Fault current First cycle

The actual value of let-through current for a given fuse will depend upon the nature and magnitude of the prospective fault current e.g. asynunetrical or synunetrical. This is because a greater current has to be reached in the symmetrical case than in the asynunetrical case to create the same amount of melting energy. This is due to the shape of the current waveform in the first cycle, which can be seen in Figure 8.1. 

The peak value of the fault current dnring the first cycle of instantaneons cnrrent. This valne determines the peak asymmetrical dnty of the switchgear connected to the generator. This valne is determined by the sub-transient reactance. 

The following discussion and worked example for an LNG plant show how to carry out simple but reasonably accurate estimates of the sub-transient fault current and its decay in the first few cycles. Following is a discussion on how to assess the fault breaking current. 

For most LV and all HV generators it is often acceptable to ignore the armature resistance as far as calculating the magnitude of first-cycle fault currents is concerned. It is usual to assume that the... 

I (rms value of symmetrical fault current during the first half-cycle) + (DC offset current) ... 

The sub-transient impedance determines the initial decay, i.e. in the first cycle or so. Therefore the emfs E" and E, together with the reactances Xj and X j, need to be used for calculating the fault currents. In a similar way to induction motors, the synchronous motors will contribute to fault-making dnty reqnirements. However, they will also contribute towards the fault-breaking duty because of the transient effects. 

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