Research On Key Aspects In Power Converter For Direct-drive Low Speed PMSG Wind Generation System

The low speed direct-drive permanent magnet synchronous generator system has a myriad of advantages, such as gearless, easy maintenance, high stability, decoupled between generator and grid. Moreover, the wind system could connect with powergrid flexibly and feed reactive power to the grid quickly. Thus, Low speed direct-drive wind generation system gains more and more attention.This paper focuses on some key techniques about direct-drive wind generation system in theory and proposes novel methods to the generation system. Simulation and experimental results verify that the feasibility and excellent performances of these methods.Comparing two topologies of the LCL filter, this paper studies on the characteristics of the converter based on carrier phase shifting parallel such as modularization, volume, circulating current, filtering performances. Based on the LCL filter physical theory, design principles about the three components are given in detail, which provide carrier phase shifting parallel LCL filter with fundamental theory.In order to suppress the circulating current caused by carrier phase shifting parallel, this paper proposes a novel control strategy based on circulating current mathematical model. Experimental results verify that the proposed control strategy could not only suppress circulating current effectively but also aquire good steady state as well as dynamic performances.Since many PWM driving signals are required for converters in the system, this paper develops an expandable PWM IP Core, which could conbine the number of PWM driving signals arbitrary. Experimental results verify that the PWM IP Core is flexible and effective in power electronics applications with complicated topologies.Due to the complicated structure of carrier phase-shifting parallel LCL filter, the mathematic model needs to be simplified to design controllers properly. This paper builds the LCL filter model based carrier phase shifting parallel and makes comparison with the single LCL filter which contributes to the fundamental theory of carrier phase shifting parallel LCL filter.In addition, this paper proposes a new grid side current feedback control method for LCL filters by adopting proper sampling frequency without active or passive damping. This control strategy is proposed based on the poles distribution and root locus diagram of the current feedback control system. By studying the stability trends in different sample frequencies, this paper gives a direction for LCL filter stability. Simulation and experiment results verify that the control method is feasible in practical application.To deal with LVRT, this paper proposes a new LVRT control strategy based on the power balance as the grid voltage drops. This control strategy stores part of power into the turbine inertia and smoothes the grid dynamic response. It also improves the system efficiency during LVRT. In addition, this paper studies the redundancy capability and the control method during faulting status, like arm faults, backup unit working, fault unit cutting off etc. Simulation and experiments indicate the converter based on carrier phase shifting parallel has very strong fault tolerant ability.In order to improve the utilization ratio of the converter capacity, this paper proposes a new unit power factor control method, which could make use of the converter capacity completely and gain good steady and dynamic state performance.The DC motor is adopted to emulate the wind turbine in the system. This paper proposes a MPPT control strategy in which the DC motor and permanent generator aims at tracking the input power and regulating rotation speed respectively. Based the control method, this paper constructs a model platform to verify the wind system performances by emulating various operating conditions, which can save developing time and cost significantly. Simulation and experiment results verify its feasibility.