Research On The Control Strategy Of Direct-driven PMSG Wind Energy Conversion System

Direct-driven wind turbine system has recently received much attention as one of preferred wind energy conversion technology, mainly because of its low maintenance cost, high energy conversion efficiency, complete decoupling from grid, and high operating reliability. This dissertation proposes the use of current source converter as the power converter for PMSG based direct-driven wind power generation system. Related research in terms of current optimizing control strategy, friendly grid-connected current control method and LVRT control strategy are carried out to meet the proposed configuration for wind turbine system, which can be concluded as following:Further investigations of the history and status of global wind industry market and wind power conversion technology are conducted based on a large amount of current literature review. Mathematical models of main components of the proposed system are set up respectively. Various SVPWM modulation methods for current source inverter are detailedly studied on the basis of theoretical analysis, and simulation studies of certain ones is also carried out for harmonic comparison.A system coordinated control strategy is proposed to optimize operating current of the wind system to maximize system overall efficiency. The calculation formula to minimize dc current for the control scheme is deduced to meet control requirement of both generator side and grid side. Based on it, the optimum dc current of back-to-back current source converter adopted in the wind turbine system is found in various operating conditions. Using the proposed control scheme the wind turbine system can be operated with low conduct losses, which is demonstrated by simulation results.A novel grid-connected control strategy is developed to improve grid-connected current waveform. Certain grid-connected control methods for inverter are compared to find out that current harmonics can be hardly suppressed using normal vector oriented control schemes. To improve grid current quality a novel control scheme with variable structure of inner current loop is proposed. A number of simulation results prove that current waveform can be generated by the propose control scheme even in distorted grid.Based on energy balance law a mode switching control strategy with the function of low voltage ride through (LVRT) is proposed. An algorithm for active and reactive current distribution during the grid fault period is provided for the realization of LVRT. The express between the dip-depth of grid voltage and rising rate of generator speed is deduced to prove the feasibility of the proposed control scheme. Simulation results using such a control strategy show that the wind turbine system can realize LVRT and work as a STATCOM when voltage sag happened. A simulated prototype of the proposed wind turbine system configuration is established at laboratory. A back-to-back current source converter adopted in the system is particularly built, software and hardware of which are designed to meet the system requirement. A number of experiments are carried out on the platform and obtained experimental results verify the validity of the theoretical analysis and simulation results done in the paper.