Generators fault currents

Z1 internal grid impedance Z2 load impedance RF fault resistance T1, T2 protective devices Q1, Q2 isolating switches K1, K2 short-circuiting devices R1, R2 resistances ofK1, K2 11 fault current generated by the grid I2 fault current generated by a reactive load. 

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. 

The philosophy to assume the impedance of the source of supply (generator or a transformer) as the impedance of the faulty circuit may be far from reality and may give a very high fault current. In actual operation, the fault intensity may be far less, as every device and component connected in the circuit will tend to add to the effective impedance of the faulty circuit and limit the magnitude of the fault current. Figure 13.15 also subscribes to this theory. But it is customary to design the systems for the worst fault conditions which, in all likelihood, may not arise, and decide the protective scheme and the current settings of the protective relays for the minimum possible fault current. 

The magnitudes of fault currents under different conditions of fault are analysed in Table 13.9. Figure 14.5 has been redrawn in Figure 13.20 for a generator circuit illustrating the sub-transient, transient and steady-state currents on an actual fault. The curve depicts the most severe fault condition which occurs when the circuit voltage is the minimum, i.e. at Vq, causing the maximum asymmetry atid the associated d.c. component. 

Switchgear and controlgear assemblies 13/355 Table 13.9 Fault currents (r.m.s.) in a generator circuit under different fault conditions... 

Since the ground fault currents in generators can be higher (Section 20.10.1) than the sub-transient state current, special care need be taken while grounding a generator to limit the ground fault current. Section 20.10.1 covers this aspect also. 

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. 

Even the grounding of the generators can be monitored through this scheme, so that only one machine is grounded at a time, to avoid circulation of fault currents (Section 20.10.1). 

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... 

If there is more than one generator operating in parallel, when the fault occurs, the reactances of all such machines will fall in parallel and diminish the effective reactance of the faulty circuit and enhance the fault current. For instance, when all four machines are operating in parallel, the effective reactance will become... 

Level of ground fault current for large generators... 

Manufacturers of large generators, 200 MW and above, recommend the ground fault current, /g to be limited in the range of 5-15 A and a fault clearing time of the order of 5-30 seconds to protect the machine and avoid overheating of the grounded steel frame. It is also... 

Figure 20.21 Grounding method and flow of ground fault current in a generator on a ground fault (Example 20.2)...

To determine the grounding parameters, consider a generator rated for 200 MW, 15 kV and the ground fault current limited to 15 A. Considering GFF as -J3, the voltage ratio of the grounding transformer with a 220 V secondary will be... 

When the generator and the switchyard grounding mats are interconnected the ground fault current will divide between the two, depending upon their ground resistances, in inverse proportions (Figure 22.11) such that... 

In this case the sources feeding the fault are generators and not generator transformers (GTs). The GTs are only current limiters and introduce their reactances into the circuit. Since the ratings of the generators and the GTs are different, it is mandatory to first convert them to a common base, say, 200 MVA in this case. Therefore the combined unit impedance of... 

With normal interrupting devices the fault current would last for only a few cycles (maximum up to one or three seconds, depending upon the system design). This time is too short to allow heat dissipation from the conductor through radiation or convection. The total heat generated on a fault will thus be absorbed by the conductor itself. 

Probably the most obvious application for superconductors is in the transmission of electrical power and it seems that that this is close to being a reality. There are many other potential applications in the power electrical engineering field, for example power generators, motors, transformers, leads and fault current-limiters. 

A similar situation exists for the particles generated by fault current arcing in flameproof enclosures after passing the flameproof joints (see Section 6.8). 

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