Short-Circuit Calculations

The short-circuit capacity of the grid is assumed to be 1000 MVA. The grid is connected to the transformers through a series inductive impedance of 8%. Losses for the substation’s transformers are not considered. The designed short-circuit capacity for the 11 kV network is assumed to be 200 MVA. As recommended by the IEC Standard [16, 17, 18, 19], the R/X ratio for the corresponding equivalent impedance is assumed to be 0.1. The voltage factor used to consider the variations of the system voltage is assumed to be 1.1 [2]. The WTs are connected to the 11 kV network through a 0.69/11 kV transformer with a series impedance of 4% and a rated resistive component of the short-circuit voltage of 1.2% [2]. In order to evaluate the WTs’ contribution to the short-circuit level at each bus, the equivalent short-circuit impedances of WTs and transformers have been computed for each bus according to the installed WTs capacities and to the parallel connections of WTs.