The main hypothesis proposed in this paper is that by controlling the harmonic and zero-sequence components of the current in nonsinusoidal permanent-magnet synchronous generators (PMGs), additional energy can be obtained, thereby increasing the machine power density without increasing the Joule effect losses. Two different strategies are proposed for three-and four-wire topologies; therefore, four different cases are analyzed. The strategies consist in controlling the PMG stator currents, following a function that depends on the waveform of the back electromotive force (EMF). The current function is obtained from the instantaneous reactive power theory. An experimental system was built to validate the proposal. Experimental results prove that it is possible to increase power in a tested PMG by 7% using the four-wire topology in comparison with the conventional block commutation and same losses. Higher power gain can be obtained for machines with almost rectangular-shaped EMF waveforms.Index Terms-Four-wire topologies, permanent-magnet (PM) machines, power control.
I. INTRODUCTIONP ERMANENT-MAGNET synchronous generators (PMGs) are characterized by high power density due to the presence of high-energy magnets, for instance, neodymium-ironboron (NeFeB) or samarium-cobalt (SmCo), commonly used to construct them. They can be also built with a high number of poles, which enables them to work at low speed. For these reasons, they are suitable for direct-driven wind energy systems [1]- [3]. PMGs usually operate at variable speed, and power flow control is carried out by imposing the waveform and amplitude of the stator currents using a power electronic converter [4], [5].While the waveform of the electromotive force (EMF) of a PMG can have an arbitrary number of constituent harmonics,
A strategy for fault detection and tolerance on the inverter that controls power flow in Variable Speed Permanent Magnet Generators is proposed in the present work. The performance of the proposed strategy is analyzed in two different 4-wire topologies. In the first topology, the neutral point of the generator is connected to the middle point of the dc-link capacitor bank while in the second topology, to a fourth leg of the inverter. The main contribution of this paper resides in the control scheme, which allows a steady operation through a fault occurrence without the need of reconfiguring neither the inverter topology nor the control algorithm. In this way, transients and the use of extra hardware components are avoided. In addition, a control loop that allows limiting the Generator losses at any operation point is presented in order to protect the machine. Finally, experimental results, obtained from a lab prototype, validate the practical feasibility of the proposed strategy.
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