1. A method for determining three-phase voltages of buses and three-phase currents of branches of an ungrounded power distribution system in response to detecting a fault occurred along a line segment of the power distribution system by a processor in communication with the power distribution system and a memory embodied thereon instructions of the method executable by the processor for performing the method, comprising: providing a normal state topology of the power distribution system from the memory; detecting the fault of the line segment of the power distribution system by the processor in communication with the ungrounded power distribution system; specifying a type of the fault, a location of the fault and impedances of the fault of the line segment of the ungrounded power distribution system using the normal state topology, wherein the line segment includes a short circuit fault caused between at least one phase to ground or at least two phases; selecting, using the type of the fault, a transformation matrix from a predetermined set of transformation matrices; determining a faulty nodal admittance matrix of the line segment after the fault using a nodal admittance matrix of the line segment constructed based on the normal state topology, and the transformation matrix corresponding to the type, the location and the impedances of the fault; forming a fault controlled zone, and generation source controlled zones for the distribution system, wherein the fault controlled zone includes buses connected to the line segment with non-transformer and non-voltage-regulator branches, and the generation source controlled zone includes all adjacent buses having connections with the generation source, but not in the fault controlled zone; initializing the voltages of buses of the fault controlled zone with initial voltages at the fault location determined by the type of the fault and voltages of the buses of the line segment before the fault; initializing voltages of the buses in the generation source controlled zone based on voltage amplifier factors of transformers and regulators along paths connecting each bus with the generation source; and determining the voltages of buses and currents of branches of the ungrounded power distribution system having the fault on the line segment through a power flow solution of the system using the faulty nodal admittance matrix and nodal admittance matrices of functional branches or line segments of the power distribution system.