Current Through-fault

Through the tap-offs of the bus, the unit auxiliary transformers (UATs) are connected to feed the station auxiliary services. For more clarity we have taken out the portion of the tap-offs from Figure 13.21 and redrawn it in Figure 13.18 to illustrate the above system and its interconnections. The tap-offs are now subject to the cumulative inOuence of the two supply sources. In the event of a fault on this section, both the sources would feed the same and the fault current through the tap-offs would add up. The tap-offs should thus be designed for the cumulative effect of both fault levels. For the sake of an easy reference, Table 13.8 suggests a few typical values of fault currents, worked out on the basis of data considered for the G and GT. One such example is also worked out in Example 13.3. 

But this may not always be true, as it is possible that one or more CTs in the faulty circuit may saturate partially or fully on a severe through-fault and create a short circuit (T 2 = 0) across the magnetizing circuits of all the CTs that are saturated. Refer to Figures 15.26(a) and (b). The CTs resistances, however, will fall across the relay circuit. Assuming that the other sets of CTs in the circuit remain functional, this would cause a severe imbalance and result in a heavy imbalanced current through the relay and an unwanted trip. Under such a condition. 

The relay has voltage as well as current settings. The former defines the stability limit against through-faults, as discussed above, while the latter determines the sensitivity of the protected zone. 

If/ , = minimum fault current through the primary (chosen on the basis of the rated full-load current of the machine or the system being protected) required to trip the relay. It may be termed the minimum primary operating current (POC) of the scheme, /pi. In term.s of the secondary... 

Relay - High impedance differential protection reiay. It operates for the fault occurring within the protection zone /pi - Fault current through ground for fault on phase 8... 

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... 

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. 

Induced residual fault current through the phase CTs. 

This relay may be used only under unrestricted fault conditions, with three CTs as shown, tf the scheme is used under a restricted fault condition, with the fourth CT in the neutral, the directional relay will remain immune to any fault occurring outside the zone of the three CTs, as the fault current through the fourth CT will offset the residual current, detected by the three CTs (.Section 2l.6.,f), rendering the whole scheme non-functional. 

We discussed in Section 21.1.1 the maximum tolerable currents through a human body and their duration. The potential difference in a ground conductor at any point where a human body may come into contact with it during the course of a ground fault should be such that the resultant current through the human body will remain within these tolerable limits. 

This realizes a near-zero or very small fault current through the capacitors and protects them besides restoring the fault level of the system to its original level. 

By comparison The currents flowing into and out of a circuit are compared. In a healthy circuit, or in a circuit in which through fault current is flowing, the two currents should be equal and the protective device does not operate. (Compensation for any transformation is necessary.) If the two currents differ, the protective device operates. 

When high voltage motors are being considered, it is usually found that the minimum conductor size of the cable is determined by the let-through fault withstand capability rather than the full-load or starting current. Cable manufacturers provide graphical data for fault withstand capabilities of their cables, which are based on practical tests. These aspects are also associated with the protection system used for the motor, e.g. a contactor-fuse combination, a circuit breaker, the protective relay characteristics (thermal, inverse time with or without instantaneous or earth fault elements). 

An Open-circuit fault in diode D4 for t > t2 means that the current through the diode, /4(f) = gdinnit), —Ua(t)), vanishes. This affects the node potential Ua- The latter one is the solution of the implicit algebraic Eq.(8.92) which in turn results in a residual 0 according to (8.87). 

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