Switching overvoltages may have times-to-crest from 20–5000 us and time to half value of less than
20 000 us. 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 a function of the wavefront. The minimum CFO occurs at the critical wavefront (CWF), which in us 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 us to 2000 us, 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 theswitching overvoltages equal or exceed E2. The shape of the distribution may be affected by the surge controlling
action of an arrester.