Research On Characteristics Of Field Modulated Brushless Doubly-fed Generators For Wind Turbine

Wind energy has always been an important part of renewable energy development.Considering China’s 18,000 km long mainland coastline and the concentration of load centres on the eastern coast,offshore wind power will become an inevitable choice for the development of clean energy in China.The harsh offshore environment and high operation and maintenance costs limit the application of the two mainstream wind turbines(doubly-fed induction generator and permanent magnet direct-drive generator).The single-stator magnetic field modulated brushless doubly-fed generator(Brushless doubly-fed generator,BDFG)is simple,reliable and can generate electricity at variable speed and constant frequency,but it works with harmonics,has limited electromagnetic load and low power density.The split-stator topology is often used to increase the torque density of the motor,but the magnetic field modulated dual-stator brushless doubly-fed generator(Dual-stator brushless doubly-fed generator,DSBDFG)places the excitation winding in the inner stator,requires a higher excitation current to achieve the same magnetic load in the outer air gap,and the outer stator yoke is highly susceptible to saturation,limiting electromagnetic performance.The magnetic field modulated single and double stator BDFGs have their own advantages and disadvantages,therefore a unified comparison of the electromagnetic characteristics of modulated BDFGs from a single and double stator perspective is required.The main studies in the full paper are as follows: 1.The topology and magnetic field modulation mechanism of the modulated single and double stator BDFGs are introduced separately,then the operating principle,power flow and speed range of the modulated BDFGs are further unified,and finally the performance indicators are summarized.2.The magnetic field modulation capability of single-stator BDFGs with different rotors is compared.Finite element models of four different rotor BDFGs are built and it is concluded that a composite rotor with a simple-salient-pole plus a short-circuited-coil(Brushless doubly-fed generator with hybrid rotor,BDFG-HR)has the best magnetic field modulation capability.The BDFG-HR is then further optimized to be representative of the single-stator topology,laying the foundation for the characterisation.3.Two new modulated dual-stator BDFG topologies are proposed: the dual-stator brushless doubly-fed generator with short-circuited-coil rotor(Dual-stator brushless doubly-fed generator with short-circuited-coil rotor,DSBDFG-SCR)and the dual-stator brushless doubly-fed generator with simple-salient-pole rotor(Dual stator brushless doubly-fed generator with simple-salient-pole rotor,DSBDFG-SPR),and the design optimization of these two generators is carried out respectively,including the derivation of power equations,pole/slot fitsand rotor structure optimization.The two new topologies of BDFG are proposed to enrich the content of modulated BDFGs,and the design optimization method provides ideas for the subsequent design research of these two DSBDFGs.4.The electromagnetic performance of single and double stator modulated BDFGs is analyzed in depth in all aspects,including no-load characteristics,rated load operation,magnetic lines of force and magnetic density clouds,unbalanced magnetic pull,overload operation and power density.The common features of modulated BDFGs are revealed to be high harmonic content and irregular pole distribution,with two pairs of magnetic poles of different pole pairs superimposed asynchronously.The differences in BDFG performance due to the single and double stator topology are given,with the BDFG-HR having the lowest saturation compared to the dual-stator structure due to the larger yoke;the DSBDFG-SCR having the best electromagnetic performance and the highest power density;and the DSBDFG-SPR having less obvious advantages in power generation operation due to the simple-salient-pole modulator.5.A small prototype of the BDFG-HR is machined to verify the validity of the finite element model and analysis methods.The parameters,no-load characteristics and load characteristics of the BDFG-HR are tested.