This paper discusses the set up of a mathematical model of the powerformer, a new type of salient-pole synchronous machine, for analyzing internal phase and ground faults in stator windings. The method employs a direct-phase representation considering the cable capacitance. To effectively implement the internal fault simulation, the magnetic axis locations of fault parts are arranged appropriately. Moreover, all machine windings supposed to be sinusoidally distributed in space and the system are magnetically linear. With the above-mentioned assumptions, the current-equivalent equations, voltage-equivalent equations, and the rotor-motion equations are formed and combined to implement the fault simulations. Simulation results showing the fault currents, during a single-phase-to-ground fault, a two-phase-to-ground fault, and a phase-to-phase fault, are presented here. With the data generated by this internal fault simulation model, the protection scheme used for the powerformer can be validated and improved accordingly.