HIGH VOLTAGE ENGINEERING (HVE)

CHAPTER 4. OVERVOLTAGES: NATURE AND PROTECTION

General characteristic of overvoltages Electrical and power systems are always subjected to overvoltages that have their origin in atmospheric discharges in which case they are called external or lightning overvoltages, or they are generated internally by connecting or disconnecting the system, or due to the systems fault initiation or extinction. The latter type is called internal overvoltages.

This class may be further subdivided into temporary overvoltages, if they are oscillatory of power frequency or harmonics, and switching overvoltages, if they are heavily damped and of short duration. Temporary overvoltages occur almost without exception under no load or very light load conditions. Because of their common origin the temporary and switching overvoltages occur together and their combined effect has to be taken into account in the design of high voltage systems insulation.

Nature of overvoltages Fundamentally, lightning is a manifestation of a very large electric discharge and spark. Several theories have been advanced to explain accumulation of electricity in clouds. The present section reviews briefly the lightning discharge processes.

Typically the negative charge centre may be located anywhere between 500 m and 10 000 m above ground. Lightning discharge to earth is usually initiated at the fringe of a negative charge centre. To the eye a lightning discharge appears as a single luminous discharge, although at times branches of variable intensity may be observed which terminate in midair, while the luminous main channel continues in a zig–zag path to earth.

The return stroke is followed by several strokes at 10 to 300 m/sec intervals. The leader of the second and subsequent strokes is known as the “dart leader” because of its dart–like appearance. The dart leader follows the path of the first stepped leader with a velocity about 10 times faster than the stepped leader. The path is usually not branched and is brightly illuminated.

(a) when the subject is close to a parallel hit or other tall object, A direct hit on a human or animal is rare; they are more at risk from indirect striking, usually: (a) when the subject is close to a parallel hit or other tall object, (b) due to an intense electric field from a stroke which can induce sufficient current to cause death, (c) when lightning terminating on earth sets up high potential gradients over the ground surface in an outwards direction from the point or object struck.

Insulation coordination Electric systems insulation designers have two options available to them: choose insulation levels for components that would withstand all kinds of overvoltages, consider, design and engineering protective devices that could be installed at the sensitive points in the system that would limit overvoltages there.

“Insulation level” is defined by the values of test voltages which the insulation of equipment under test must be able to withstan The “protection level” provided by arresters is established in a similar manner to the “insulation level”; the basic difference is that the insulation of protective devices (arresters) must not withstand the applied voltage.

Protection devices Special protective devices from overvoltages are called arresters. The development of metal oxide arresters (MOA) represented a breakthrough in overvoltage protection devices. It became possible to design arresters without using gaps which were indispensable in the conventional lightning arresters, which utilized non–linear resistors made of silicon carbide (SiC) and spark gaps.

An ideal lightning arrester should: conduct electric current at a certain voltage above the rated voltage; hold the voltage with little change for the duration of overvoltage; substantially cease conduction at very nearly the same voltage at which conduction started. In SiC arrester type the three functions are performed by the combination of the series spark gaps and the SiC elements. In the case of MOA arrester the metal oxide valve elements perform all three functions because of their superior non–linear resistivity.