Electrical Power II Unit 4-Overvoltages Unit 5-HVDC Unit 6-FACTS

Unit 4-Overvoltages Definition Effects Causes Types Voltage Surge Basic Insulation Level (BILs) Volt-Time curve

Lightning Phenomenon: Discharge of cloud to the ground Lightning Mechanism:

Protection against overvoltages: 1.Shielding Methods  Don’t allow an arc path formation between the line conductor and ground  Protection against external overvoltages due to lightning strikes  Ground wires, Earthing screens, Surge absorbers 2.Non-Shielding Methods  Allow an arc path formation between the line conductor and ground  Protection against internal overvoltages  Can also be used for protection against lightning strikes  Spark gaps, Lightning arresters

 Ground Wires  Earthing Screens  Surge Absorbers  Surge Diverters  Arcing Horn or Horn Gap

Surge Absorbers  Type of shielding method  Absorbs energy contained in a travelling wave  An air core inductor connected in series with line conductor and surrounded by earthed metallic sheet called dissipater  Acts like an air cored transformer with primary as low inductor and secondary as dissipater  For the travelling wave incident on Surge Absorbers the part of energy content is dissipated as heat due to transformer action and eddy currents  The steepness of wave is reduced due to series inductor Line conductor

 Type of Non-shielding method  Bypass the abnormal high voltage to the ground without affecting continuity of supply  Also called as Lightning arrester  Three types 1.Rod gap 2.Expulsion type of lightning arrester 3.Valve type of lightning arrester Surge Diverter

Duties of surge diverters:  Should not pass any current at normal or abnormal (5% more than normal voltage) power frequency voltage  Should breakdown quickly at high frequency voltage  Should protect equipment as well as should discharge surge current without damaging itself  Should not allow current at power frequency to bypass to the ground after the surge discharged  The non-shielding methods are based on insulation breakdown  Breakdown of insulation directly proportional to voltage magnitude and time of application  Also depends upon shape & size of conductors used in protective device

1. Rod gap