Book at a Glance

PART III- Voltage Surges, Over- voltages, Circuit Interrupters and Grounding Practices

Chapter 20. Temporary over-voltages and system grounding

• Theory of over voltages
Analysis of ungrounded and grounded systems
The necessity of grounding an electrical system
Analysis of a grounded system
Arc suppression coil or ground fault neutralizer
Ground fault factor (GFF)
Magnitude of temporary over-voltages
Insulation coordination
Application of different types of grounding methods (for MV, HV and EHV systems)
Important parameters for selecting a ground fault protection scheme

Theory of over-voltages
A three-phase balanced system has all the three phasors of voltage and current 120° apart, as illustrated in Figure 20.1(a) for a conventional anti-clockwise rotation. These phasors are known as positive sequence components. During a fault, this balance is disturbed and the system becomes unbalanced being composed of two balanced components, one positive and the other negative sequence (Figures 20.1(a) and (b)). For a description of the effects of these components, refer to (Section 12.2(v)). During a ground fault, zero phase sequence components also appear, which are single phasor components and combine three equal phasors in phase, as shown in Figure 20.1(c). This is the residual voltage, Vg, that appears across the ground circuit, i.e. between the neutral and the ground as illustrated in Figure 20.12. This voltage is responsible for a fault current, Ig · Ig will flow through the grounded neutral when it is a three-phase four-wire neutral grounded system, as shown in Figure 20.12. It will also flow through a three-phase three-wire artificially grounded system when it is grounded through a neutral grounding transformer (Section 20.9) as illustrated in Figures 20.17 and 20.18. In a three-phase three-wire system, which has neither its own grounded neutral nor an artificially created grounded neutral, there will be no direct ground fault current. But charging currents through the ground leakage capacitances, particularly on an HV system, may still exist, as illustrated in Figures 20.2–20.4.

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