Ground fault neutralizers

Arc suppression coil or ground fault neutralizer Ground fault factor (GFF) Magnitude of temporary overvoltages Insulation coordination Application of different types of grounding methods (for HT, HV and EHV systems) Important parameters for selecting a ground fault protection scheme... 

When this balance is disturbed, due to either an unbalance in the loads or due to a ground fault, a residual or zero phase sequence voltage in the neutral circuit will appear. When one of the phases in the secondary of a three-phase transformer is open circuited and a three-phase supply is applied to its primary windings, there will appear... 

Ground fault on one phase System neutral grounded... 

Figure 15.5(b) An RVT under ground fault on a 3-p four-wire grounded neutral system... 

Where three CTs for unrestricted or four CTs for restricted ground fault or combined O/C and G/F protections are employed in the protective circuit, the VA burden of the relay is shared by all the CTs in parallel and a normal VA CT may generally suffice. Such is the case in most of the protective schemes discussed in Sections 21.6 and 15.6.6(1), except for those employing only one CT to detect a ground fault condition, such as for a generator protection with a solidly grounded neutral (Figure 21.12). 

When the scheme is required to detect the ground fault as well as the phase faults, a triple-pole relay is used, eaeh pole of which is connected between the shorted terminals of the two same phase CTs and the neutral formed by shorting the other terminals of all the CTs, as shown in Figure 15.22. The setting of all the poles is kept the same. In other words, the sensitivity level remains the same for all types of faults. 

During faults such as during a ground fault in a resonant grounded system or an isolated neutral grounded system. 

Figure 20.10 Delecting a ground fault in an isolated neutral system...

If /g = ground fault current and /g = fault current through the healthy phases due to neutral impedance Zg... 

The more recommended practice is to ground the neutral solidly or through an impedance, commensurate with the requirements of the protective scheme and the fault current limited to a desired level. The terminal equipment and the windings of all the machines may now be designed for a voltage corresponding to the relevant GFF. 

This is not a method of providing an artificial neutral, as in the previous case, but to detect an unbalance or residual voltage (zero sequence voltage) in a three-phase three-wire or a three-phase four-wire ungrounded system. The residual or zero sequence voltage that may appear across the open delta will be the reflection of an unbalance or a ground fault in the system (Figure 20.10). Also refer to. Section 15.4.1 for more details. 

Figure 20.19 In a healthy system the unbalanced current (other than a ground fault or phase to phase and ground fault) will flow through the neutral and not the ground...

From this equaiion one can determine the required value of neutral circuit impedance for a particular level of ground fault current. The external impedance will be Z, less the ground impedance. In HT systems one c in also delermine the likely value of a ground inductor coil to achieve a near-resonance condition, to eliminate the arcing grounds, on the one hand, and facilitate a strike-free extinction of an arc hy the interrupting device, on the other. 

The above problem can also be overcome by impedance grounding rather than solid grounding. It can be a resistance R or inductance L or both, as discussed above. In the present case, if we consider a p.u. resistance / of Just 9% in every neutral, the improved ground fault current... 

The theory of operation of such a protection scheme is based on the prineiple that in a balanced cireuit the phasor sum of currents in the three healthy phases is zero, as illustrated in Figure 21.7, and the current through the grounded neutral is zero. In the event of a ground fault, i.e. when one of the phases becomes grounded, this balance is upset and the out-of-balance current flows through the grounded neutral. A healthy three-phase circuit, how ever. 

The ground fault current may be detected through three or four CTs, one in each phase and the fourth in the neutral circuit (Figures 21.5(a) and (b)). Through the neutral to discriminate the fault, as discussed later. 

Single-or three-phase Single-phase reactors are used in the neutral circuit either to limit the ground fault currents or as arc-suppression coils (Section 20.5). Similarly, three-phase reactors are used for three-pha.se applications. 

These are meant to limit the ground fault current and are used between the neutral of the system and the ground. They are single-phase and may be short-time rated, otherwise they are the same as the current limiting reactors (Figure 27.12). Their ratings can be calculated by... 

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