Studies on the Internal Fault Simulations of a High-Voltage Cable-Wound Generator

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.

I. Introduction

A Completely new type of generator, the powerformer, presented by ABB, offers a direct connection to the power network without the need for a step-up transformer. The windings of this new high-voltage generator consist of insulated cables similar to the conventional solid dielectric ones. The cable of the powerformer can be considered as a capacitor with charges on the electrodes that are, in this case, the inner and the outer semiconducting layers [1]. The electrical charge on a phase winding of the powerformer at voltage maximum is 30 times larger than the charge on a phase winding of a conventional generator with the same rated apparent power [2]. The internal short-circuit current should be investigated and simulated in high accuracy for the purpose of protection of the powerformer. Hence, an accurate modeling method, taking account of the distributing capacitance, is worth studying.