PERMANENT MAGNET MULTIPHASE MACHINE MODELING AND CONTROL FOR MV WIND ENERGY APPLICATIONS

Due to the rapid development of the power electronics in the second half of the twentieth century, a significant research effort has been put into the modelling of electrical machines to provide mathematical models for control purposes. As the power electronics isolate the machine from the grid, the number of phases on both sides no longer needs to be the same, thus allowing for use of multiphase machines. Several studies have shown that multiphase machines can yield lower torque ripple, provide higher torque per phase current, and that they can continue to operate with one or more faulty phases, thus increasing the robustness of the power stage. This, amongst other benefits, has led to increased interest in multiphase machine employment for critical applications, such as more-electric aircraft, electrical propulsion systems for ships and offshore wind, etc. Amongst the different multiphase machine constructions, the multiple three-phase winding structure with isolated neutral points is of special interest. It can be operated using multiple three-phase converters, so that almost no modification of hardware is needed. Furthermore, with high power machines (above the 5 MW class), several converters in parallel should be used when increased availability is desired. This is where multiple three-phase winding machines show an additional benefit, galvanic isolation between the windings. By connecting one three-phase converter to each of the three-phase windings of the machine, the increased availability of paralleling converters is obtained while the problem of the circulating current between paralleled converters is practically eliminated thanks to said galvanic isolation. The control schemes of three-phase machines should not be directly applied to multiple three-phase winding machines, since these show internal cross couplings between the different three-phase windings that may affect dynamic performance. To examine the behaviour and design control schemes for multiple three-phase winding machines, modelling approaches based on vector space decomposition, multiple dq modelling approach and a novel approach, specifically developed in this thesis for the independent power flow control in individual three-phase windings, are studied. It is demonstrated that, by including appropriate decoupling terms in the traditional three-phase control structure, a completely decoupled operation can be obtained in all the three-phase windings in the machine when control scheme is based on the multiple dq modelling approach. With this control approach, the control of these machines is accomplished using control structures and model transformations familiar to those skilled in the art of the three-phase machines. For six-phase machines the existing transformations are sufficient for all control purposes, while the novel transformation becomes a useful tool when there are three or more three-phase windings. The influence of a low switching to fundamental frequency ratio on behaviour of the controlled object is also covered in this work. This has a great impact on the modelling of current control loops, especially when using the synchronously rotating reference frame in variable fundamental frequency applications, such as motor drives. The precise modelling of the actual control loops is of vital importance since it allows development of faithful control tuning techniques. With these, the regulator parameters, which ensure certain specified dynamic performance of the loops, are obtained and their behaviour can be precisely described and predicted by simulations. The machine’s parameter identification has also been approached in this work; accurate parameter knowledge is of essential importance to ensure the correct match between experimental and simulation results. All the experimental work has been done using a 150 kW permanent magnet synchronous generator in six-phase configuration with two three-phase winding placed spatially in phase. Unequal power sharing between different three-phase windings is studied further, including the simultaneous operation of one winding in motoring and the other in generation for a six-phase machine. This particular mode of operation has been found as very useful in development of a novel testing method for the machines with multiple three-phase windings, of synthetic loading type, which is fully verified by experimentation. A corresponding theoretical/simulation work has been performed for a nine-phase (triple three-phase) machine