07-04-2009, 11:13 PM
This paper is an overview of the sources and nature of various transient over voltages and the problems that may result in a low voltage ac distribution system. Voltage transients may be repeatable transients or random impulses and they have got remarkable effects on semiconductors, electrochemical contacts, insulation etc. Switching parallel loads on the same branch of distribution system as well as lighting are responsible for these transient overvoltages. Low voltage ac mains system should be protected from these transient overvoltages. For this we have got various surge suppression techniques such as MOVs, SADs, gas tubes, filters and transformers. For the transient overvotlage protection to be effective the impulse energy should be dissipated in the suppressor and the voltage should be held low enough to ensure the survival of the circuit components. The above surge suppression technologies differ in their current handling, energy dissipating and surge transient capabilities and hence they also differ in their efficiency. Comparing the performance of these devices it can be seen that a combination of gas tube surge arrester and metal oxide varistor is more efficient. We have seen that, transients are of two types, repeatable transients and random transients; random transients being more severe. The adverse effects of these transients on semiconductors, electromechanical contacts and insulation also have been discussed. Inorder to suppress the voltage transients we have got various surge suppression technologies. An ideal surge arrester should have the ability to overcome the current handling and energy diverting of the crowbar type device with the speed of the clamping device. Filters usually require require a series resistor to reduce undesirable effects. Isolation transformer blocks common mode voltage efficiently, but it fails to attenuate different mode voltages. In spark gap technology, the carbon block surface may get pitted and also control of breakdown characteristics is difficult.In dc applications,gas tubes are seem to be more advantageous.When used in ac applications, they require a current limiting device in series between the gas tube and the source of follow-on current. SAD has improved VI characteristics, but poor peak current handling capability. MOVs seem to be ideal in low voltage ac power lines.
