A new approach to calculate the decaying AC contributions to short-circuit: the "characteristic" currents method

For the near to generator short-circuit analysis, it is well known that a rigorous calculation procedure requires the use of complex dynamic models of power system components. In most practical cases, however, it is sufficient to adopt simplified and standard based calculation methods. For these cases the paper suggests a new approach of the short-circuit models of power system components based on the “characteristic” currents method (CCM). The paper, at first, presents a short overview of the IEC Standard 909, assumed as reference for the proposed approach. The calculation assumptions and the simplifications of the IEC Standard are pointed out in order to evidence how the new approach proposed allows one to obtain conservative, but more accurate results. The “characteristic” currents procedure is successively applied to a sample network with external feeder and internal generation and with motor load. The same network is analyzed by means of the IEC procedure and by means of a computer program able to consider dynamic models of electrical equipment. A comparison among these approaches is carried out in order to validate the “characteristic” currents method