Temporary overvoltages caused by load rejection are a function of the load rejected, the system topology after disconnection, and the characteristics of the sources, i.e., short-circuit power at the station, speed and voltage regulation of the generators, etc.

These overvoltages are especially important in the case of load rejection at the remote end of a long line due to the Ferranti effect. Primarily, it affects the apparatus at the station connected on the line side of the remote circuit breaker.

A distinction should be made between various system configurations when large loads are rejected. A system with relatively short lines and high short circuit power at terminal stations will have low overvoltages.

A system with long lines and low short circuit power at generating sites will have high overvoltages. In a symmetrical three-phase power system the same relative overvoltages occur phase-ground and phasephase.

The longitudinal temporary overvoltages depend on whether phase opposition is possible. Phase opposition can occur when the voltages on each side of the open switching device are not synchronized.

A description of the degree and duration of phase-ground and longitudinal overvoltages for two types of substations follows:

— System substation: In a moderately extended system, for a full load rejection, the temporary overvoltage is usually less than 1.2 pu. The duration depends on the voltage control operation and may be up to several minutes. In extended systems, the overvoltages may reach 1.5 pu or even more when Ferranti effects or resonance occur.

The duration may be in the order of seconds. The longitudinal overvoltage across a switching device is usually equal to the phase-ground overvoltage unless motors or generators, connected to the rejected side, produce phase opposition.

— Generator station: For a full load rejection, the overvoltage at the substation may reach up to 1.5 pu. The duration may be up to 3 s depending on the generator characteristics and control.

The longitudinal temporary overvoltage is the difference between the phase-ground operating voltage at one terminal and the phase-ground temporary overvoltage on the other terminal. In the case of phase opposition the longitudinal overvoltages could be as high as 2.5 pu. Shunt reactors, static VAR compensators, or special (grouped) arresters can control these overvoltages.

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