PART III- Voltage Surges, Over- voltages, Circuit Interrupters and Grounding Practices
Chapter 17. Voltage surges – causes, effects and remedies
• Temporary over-voltages
• Voltage surge or a transient
• Transient stability of overhead lines
• Causes of voltage surges
• Causes of steep rising surges
• Effect of steep-fronted TRVs on the terminal equipment (motor
as the basis)
• Determining the severity of a transient
• Protection of rotating machines from switching surges
• Theory of surge protection (insulation coordination)
• LV surge protection and surge protection devices (SPDs)
Voltage surges are generally a phenomenon of high voltage (HV) power systems and can be considered as the most severe pollutant to the insulation of the power system and the terminal equipment. In this chapter we analyse the likely amplitude and steepness of surges that may arise under different system conditions and the most appropriate insulation co-ordination between the equipment connected on the same system. Insulation co-ordination provides a criterion in selecting the right equipment with a more economical insulation level for different applications and locations. Generally, locations away from the source of voltage surges, i.e. equipment installed in the downstream of a power system is subject to diminishing surge effects. For example, a rotating machine, which may be a motor or a generator, would rarely be subject to a direct lightning stroke as it would seldom be connected on a bus exposed to direct strikes. It is usually connected through a bus or a cable which is fed through a transformer. All these inter-connecting devices would withstand most of the severity of a lightning stroke and it would be only somewhat attenuated and damped surge which the terminal equipment would be subject to.
This concept of diminishing value of voltage surges is a logical parameter to economize on the cost of insulation as far as permissible, without jeopardising the adequacy of protection to the system or the associated equipment. Different equipment installed at different locations on the same power system may thus have varying degree of basic insulation level (BIL), as discussed in Section 18.3. One may notice the variation in BIL from Tables 11.6, 13.2, or 14.1, and 32.1(a), for motors, switchgears and bus systems respectively when installed on the same power system. Similar variations would apply for other equipment also connected on the same system. The aim here is to cover the subject for a proper understanding without going into extensive details.
High over-voltages occur on the healthy phases during a ground fault:
• When the system is grounded through an arc suppression coil and is under-compensated. Whereas the arcing grounds give rise to voltage surges.
• When the system has an isolated neutral.
• When the system is impedance grounded.