The Stability Research For Load Side Converter Of Brushless Doubly Fed Generator Based Shaft Power Generation System

As the fossil energy gradually dry up, developing new energy sources and enhancing the efficiency of energy utilization has attracted more and more attention. Ship shaft power generation system can take full use of the ship host power which can reduce the using of the light diesel oil. So it can reduce the ship’s operating costs. At the same time, brushless doubly fed generator(BDFG) only needs slip power capacity converter and it further reduces maintenance cost by canceling the electric brush and slip ring. So it is more suitable for the ship shaft power generation system. But in such a system, the load side converter is connected to the power winding through the filter. Because of the leakage inductance, the filter performance and the stability of the load side converter is influenced. This paper analyzes the damping characteristics and the stability problem of the LC filter based load side converter.First, the harmonic of the output current and the load voltage of the converter is analyzed. It points out that the LC-L structure is formed when the LC filter combined with the not neglectable stator power leakage inductance. For the stability control of LSC, the mathematical model under static abc reference frame and dq reference frame has been build. And also the two loop control block diagram of the voltage and current has been deduced under dq reference frame. According to the analysis of the control block diagram for the current control, this paper reveals that the conventional PCC voltage feed-forward becomes a feed-back path in this system which changes the transfer function of the controlled plant. Then the relationship between the damping characteristic of the controlled plant and the filter parameters has been deduced from the perspective of both nyquist diagram and the physical significance.In order to realize the stability control of the load side converter, the stability limitation of the controller parameters and the robustness of the system under different damping characteristic situations have been compared. It points out that the better robustness has been achieved under the positive damping of the controlled plant. Direct at this conclusion, only the inherent damping characteristic is proposed to use to achieve the stability control when the controlled plant is at positive damping. Then the relationship between the THD of PCC voltage and the filter parameters has been deduced and the filter design principle has been put forward based on the conventional design principles. When the controlled plant is at negative damping with the inherent damping, the active damping method of capacitor current feedback has been proposed to use based on the analysis of the complementary characteristic between the capacitor current feedback and capacitor voltage feedback.For the stability of the current control, the other factor that can influence the stability of the current has been analyzed: the phase lock loop(PLL). The mathematical model has been built in this paper. Then the influence of the PLL on the current loop has been analyzed and the transfer function of the current loop taken the PLL into account has been deduced. Based on this transfer function, the influence of the PLL parameters on the stability of the current loop has been analyzed. The validation of the nyquist diagram and the simulation based on PLECS has also been given. It provides certain guidance for the design of the phase lock loop(PLL) and ensures the integrity of current loop stability analysis.In order to illustrate the correctness of the theory, after each chapter, the simulation results based on PLECS have been given. Finally, a filter design example for a 64 kW brushless doubly fed generator without extra passive or active damping methods has been given at the positive damping of the controlled plant. And also the capacitor current feedback at the negative damping of the controlled plant has been designed. The experiment is carried out to verify the correctness and reliability of the theoretical analysis.