Analysis On Converter Dominated Small-Signal Oscillatory Stability Of Microgrid

As an effective means to improve the utilization efficiency of renewable energy and enhance the reliability and flexibility of power supply,the microgrid system suffers from stability issues due to high penetration of power electronics and low equivalent inertia.In addition,the coexistence of complex and diverse control strategies and various types of dynamic components leads to involute dynamic interactions that adversely affect the dynamic stability of microgrid system.Therefore,this paper investigates the small-signal oscillatory stability and dynamic interactions caused by converter integration.Based on the comprehensive summary of the microgrid stability research and dynamic interaction research,this paper focuses on three types of interactions:the strong dynamic interaction induced by specified modal conditions,the aggregation dynamic interaction caused by aggregated same type of dynamic components,and oscillatory interaction resulting from the difference of dynamic characteristics.Aiming at the existing stability problems and challenges,the dynamic interaction and stability of microgrid are studied from the aspects of mathematical model establishment,instability mechanism and simulation verification,then corresponding stability enhancement measures are proposed to improve the dynamic stability characteristics,providing useful reference for mcirogrid design,analysis and operation.The main research contents and innovations of this paper are summarized as follows:(1)In the current works,there is no clear conclusion on whether the strong dynamic interaction induced by the modal conditions exists in microgrids.Based on the establishment of closed-loop interconnection model of microgrid,the open-loop modal analysis method is applied to the analysis of strong dynamic interaction in microgrid,and the strong dynamic interaction caused by modal proximity is proved to be widespread inside microgrid system.Aiming at the deterioration of microgrid stability caused by strong dynamic interaction,a quantitative method is proposed to gauge the interaction intensity and the stable operation parametric domain is then described,while the small-signal stability of microgrid can be improved by proposed stability enhancement measures.Then,the correctness of instability mechanism analysis and effectiveness of control methods are verified by DC microgrid,AC multi-microgrids and hybrid microgrid study cases.(2)In current microgrid researches,the investigation on the interaction among a group of dynamic components is insufficient.For instance,a group of dynamic loads usually be modeled as an equivalent load based on power multiplication.Based on matrix equivalent transformation theory and modal analysis method,the aggregated interaction caused by dynamic components aggregation is investigated and instability mechanism is revealed.Then an order reduction analysis model and the improved stability criterion are proposed considering aggregated interaction.Due to some assumptions in former analysis might not be satisfied,the impedance analysis method is used to extend the analysis of aggregation interaction to practical situations of generalization,and the damping control strategy is proposed to suppress the influence of aggregated interaction on the stability of microgrid,which are demonstrated by microgrid examples.(3)Researches on the oscillatory interaction among dynamic components or the sub-microgrids are not sufficient enough,relevant studies are mainly based on case analysis.Aiming at the oscillatory interaction caused by the difference of dynamic characteristics of components,the instability mechanism resulting from interaction oscillation is clarified by introducing a reduced-order analysis model,and then the instability mechanism analysis of interaction oscillation is extended to multimachine/multi-microgrid.By using the characteristics of fast response of converters,a supplementary stability control method to suppress the interaction oscillation is proposed,which can effectively suppress the adverse impacts of the oscillatory interaction on the stability of the microgrid,and is verified by the single microgrid and multi-microgrid cases.