Modelling, Analysis, and Control Aspects of a Rotating Power Electronic Brushless Doubly-Fed Induction Generator

This thesis deals with the modeling, analysis and control of a novel brushless generator for wind power application. The generator is named as rotating power electronic brushless doubly-fed induction machine/generator (RPEBDFIM/G). A great advantage of the RPE-BDFIG is that the slip power recovery is realized in a brushless manner. This is achieved by introducing an additional machine termed as exciter together with the rotating power electronic converters, which are mounted on the shaft of a DFIG. It is shown that the exciter recovers the slip power in a mechanical manner, and delivers it back to the grid. As a result, slip rings and carbon brushes can be eliminated, increasing the robustness of the system, and reducing the maintenance costs and down-time of the turbine. To begin with, the dynamic model of the RPE-BDFIG is developed and analyzed. Using the dynamic model, the working principle of the generator is understood and its operation explained. The analysis is carried out at speeds, ±20% around the synchronous speed of the generator. Moreover, the dynamics of the generator due to external load-torque disturbances are investigated. Additionally, the steady-state model is also derived and analyzed for the machine, when operating in motor mode. As a next step, the closed-loop control of the generator is considered in detail. The power and speed control of the two machines of the generator and the dc-link voltage control is designed using internal model control (IMC) principles. It is found that it is possible to maintain the stability of the generator against load-torque disturbances from the turbine and the exciter, at the same time maintain a constant dc-link voltage of the rotor converter. The closed-loop control is also implemented and the operation of the generator with the control theory is confirmed through experiments. In the third part of the thesis, the impact of grid faults on the behaviour of the generator is investigated. The operation of the generator and its response is studied during symmetrical and unsymmetrical faults. An approach to successful ride through of the symmetrical faults is presented, using passive resistive network (PRN). Moreover, in order to limit the electrical and mechanical oscillations in the generator during unsymmetrical faults, the dual vector control (DVC) is implemented. It is found that DVC to a certain extent can be used to safeguard the converter against large oscillations in rotor currents. Finally, for completeness of the thesis, a preliminary physical design of the rotating power electronic converter has been done in a finite element software called ANSYS. The thermal footprint and the cooling capability, with estimates of the heatsink and fan sizes, are presented. Besides, another variant of a rotating electronic induction machine which is based on the Lindmark concept and operating in a single-fed mode is also