Distribution System Training

The objective of this module is to look at the causes and effects of excessive overvoltage on the distribution system, with particular emphasis on lightning surges. The features of different types of lightning arrester are presented, including discussion of insulation coordination with relation to insulation withstand voltages and the BIL (Basic Insulation Impulse Level).

After completion of this module, the participant should be able to understand the following concepts and apply them in day-to-day work practice.

• The effects of excessive high voltage on the distribution system.
• Causes of excessive overvoltage

1. Capacitor switching
2. Ferroresonance
3. Current chop
4. Lightning surges

• The characteristic of lightning surges including typical values of surge current, voltage, and duration
• The standard current surge waveform
• Line insulator flashover and follow on 60 Hz current
• Function of the lightning arrester and typical installation features
• Features of the silicon carbide lightning arrester
• The need for spark gaps in the silicon carbide arrester
• Arrester voltage characteristics:

1. Spark overvoltage,
2. surge discharge voltage, and
3. Re-seal voltage (rated voltage)

• The characteristics of zinc oxide arresters
• Equipment insulation characteristics
• Insulation withstand voltage
• Basic insulation impulse level (BIL)
• Equipment full wave test and chopped wave test
• The BIL standard waveform
• Selection of arresters
• Considerations of neutral voltage shift under ground fault conditions
• Insulation coordination
• Selection of arresters for URD systems