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Overvoltage protection

Overvoltage protection

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Overvoltages due to lightning have their cause external to the system, therefore they are often referred to as external overvoltages.The mechanism of the lightning discharge has been studied with rotating cameras, it illustrates a typical record by Schonland obtained with such a camera. It has been found that the discharge is normally initiated from the cloud at a point of high electric stress, and in the majority of cases, a negative charge from the cloud proceeds towards the ground in a series of jerks of 'steps'. This is the 'leader stroke'. When the leader stroke approaches the ground, high electric stresses develop above the ground and an upward streamer of positive charges develops from the ground.

Chapter Contents:

  • 11.1 Overvoltage phenomena in power systems
  • 11.1.1 External overvoltages (lightning)
  • 11.1.2 Internal overvoltages
  • 11.2 Travelling waves
  • 11.2.1 Wave propagation along a transmission line without losses
  • 11.2.2 Reflections at the end of the line
  • 11.2.3 Discontinuities in surge impedance and junctions with infinitely long lines
  • 11.2.4 Effect of waveshape and of finite length of lines
  • 11.3 Insulation co-ordination
  • 11.3.1 Fundamental principles of surge protection and insulation co-ordination
  • 11.3.2 Basic requirements
  • 11.3.3 Insulation and protective levels
  • 11.3.4 Relation between overvoltage tests and service conditions
  • 11.3.5 Practical choice of insulation levels
  • 11.4 Protection against external overvoltages
  • 11.4.1 Shielding of overhead lines and substations
  • 11.4.2 Surge protection by effective system layout
  • 11.4.3 Voltage limiting devices
  • 11.5 Protection against internal overvoltages
  • 11.5.1 Protection against switching transients
  • 11.5.2 Protection against sustained internal overvoltages
  • 11.5.3 Protection against internal temporary overvoltages
  • 11.6 Practical aspects and some special problems of insulation co-ordination and surge protection
  • 11.6.1 Effect of system neutral earthing on insulation requirements
  • 11.6.2 Choice of surge arresters and derivation of basic impulse insulation levels
  • 11.6.3 Clearances to earth between phases and across isolating gaps
  • 11.6.4 Standard insulation levels, clearances with recommended co-ordinating gap settings, or surge arrester ratings, or both
  • 11.6.5 Effect of rain, humidity and atmospheric pollution
  • 11.7 Probabilistic or statistical approach in insulation co-ordination
  • 11.7.1 Statistical aspects of overvoltages and insulation strength
  • 11.7.2 Application of statistical distribution to insulation co-ordination
  • 11.8 Economic aspects
  • 11.9 Bibliography

Inspec keywords: overvoltage protection; lightning protection; discharges (electric); earthing

Other keywords: positive charges; overvoltage protection; electric stress; rotating cameras; grounding; leader stroke; lightning discharge

Subjects: Power system protection

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