In-situ post-assembly magnetization of large rare-earth permanent-magnet machines

Abstract

Auxiliary permanent magnet generator (PMG) represents an essential part of the large steam generator excitation system. It provides energy for main exciter (rated power 1600 kVA) which supplies excitation for steam generator (rated power 300 MW). While auxiliary PMG is out of service, its permanent magnets are demagnetized. In order to put auxiliary PMG into operation, magnetization of permanent magnet poles is needed. This paper presents a method for in-situ post-assembly magnetization (PAM) of rare-earth PMG by applying DC current to all three phases of its stator windings. Magnetization process is performed in the power plant, and all equipment used in experiment is readily accessible to plant maintenance personnel (transformer, rectifier, circuit breaker, cables, etc.). The main advantage of this technique is that magnetization is performed without demounting the PMG off the main rotor shaft, and as a result, generator overhaul duration is significantly reduced, and magnetization costs are decreased. Throughout the procedure, magnetizing currents, voltage on PMG stator terminals and magnetic induction (flux density) are measured. Prior to magnetization, DC currents, current pulse duration and rotor position are calculated. Based on the calculated values, the temperature rise in stator windings has to be checked. The technique is validated through case study in the real power plant at the PMG, rated power 35 kVA, which was successfully magnetized. Subsequently, the simulation is performed, in order to determine the unknown parameters of the equipment used.