Introduction 
Examination of over voltages at the power tool consists of a have a check in their magnitudes, shapes, durations, and frequency of occurrence. The have a check want to be finished in any respect factors alongside the transmission community to which the surges also can furthermore travel. 
Types of Overvoltage 
• The voltage stresses on transmission community insulation are determined to have quite a few Origins. 
• In normal operation AC (or DC) voltages do now now not strain the insulation severely.

• Over voltage stressing a electricity tool can be labeled into 
essential types: 
External overvoltage: generated through atmospheric disturbances of these disturbances, lightning is the most not unusualplace and the most severe. Internal over voltages: generated through changes withinside the strolling conditions of the network. Internal over voltages. 
Lightning Over voltages Lightning is produced in an attempt through nature to hold dynamic balance the various without a doubt charged ionosphere and the negatively charged earth. 
Over fair-weather areas there can be a downward transfer of incredible costs through the global air-earth current. This is then counteracted through thunderstorms, in the course of which incredible costs are transferred upward withinside the form of lightning. During thunderstorms, incredible and horrible costs are separated through the actions of air currents forming ice crystals withinside the better layer of a cloud and rain withinside the lower part.

The cloud turns into negatively charged and has a larger layer of amazing charge at its pinnacle. As the separation of charge proceeds withinside the cloud, the functionality difference a number of the centers of charges ‗will boom and the vertical electric powered powered powered powered place along the cloud moreover will boom. The fashionable functionality difference a number of the 2 fundamental charge centers can also variety from l00 to 1000 MV. Only a part of the complete charge-severa hundred coulombs is released to earth thru lightning; the rest is consumed in inter cloud discharges. The pinnacle of the thundercloud dipole above earth can also reach5 km in tropical regions. 

The Lightning Discharge 
The channel to earth is first set up via a stepped discharge called a frontrunner stroke. The chief is initiated via a breakdown among polarized water droplets on the cloud base because of the excessive electric powered powered powered field, or a discharge some of the poor price mass withinside the decrease cloud and the immoderate high-quality price pocket below it. As the downward chief strategies the earth, an upward chief starts offevolved offevolved to hold from earth earlier than the previous reaches earth. The upward chief joins the downward one at a detail called the placing detail. This is the begin of the pass decrease lower back stroke, which progresses upward like a travelling wave on a transmission line.