Multi-loop model for fractional pole-path ratio synchronous generators with stator winding internal faults

Fractional pole-ratio winding is a new type of AC winding, consisting of coils with different pitches. The application of fractional pole-path ratio windings in synchronous generators will bring new problems to the modelling and simulation of internal faults. It is important to establish a mathematical model for the fractional pole-path ratio synchronous generators with internal faults and accurately calculate the fault currents. In this study, the multi-loop model of fractional pole-path ratio synchronous generators is first proposed. The method for calculating mutual inductances between stator coils with arbitrary pitch is given, and all the space harmonics, including the fractional ones, are considered in the inductance calculation. In order to improve the simulation accuracy of turn-to-turn faults, the effect of core localised saturation is modelled by modifying the air gap function of fault coils. A 300 MW fractional pole-path ratio synchronous generator is set as an example, and three types of internal faults are simulated. The comparisons of simulation results are made between the multi-loop model and the finite element model to verify the validity of the multi-loop model proposed in this study.