Switching overvoltages may have times-to-crest from 20–5000 ms and time to half value of less than
20 000 ms. They are generally a result of the following:
— Line energization,
— Faults and fault clearing,
— Load rejections, or
— Switching of capacitive or inductive currents.
In general, the time to crest (wavefront) is of more importance since the critical flashover voltage (CFO) is function of the wavefront. The minimum CFO occurs at the critical wavefront (CWF), which in ms is equal to about 50 times the strike distance in meters (m). For a wavefront smaller or greater than the CWF, the CFO increases.
The CFO increases by about 10% when the wavefront is in the order of 1000 ms to 2000 ms, which usually occurs when employing low-side transformer switching. The distribution of switching overvoltages is obtained using a transient program where the breakers are randomly closed or reclosed 200 to 400 times.
These overvoltages are then statistically analyzed to obtain a probability distribution, which approximates the data. Several distribution functions have been used, e.g., Gaussian, extreme-value Weibull. However, a Gaussian or normal distribution is used most frequently.
This distribution is defined by its 2% value, called the statistical switching overvoltage (E2), and by its standard deviations in pu of E2. The standard deviation pu is:
s/E2. To clarify the definition, E2 means that 2% of the switching overvoltages equal or exceed E2. The shape of the distribution may be affected by the surge controlling action of an arrester. Typically the gapless arrester modifies the reflected wave and reduces the voltage stress caused by the return wave.