Synchronization Stability And Control Of Power Systems With High-Penetration Power Electronics

With the development of renewables,HVDC systems and microgrids,more and more power-electronic devices(i.e.,power converters)are integrated into modern power systems.Hence,the dynamics of modern power grids are dominated by both synchronous generators and power con-verters.Due to the diversity in the application scenarios of power converters and the various control structure,the power system dynamics are becoming more and more complex,especially consider-ing the strong couplings among different control loops,different devices and the power network.In fact,the synchronization mechanism of power converters can be very different from conven-tional synchronous generators,and it still remains to be fully understood how the high penetration of converters affects the synchronization stability mechanism as well as the dynamic behaviors of modern power grid.Currently,this lacking of understanding impedes the“grid-friendly”integra-tion of power-electronic devices and also poses great challenges to the stable and secure operations of power systems.To alleviate this situation,this dissertation is centered around the synchroniza-tion behaviors of power-electronic devices with various control structures,aiming at revealing the synchronization stability mechanism and guiding the control design of“grid-friendly”synchro-nization unit to improve the synchronization stability margin of modern power grids.The main contributions of this dissertation are summarized as follows.1)We propose a synchronization-dominated loop model to analyze the small-signal synchro-nization stability of power converters with phase-locked loop(PLL)based or grid-forming dy-namics.We investigate the synchronization stability mechanism and the variations of the stability margin under PLL-based and grid-forming control structures.We show that the PLL will strongly couple with other loops under weak grid conditions and result in complex small-signal synchro-nization instabilities.By comparison,the grid-forming control has much more robustness against the variable grid conditions and thus is a better choice to accommodate large-scale power-electronic devices in modern power grids.2)We propose the concepts of virtual power angle(VPA)and virtual power angle charac-teristic(VPAC)curves to analyze the synchronization stability of grid-forming converters under large disturbances.We focus on two typical disturbance scenarios,i.e.,active power fluctuations and voltage sags,and show that synchronization instabilities may arise in grid-forming converters under large disturbances.Moreover,the stability margin is significantly reduced due to the cur-rent limitation in power converters.The converter may run into an unexpected equilibrium after the disturbance is cleared,which should also be prevented.Based on the VPA analysis,we pro-pose a control method to shape the VPAC of power converters and thus improve the large-signal synchronization stability of grid-forming converters.3)In terms of stabilizing control design,we instigate how to improve the small-signal and large-signal synchronization stability of power converters based on the aforementioned stability analysis,as stated below.(?)Based on the synchronization-dominated loop modeling and stability analysis,we propose a loop shaping method for the grid-synchronization unit to improve the small-signal syn-chronization stability.Moreover,we investigate how the design of grid-synchronization unit affects the electromechanical oscillations in power systems,and point out that the inertia em-ulation makes the converters significantly participate in the oscillations.We illustrate how loop shaping method takes effect in eliminating the negative damping torque in the synchro-nization unit.(?)To enhance the robustness of power converters(with dc-link voltage control)against variable power grid strength,we propose a virtual synchronous control(Vi Syn C)for voltage source converters which utilizes the dynamics of dc-link capacitor to realize self-synchronization,thereby avoiding using PLL for grid synchronization.We show that the proposed Vi Syn C greatly improves the robustness of power converters.We further propose an unified Vi Syn C to be applied in multi-terminal dc(MTDC)systems which enables the MTDC system to interconnect multiple very weak ac grids without using master-slave control strategies.(?)To explore a more generalized synchronization control structure and improve the robustness of power converters against variable grid conditions,we propose a H_?-control design frame-work for grid-connected power converters.We illustrate how to specify weighting functions to achieve expected control objectives and dynamic performance.Moreover,we propose a decentralized stability criterion which can be integrated in the H_?-control design framework and guarantee the global stability of a multi-device system through local control design.