Transient Modeling And Synchronization Stability Analysis Of PLL-based Converters Under Short-circuit Faults

Taking great efforts to develop renewable energy generation(REG)technology is an important measure to achieve carbon peaking and carbon neutrality goals and energy transition.REGs such as photovoltaics and wind power are extensively integrated to distribution network,and their grid-side converters commonly utilize phase-locked loop(PLL)for synchronization(i.e.PLL-based converters).PLL-based converters and synchronous generators are different in synchronization mechanism,and there has been coupling among different control loops,which results more complicated dynamic behavior of power system.Also,with ever-increasing penetration of REGs,the strength of AC grids is weakening,and interactions among different converters,converters and the grid are strengthening.Under large disturbances such as shortcircuit faults,converters are prone to transient instability and trip from the grid,which poses great challenges for the safe and stable operation of the system.Hence,this paper focuses on transient synchronization issues of PLL-based converters.The research work is carried out from two aspects: transient modeling and synchronization stability analysis,and the relevant conclusions can provide reference for parameter design and stability control of high-penetration REG-integrated distribution network.The main work of this paper is as follows:(1)With respect to PLL-based converter infinite-bus system,the transient model of the system considering the interaction with the grid is established,and the inherent synchronization mechanism of PLL is revealed.Also,the influence of key parameters on transient synchronization stability is quantified by numerical calculation method.The research results show that the imbalanced voltage input of PLL is the root cause of transient synchronization instability.Moreover,transient synchronization stability of the system would be deteriorated with the magnitude of current injection increasing or the grid strength reducing,and transient synchronization stability would be improved with the damping ratio of PLL increasing or bandwidth of PLL decreasing.(2)An aggregation reduced-order mothed based on coherency equivalence for multiple PLL-based converter system is proposed.First,mathematic model of the multi-converter system is established and rewritten in the form of swing equations.It is found that the difference of line impedance voltage drop leads to the diversity of coherency properties between the same bus and different bus converters.On this basis,impedance voltage drop-based coherency criterion is proposed.Then,equivalent circuit parameters are calculated on the premise of constant output power before and after aggregation.And by adopting the concept of center of inertia,equivalent control parameters are obtained by aggregating the swing equations of PLLs.(3)Based on the developed coherency equivalence model,transient interaction mechanism is revealed,and the impact of key parameters on transient synchronization stability is analyzed.First,transient model of the multi-converter system is established,and interaction coupling terms among converters are identified.It is found that the interaction among converters is dominated by mutual impedance voltage drop,which is mainly affected by current injection angles.On this basis,the influence of current injection angles and bandwidth difference on transient synchronization stability is further analyzed.The research results indicate that transient synchronization stability of the system can be significantly enhanced by appropriately allocating active and reactive current components among converters or reducing PLL’s bandwidth of large capacity converters.