Generators fault levels

Switchgear and controlgear assemblies 13/353 Table 13.8 Typical parameters of a power generating station and Its likely fault levels... 

A power system is connected to a number of power supply machines that determine the fault level of that. system (e.g. generators and transformers). The impedances of all such equipment and the impedances of the interconnecting cables and overhead lines etc. are the parameters that limit the fault level of the system. For ease of calculation, when determining the fault level of such a system it is essential to consider any one major component as the base and convert the relevant parameters of the other equipment to that base, for a quicker calculation, to establish the required fault level. Below we provide a few common formulae for the calculation of faults on a p.u. basis. For more details refer to a textbook in the references. 

We illustrate a typical powerhouse generation and transmission system layout in Figure 13.21, and reproduce in Table 13.10 the typical fault levels of different transmission and distribution networks in practice for different voltage systems. 

Since this represents the generator voltage, therefore the fault level will be governed by the generator and the generator transformer as indicated in Table 11.8,... 

Considering the system fault level at 32.8 kA, calculate the ground fault contribution by the generating units ... 

The peak value is a function of fault level Section 13.4.1 (7), Table 13.1 1. Which in turn, is a function of size and impedance of the feeding source, such as a transformer or a generator, Section 13.4.1 (5). Table 13.7. The values prescribed in the above table are thus based on these parameters. 

As discussed in Section 13.4.1(5). these sections are under the cumulative influence of two pow er sources and may be tested for a higher short-time rating, which would be the algebraic sum of the two fault levels, one of the generator and the other of the generator transformer as noted in Table I 3.8. Also refer to Figures 31,1 and 13.18 for more clarity. 

Modem machines are connected to power systems that often have relatively high prospective fault levels and so the generators and motors need to have their windings and terminations robustly braced to avoid movement during a major fault. General-purpose industrial machines may not be robust enough for such high fault level service. 

In the oil industry it is often found necessary to increase the number of generators on an existing system. Sometimes this causes fault level problems at the generator switchboard. Rather than replace the switchboard it may be possible to insert one or more reactors. Several solutions are possible -... 

Very and extremely inverse relays are used in systems where the fault level downstream is low when compared with that available at the point of main supply, e.g. a main generator switchboard. They are also nsed where coordination with upstream or downstream fuses is necessary. 

Based on user feedback obtained from BMD subject-matter experts, we recognize the importance of generating mission-level awareness. It is of little value to a commander to know that a service has failed it is of great value to know that the BMD mission is currently compromised during a missile attack because a critical service has failed. Mission-level awareness can be embedded in the NCDS architecture by implementing analyzers that correlate system faults with mission impact. It remains an open problem though how such analyzers should be implemented. 

Therefore, the level of phase-to-phase asymmetrical faults will he generally of the same order as the three-phase symmetrical faults. The ground faults, however, will he higher than the symmetrical faults. Special care therefore needs he taken while grounding a generator, when they are solidly grounded, particularly to limit the ground fault currents See also Section 20.10.1. 

For a switching device (which has not been previously tested for a short-circuit test). This should be closed and held in the normal service position. The test voltage (that would generate the required level of fault current) may be applied on one set of terminals, the other terminals being shorted. The test may be continued until the short-circuit device operates to clear the fault, but in no case for less than 10 cycles. In LT assemblies the point where the short-circuit is created should be 2 0.4 m from the nearest point of supply. 

At low p.f.s the generator operates at a low level of excitations (armature reaction demagnetizing). During a fault, therefore, when the p.f. of the circuit falls it will also cause a fall in the excitation level and in turn in the terminal voltage. A low voltage, however, would reduce the severity of the fault. 

Level of ground fault current for large generators... 

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