Decentralized Control of a Nine-Phase Permanent Magnet Generator for Offshore Wind Turbines

This paper presents a decentralized current control approach for a nine-phase wind turbine generator. This type of generator has three different three-phase stators sharing the same machine yoke and connected to the grid by means of three different voltage source back-to-back power converters. Due to the machine configuration, magnetic couplings are present between the three stators, complicating the design and implementation of the machine current controllers. Rather than a centralized control approach, this paper proposes a methodology to design a decentralized machine control to regulate the active and reactive power flowing through each stator independently. A complete dynamic analysis is performed in order to design the controller to reduce the coupling effects within the machine, while ensuring a proper dynamic performance. The control strategy is validated through simulation and experimental results.

I. Introduction

Offshore wind power is one of the most promising renewable energies [1]. In the North Sea, a number of wind power plants will be commissioned in the next years. Offshore wind has advantages compared to onshore wind, such as abundant wind resources [2], no size limitation of the turbines and a reduced visual impact on population. On the other hand, offshore installations have some financial and technical challenges [3] such as long distance transmission systems required for the energy grid integration, the complex and expensive installation, operation and maintenance tasks, besides the limitations in footprint and weight of the offshore structures. Therefore, offshore turbines must be designed to be highly efficient, light and reliable [4], [5], with reduced maintenance requirements, in order to increase the cost-effectiveness of the system. One of the most interesting topologies proposed for offshore wind is the direct driven multipole permanent magnet synchronous machine (PMSM) with full power converter (FPC) [5], a turbine concept which does not include the gearbox. Based on this idea, different advanced generator concepts as the direct driven multipole triple three-phase stator PMSM have been proposed for wind energy [6]–[8].