SOLUTION

The insulation of modern EHV and UHV lines is designed based on switching voltage.

Switching Overvoltage

Switching overvoltage is characterized by large amplitude, high-frequency oscillation, strong damping, and short duration. It is caused by switching off circuit breakers or system fault, including the overvoltages after normal switching operations, such as the closing of lines, transformers, reactors, and post-fault reclosing of a line, and the overvoltages as a result of tripping, a fault, or fault clearing. Switching overvoltages may have different degrees of effects on the insulation of electrical equipment and protection equipment, primarily depending on their amplitude, wave shape, and duration.

Insulation Co-ordination

• The amplitude of switching overvoltages is a key factor for designing the insulation of EHV and UHV equipment. The insulation coordination procedure is based on the insulation strength characteristics.
• Insulation strength is characterized by a discharge probability function since every applied impulse magnitude is associated with a probability of flashover.
• Switching overvoltage waves in real systems have irregular waveshapes.
• The switching surge overvoltage is thought of two components, one basic wave having a low-frequency component and the other one is the superimposed wave with rapid variations with time.
• The effect of different arresters needs to be studied, since installing arresters is an important way to suppress switching overvoltages in EHV and UHV transmission lines.
• The most common temporary overvoltages occur in the healthy phases of a system during phase-to-earth faults.
• Switching overvoltages appear in the power systems due to switching of load and/or fault currents, and they cannot be avoided.
• Insulation Coordination is a series of steps used to select the dielectric strength of equipment in relation to the operating voltages and transient overvoltages which can appear on the system for which the equipment is intended.
• Thus Insulation coordination is an art to decide the insulation level of the equipment.