Analyzing of Relevant Fault Type for Calculation of Minimum Fault Current Magnitude on Distribution Transformer Primary Side

In many text books and papers such as [1]-[5], it is possible to find descriptions about shunt fault calculations and about the influence of the three-phase transformer connection Dyn on the fault currents distribution through distribution transformers. In [6] is described a method for digitally simulating a three-phase power system which is subjected to a fault at one location with blown fuse or open conductors at another location. The method is applicable when the simultaneous unbalances are on opposite sides of a delta-grounded star transformer. From these books and paper it can be concluded, that in the common case of a delta primary-grounded star secondary connection (Dyn), the currents on the source side of the transformer differ from the corresponding currents on the fault side for the unbalanced secondary shunt fault and for the simultaneous fault. This paper analyzes influence of three-phase transformer connections (Dyn5 and Yzn5) and several type short circuits and simultaneous faults in determining of relevant fault type for calculation of minimum fault current magnitude on the distribution transformer primary side, which will be relevant for the selection of fuse-links minimum breaking current of high-voltage fuses for transformer circuit applications. In the carried out analysis it is assumed, that a secondary short circuit (shunt) fault occurs between the transformer terminals and the lowvoltage side protective device and that the fault must be cleared by the high-voltage side fuses, which are used as transformer primary-side protective devices (incoming protection). In this paper it is considered that a simultaneous fault represents a multiple fault which involves at the same