Analysis of Inherent Unbalanced Currents in Three-Phase Multi-Branch Brushless Exciter

Abstract

The large-capacity three-phase brushless exciter, constituted by multi-branch armature winding generator and the bridge rectifier, usually operates under ‘diode-parallel structure’ or ‘rectifier-parallel structure'. Ideally, currents of all paralleled diodes in the same bridge leg or branch currents of the same phase should be equal in normal operation. However, due to inconsistence of the diodes parameters and errors of generator manufacture, currents of the paralleled components are always different in practice. Even in normal operation, there are some certain inherent unbalanced currents among the paralleled components. This paper presents a study on the inherent unbalanced currents in the large-capacity three-phase brushless exciter. Two experimental platforms, called diode-parallel system and rectifier-parallel system, are built up one after another, with the same three-phase dual-branch synchronous model machine and the same-batch diodes. It is found that the corresponding diode currents in the two systems, which should be equal in ideal condition, are significantly different in real systems. Neither frequencies nor effective values of the inherent unbalanced currents in the two systems are the same. Mechanism of the phenomenon is explained based on analysis of the electromotive force and circuit of the unbalanced current. Finally, combining the theoretical analysis with FFT results of experiment data, advantages and disadvantages of two systems are clarified. The inherent unbalanced currents in the rectifier-parallel structure are smaller than those in the diode-parallel structure, which results in a relative lower requirement for the diodes. The work in this paper could provide a basis for topology selection of large-capacity three-phase multi-branch brushless exciter system.