1. A method for analyzing a fault of an ungrounded power distribution system, comprising: determining a type of a fault in a line segment of the ungrounded power distribution system; modifying a nodal admittance matrix of the line segment before the fault using a transformation matrix corresponding to the type of the fault to produce a faulty nodal admittance matrix of the line segment after the fault; and analyzing the ungrounded power distribution system using the faulty nodal admittance matrix and nodal admittance matrices of functional branches or line segments of the power distribution system, wherein steps of the method are performed by a processor.