over-speed and are designed to limit over-speed to low values.
Steam turbine generator units are not always provided with over-speed or overvoltage protection, but this type of protection is often recommended for hydro units or combustion turbine units. In many cases, the desired protection is provided by the voltage regulating equipment.

If not, it can be provided by overvoltage relays or over-frequency relays. Overvoltage relays should have a time delay and a pickup of about 110% of rated voltage. An instantaneous unit is sometimes provided with a pickup of 130%-150% of rated voltage. Some types are compensated for varying frequency and should be supplied from a voltage transformer that is different from that supplying the voltage regulator. Some of these relays are used to simply insert a large resistance in the exciter field circuit.