The following points highlight the neutral grounding practices at every voltage level of an electric circuit.
1. Generally one-neutral ground is provided at each voltage level. Between generation and distribution there are several voltage levels. One ground is provided at each voltage level [Fig. 12.12 (b)]
2. The grounding is provided at source and not at load end.
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3. Each major bus section is grounded.
4. (a) The neutrals of generators are grounded through a resistance to limit the stator fault current. The value of resistor determines the percentage of generator winding left unprotected. Synchronous motors and synchronous capacitors are provided with reactance grounding.
(b) Where several generators are connected to a common neutral bus, the bus is connected to ground through a single grounding device. Disconnect switches can be used to ground the desired generators to the neutral bus.
(c) Where several generators are operating in parallel, only one generator neutral is earthed. This is done to avoid the interference of zero sequence currents.
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(d) In generating station there is a provision to ground neutral of at least two generators, though only one is used at a time. The other generator neutral is grounded when the first generator is out of service.
(e) When there are one or two supply sources, no switching equipment is used in the grounding circuit.
5. For protection purpose, the neutral point of star side of power transformer is usually grounded.
6. The star-connected secondary sides of protective CTs and PTs are grounded at one point. This ensures stable neutral, proper measurement of voltages and currents, kWh and kVA on secondary side measuring instruments and controls.
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7. For circuits between 3.3 kV and 33 kV resistance or reactance grounding are used. But for low voltages up to 600 V and for high voltages above 33 kV solid or effective grounding is used. Effective grounding limits the voltages of healthy phases to line-to-neutral values in the event of ground faults and also eliminates the arcing grounds. The effective neutral grounding causes ground fault currents of high magnitudes but the modern protection systems are very sensitive and fast operating so that the faults are cleared in very short times.