Neutral fault

For ungrounded star capacitor units In this case when it causes a line to neutral fault the impedance of the other two phases will limit the fault current to almost three times the rated current. It is still easy to choose HRC fuses so that they would reinain inoperative... 

The burn accidents invariably result from manual excavation when, for example, an operator using a pneumatic drill penetrates a concealed, buried cable with the tool. A short circuit will occur when the bit comes into contact with one or more of the live conductors and the metallic armouring and/or sheath of SNE cables or the CNE conductor where a PME distribution system is in use. The initial phase/earth or phase/neutral fault usually develops and involves the other phases, causing arcing which may emerge as a flame arc and blast from the hole made by the tool, injuring the operator. 

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...

Consider the system shown in Figure 20.12 and introduce some impedance Z in its neutral circuit as shown in Figure 20.13. Now it is possible to vary the magnitude and characteristic of the fault current through the neutral circuit. 

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

Tlte value of / can thus be varied in magnitude and phase displacement to suit a particular location of installation or pi otective scheme by introducing suitable R and /Y into the neutral circuit. When the impedance is inductive, the fault current will also be inductive and will offset the ground capacitive current /". In such a grounding, the main purpose is to offset the fault current as much is possible to immunize the system from the ha/ai ds of an arcing ground. This is achieved by providing an inductor coil, also known as an arc suppression coil, of a suitable value in the neutral circuit. 

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 instant (sub-transient) fault current, /jjgf, through a generator in a symmetrical three-phase system, irrespective of the condition of neutral as defined in Table 13.9 will be... 

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. 

---