Research On The Control Strategy Of PV Grid-Connected Converter Considering AC System Operating Conditions

In the context of a high proportion of renewable energy and power electronics,the construction of the new power system with renewable energy as the mainstay is continuing to be promoted.However,the continuous rise in the penetration of renewable energy,such as photovoltaic(PV),also brings new operational challenges to the traditional grid.With the large-scale integration of PV,the power system shows more and more obvious characteristics of weak grid,and the volatility of PV generation will lead to a more frequent and wider range of fluctuations in system operating conditions.PV grid-connected converter is the key power electronic device for grid connection of PV plants,and the sensible design of its control strategy is crucial to enhancing the stability and reliability of PV grid-connected system.To promote the construction of ’grid-friendly’ PV plants,the future grid requires that PV can operate stably when the AC system conditions fluctuate widely and provide necessary frequency support for the system.To address the aforementioned issues,this work uses the PV grid-connected converter as the research object,analyzes the stable operation boundaries and stable operation zones of PV grid-connected system under different control strategies and different operating conditions,clarifies the switching criteria of current vector control(CVC)and power synchronization control(PSC),and proposes a dual-mode smooth switching control strategy of PV grid-connected converter with continuous frequency support.The main work includes:(1)For a typical PV grid-connected system,the operation principle of PV,the topology of PV grid connection,the control mode of DC/DC converter and the mathematical model of DC/AC converter are introduced,and the principle of CVC and PSC of PV grid-connected converter are emphatically studied.In addition,considering the changes in the dynamic characteristics of the system after a significant amount of renewable energy and power electronics are connected,the AC system is equivalent.Finally,a two-stage PV grid-connected system model is built on the PSCAD/EMTDC simulation platform to verify the correctness of the model when different control strategies are adopted.(2)Under various AC system strengths,the trend of small-signal stability and transient stability of two common grid-following and grid-forming control strategies,CVC and PSC,are compared,and the impact of impedance angle on the operating characteristics and stability boundaries of the two control modes is analyzed.Then,through the theoretical calculation and simulation verification of the critical short circuit ratio(CSCR)of the PV grid-connected system under different control modes,the operating boundaries are extracted.Based on this,the stable operation zones of the system are obtained,and finally,the suggested switching criterion of a hysteretic switching method based on CVC and PSC is provided.(3)Based on the complementary advantages of CVC and PSC under different system strengths,a dual-mode smooth switching control for PV grid-connected converter is realized by suppressing the sudden changes in phase angle and current reference values,which improves the grid-connected stability of PV under changing system strength conditions.On this basis,a power correction part is added to achieve the goal of no sudden change in the operating point at the moment of switching by translating the active-frequency characteristic curve,and a dual-mode smooth switching control strategy with continuous frequency support is proposed to maintain the continuity of frequency support effect of PV when the control modes are switched.The effectiveness of the proposed control strategies is confirmed based on a model of a PV grid-connected system connected to the IEEE 33 bus case,and the superiority of the advanced smooth switching strategy in low inertia scenarios is verified through sensitivity analysis.