4.1 Types of short-circuits

In three-phase a.c. systems it has to be distinguished between different types of short-circuits (s.-c.), as outlined in Figure 4.1.

Figure 4.1: Types of short-circuits and short-circuit currents. (a) Three-phase short-circuit, (b) double-phase short-circuit without earth/ground connection, (c) double-phase short-circuit with earth/ground connection and (d) line-to-earth (line-to-ground) short-circuit

Short-circuit currents can be carried out with different methods and in different details, depending on the available data and the technical needs. IEC 60909-0 calculates characteristic parameters of the short-circuit current, which are necessary for the design of power system equipment. The course of time of short-circuit currents is outlined in Figure 4.2. Generally, one has to distinguish between near-to-generator and far-from-generator short-circuits. A near-to-generator short-circuit exists if the contribution to the short-circuit current of one synchronous generator is greater than twice its rated current, or if the contribution to the short-circuit current of synchronous or asynchronous motors is greater than 5 per cent of the short-circuit current without motors.

Figure 4.2: Time-course of short-circuit currents. (a) Near-to-generator short- circuit (according to Figure 12 of IEC 60909:1988), (b) far-from- generator short-circuit (according to Figure 1 of IEC 60909:1988). I ? _k initial (symmetrical) short-circuit current, i _p peak short-circuit current, I _k steady-state short-circuit current and A initial value of the aperiodic component i _dc

The analysis of the short-circuit current in the case of near-to-generator short- circuit as per Figure 4.2(a) indicates two components, besides the decaying d.c. component,...