Small Signal Stability Analysis And Control Technology Of Renewable Energy Grid-connected System With Different Synchronous Modes

Synchronous modes and control characteristics of renewable energy equipment are significantly different from those of synchronous generator.The dynamic characteristics of renewable energy grid-connected system(REGS)are determined by the dynamics of multiple physical energy storage components and corresponding controllers.As a result,the dynamic behavior of REGS during small disturbance is complex,which presents the characteristics of multiple time scale and strong coupling in the system.Moreover,the forms of small signal instability are significantly different in different synchronous modes system,which makes the coordinated control of diversified control loops more complicated.Therefore,the small signal instability problem in different synchronous modes control system will seriously threaten the stable operation of REGS.Although the oscillation mechanism and influencing factors of REGS with different synchronous modes have been studied,the detailed analysis considering the actual operating conditions of REGS is lacking.Such as the stability analysis in the case of multiple converters system and short circuit faults in the power grid is rarely studied.In addition,there is a lack of relevant research on the small signal stability analysis and damping controllers of hybrid REGS.Therefore,it is urgent to comprehensively analyze the small signal stability of hybrid system and propose stabilization controller to improve the safety and reliability of hybrid REGS.In order to improve the small signal stable performance of REGS with different synchronous modes.Firstly,doubly fed induction generator(DFIG)is a typical mainstream power generation equipment of REGS.Therefore,this thesis takes the gridfollowing DFIG grid-connected system as an example,which aimed to analyze small signal stability of grid-following system during low voltage ride-through.Secondly,the virtual synchronous generator(VSG)grid-connected system with multiple converters is considered as the research object in this thesis.Furthermore,the oscillation instability mechanism,oscillation influencing factors and multi-converters optimal control strategy is studied.Finally,the interaction between the control loops is analyzed in the hybrid system with VSG and grid-following DFIG,and a damping control strategy is proposed to improve the oscillation suppression performance of the hybrid system.The contents of this dissertation are as follows:(1)When power grid presents the characteristic of weak power grid,the adaptability of grid-following control system may deteriorate due to the strong coupling between the generation equipment and the power grid.Therefore,this thesis takes the grid-following DFIG grid-connected system as an example to analyze small signal stability of grid-following system during low voltage ride-through.Furthermore,the output impedance of grid-side and generator-side are established respectively.The influence law of different parameters on the dynamic stability of the system is explored through comparing the frequency domain characteristics of subsystems.Finally,a virtual capacitor control strategy is proposed from aspect of reducing the electrical connection distance,which can enhance the system small signal stability in the weak power grid.(2)The linearization model of multiple VSG grid-connected system is established.The influence of grid impedance,active power control parameters and output power on the dominant characteristic root is analyzed.Furthermore,combined with residue index,the additional damping control strategy based on hybrid particle swarm optimization algorithm of chaos and differential evolution algorithm is proposed to improve the small signal stability of multiple VSG grid-connected system.The time domain simulation and experimental results verify the correctness of the theoretical analysis and the effectiveness of the proposed control schemes.(3)The hybrid grid-connected system with VSG and grid-following DFIG is decomposed into two open-loop subsystems by using open-loop mode resonance theory.Furthermore,the relationship between the open-loop mode analysis and the dynamic interaction is established.From the view of mode interaction,the small signal instability mechanism of hybrid system caused by controller interaction is revealed.Finally,a damping controller is proposed to make the state matrix of the closed-loop system behave as a normal matrix.The closed-loop system has the minimum eigenvalue sensitivity and closed-loop eigenvalue is not sensitive to parameter changes.Therefore,the controller interaction and small signal instability in hybrid grid-connected system can be solved.In this thesis,oscillation instability mechanism,oscillation influencing factors and dynamic interaction of REGS with different synchronization modes are studied in depth.Meanwhile,the optimized control strategies are proposed.The research work in this thesis provides a theoretical basis for further improving the consumption and stable operation ability of REGS,which also has the prospect of engineering application.