Switching in Electrical Transmission and Distribution Systems

11 Switching Overvoltages and Their Mitigation

11.1 Overvoltages

Overvoltages, stressing a power system, can generally be classified into two categories regarding their origin:

• External overvoltages, generated by lightning strokes, which are the most common and severe atmospheric disturbances.
• Internal overvoltages, generated by changes in the operating conditions of the network, like switching.

Voltages and overvoltages are classified in IEC 60071-1 [1] according to their shape and duration as follows:

Low-frequency voltages and overvoltages (of constant r.m.s. value):

• Continuous voltages having a power frequency of 50 or 60 Hz and duration of at least 1 h.

• Temporary overvoltages (TOV) are power-frequency and harmonic overvoltages of relatively long duration, from 20 ms to 1 h. They may be undamped or weakly damped. Frequencies in practice are in the range from 10 to 500 Hz. The standardized voltage shape for testing is 48 to 52 Hz with duration of 60 s.

  The most common temporary overvoltages occur on the healthy phases of a system during phase-to-earth faults. Some other well-known events leading to the generation of temporary overvoltages are load rejection, Ferranti rise, resonance and ferroresonance phenomena (see Section 10.6). The temporary overvoltages may lead to overstressing of surge arresters and magnetic saturation of transformers and shunt reactors. Temporary overvoltages are considerably lower than transient overvoltages.

The following means can be used ...