Oscillation Analysis And Suppression Strategy Of Pmsg With High Shares Of Renewables And Power Electronics

With the development of novel power system dominated by new energy sources,the existing AC system is gradually being electrified,exhibiting the characteristics of a high proportion of renewable energy and a high proportion of power electronic devices(‘dual-high’).The grid features low damping,weak inertia,and strong nonlinearity,making it prone to excite wideband oscillations with multiple types and multiple frequency bands intertwined,posing a serious threat to the stability and safety of direct drive wind power system.Establishing a wideband oscillation model and analysis system,revealing the oscillation mechanism of direct drive wind power system,and researching wideband oscillation suppression strategies suitable for various grid conditions,has become an urgent task.Therefore,this paper takes the direct drive wind power system as the research object,focusing on the research of the mechanisms and suppression method of wideband oscillation.It discusses the impact mechanisms of multi-loop control coupling,“source-grid”interaction,control delays,and “source-source” interaction coupling on wideband oscillations,and studies the oscillation suppression methods under different conditions.The main research work of the paper is as follows:(1)Existing research typically assumes decoupling of multi-loop control,making it difficult to quantitatively analyze the impact of multi-loop control coupling on the stability of permanent magnet synchronous generator(PMSG).Most typical PMSGs adopt a multi-loop cascaded control structure,and the decoupling conditions of complex multi-loop control may not be guaranteed under multiple grid operating conditions.Therefore,drawing on the theory of complex torque analysis,this paper considers complex control coupling and establishes a complex torque model of the grid-side converter(GSC)of PMSG.It quantifies the impact of factors such as multi-loop control coupling,line impedance,and controller parameters on system damping torque,and reveals the oscillation instability mechanism from the perspective of damping.Based on theoretical analysis,a phase compensation-based phase-locked loop damping reshaping control strategy is proposed to enhance system stability by compensating for the negative damping introduced by multi-loop control coupling.Finally,corresponding simulations and experiments are conducted to verify the correctness of the theoretical analysis and the effectiveness of the improved control strategy.(2)For PMSG,the coexistence of internal multi-loop control coupling and external coupling with the ‘source-grid’ interaction may introduce additional paths for the phase-locked loop(PLL),affecting its synchronization performance and unit stability.Therefore,this paper establishes a synchronous model for PMSG considering both internal and external coupling.It takes into account the interaction between the wind turbine and the grid,elucidates the PLL as a coupling bridge between the turbine and the external grid,and investigates the impact of multi-scenario grid operating conditions on system stability.Additionally,it considers the dynamics of the DC bus voltage and studies the intrinsic mechanism of interaction between the grid-side subsystem and the machine-side subsystem of PMSG induced by the DC bus.Based on the small signal modeling,it analyzes the impact of internal multi-loop control parameters and external grid operating conditions on the equivalent damping of the system.In response to the influence of internal and external coupling,a coordinated control strategy for machine/grid coordination is proposed to enhance the equivalent damping torque of the wind turbine.The research findings indicate that the additional paths introduced by internal and external coupling alter the structure of the PLL,weakening its equivalent damping.By compensating for the negative damping introduced by this coupling,the equivalent damping can be reshaped,enhancing the synchronization of direct-drive wind turbine units and achieving suppression of low-frequency oscillations.(3)To address the high-frequency resonance issue caused by the direct parallel connection of the LCL output filter of the GSC of PMSG with the grid,this paper first establishes a circuit model for high-frequency oscillation analysis.It elucidates the mechanism of the resonance circuit formed by the direct parallel connection of the grid impedance and the LCL filter,analyzes the influence of a wide range of variations in grid impedance on the resonance characteristics of the filter,and reveals the mechanism by which delay introduces negative impedance to suppress resonance.Based on the mechanism analysis,compensation controllers are designed to improve the conventional active damping strategy based on capacitor current feedback,proposing a robust active damping control strategy with delay compensation.This improved control strategy broadens the positive impedance region for suppressing resonance,ensuring positive impedance characteristics of the system under time-varying grid strength,achieving the suppression of high-frequency oscillations,and enhancing the stability of the system under weak grid conditions.Finally,simulations and experiments are conducted to validate the feasibility and effectiveness of the optimized control strategy.(4)The PMSGs are usually tied to the grid with synchronous machines,and there are interactions,which reduces the system margin of the system.Addressing the oscillation issues arising from ‘source-source’ coupling,this paper focuses on the interaction between PMSGs and traditional synchronous machine systems.From the perspective of synchronization,it analyzes the distinct dynamic characteristics of wind turbines and synchronous machines.Considering the ‘source-source’ interaction characteristics,the paper investigates the coupling mechanism and synchronization oscillation issues of wind turbine and synchronous machine interconnected systems.Firstly,a detailed small-signal model of the heterogeneous interconnected system is constructed to analyze the coupling paths of ‘source-source’ and reveal the mechanism by which this coupling induces system oscillations.The paper analyzes the variation of system equivalent damping under different grid strengths,control parameters,and different voltage drop depths.Based on theoretical research,an oscillation suppression control strategy considering‘source-source’ coupling is proposed.By designing corresponding optimized control strategies to compensate for the negative damping introduced by“source-source” coupling,the oscillations of the interconnected system are suppressed.Finally,through corresponding RT-LAB semi-physical simulation experiments,the correctness of the theoretical analysis and the effectiveness of the suppression strategy are verified.This paper addresses the wideband oscillation issue of grid-connected direct-drive wind turbine systems.Starting from the single-machine level,it considers the interaction of multi-loop control coupling,internal and external complex coupling,and control delays,studying both low-frequency and high-frequency oscillation problems in the PMSG.Furthermore,taking into account the ‘source-source’ interaction,it investigates the oscillation problems of the PMSG and synchronous machine multi-machine interconnected system.The research results further reveal the oscillation instability mechanism of both the PMSG single-machine infinite system and the PMSG-synchronous generator multi-machine interconnected system,addressing the challenges of oscillation mechanism analysis,oscillation suppression,and stable operation under the “dual-high” background.This research provides a certain reference for the research of wideband oscillation of wind power system.