Research On Transient Characteristics And Synchronization Stability Issue Of AC Microgrid

Environmental pollution and energy crisis promote the development of renewable energy resources,such as wind and solar energy.Microgrid is an important way to realize efficient and orderly accommodation of distributed renewable energy resources.Due to inevitable reasons,such as natural disasters,external force damage,sudden increase of load and human error operation,microgrid is usually subjected to large disturbances.The limited thermal capacity of power devices,different synchronization control strategies and weak anti-disturbance ability make the microgrid suffer from severe device-level and system-level stable operation problems.How to ensure the safe and stable operation of microgrid without off-grid has become a bottleneck problem to be solved in the promotion of microgrid technology.Th is paper focuses on safe and stable operation of microgrid under large disturbances.The research work is carried out from two aspects: transient characteristic analysis and control and transient synchronization stability analysis.With respect to the device-level safe operation problem,transient characteristics analysis of power electronic converters with different control strategies is carried out,and then differential current limiting control strategy is proposed.The transient characteristics of microgrid are determined by the adopted control strategie s.However,the diversification of control strategies makes it difficult to carry out transient characteristics analysis.The typical control strategies are chosen and corresponding equivalent circuit models are established respectively.Based on the circu it models,transient output current of converters with different control strategies are described either by analytical solution and numerical calculation method.It is found that the voltage-controlled converters are faced with serious inrush current probl em.Based on the conclusion,a differential current limiting control method is proposed to ensure the device-level security and system-level frequency stability requirement.With respect to the system-level stable operation problem,the instability mechanism of power-synchronized converters is revealed,and the attraction-domain of microgrid is quantitatively described using Lyapunov’s function.The powersynchronized converters,which are controlled to mimic the output characteristics of synchronous generators,are also confronted with the transient instability under large disturbances.Considering the differences between power-synchronized converters and the synchronous generators,the deterioration effect of the reactive power-voltage droop control loop on the transient stability of power-synchronized converters is revealed.Based on Lyapunov theory,an effective energy function for transient stability analysis of power-synchronized converters is constructed,which is used to describe the dominant factors and attraction-domain of microgrid quantitatively.Based on the mechanism analysis,stability improvement control method,which is based on reference active power adjustment,is proposed to ensure the stable operation of microgrid and restrict output current at the same time.With respect to the transient stability problem under mode switching control,the instability mechanism is revealed.Then,the re-synchronization phenomenon after the first swing instability is found in strong-damping system.Mode switching control is an important method to realize current limiting.However,the system under mode switching control is likely to be driven into instability area.Taking the powersynchronizaed converters as the research target,the synchronous stability mechanism after fault clearing and control mode switching is revealed,and the Lyapunov’s energy function,which is suitable for switching system,is proposed to quantitatively judge the stability of the system.Since the line impedance of the microgrid is resistive and the system damping is large,the re-synchronization phenomenon is identified in microgrid,and the key factors,which affect re-synchronization capability,are found.With respect to the transient stability problem with different synchronization control strategies,the transient interaction mechanism of the paralleled system is revealed,and transient stability improvement control is proposed by controlling PLL-synchronized converters to track angular frequency of power-synchronized converters.Taking the paralleled system with power-synchronized converter and PLLsynchronized converter as the research target,the transient interaction model is established,and the dominant effect of coupling phase angle is found.Then,considering the conditions of pure inductive and resistive-inductive network,it is pointed out that the system stability is optimal when the injection current angle and the impedance angle of parallel lines meet a certain relationship.Based on the conclusions,the transient stability optimal control method is proposed,which can greatly improve the transient stability of the paralleled system.This thesis focuses on the safe and stable operation of microgrid under large disturbances and the researches are carried out from two perspectives: device-level safe operation problem and system-level stable operation problem.In detail,four research contents are carried out: transient characteristics analysis and current limiting control,transient stability criteria,stability of switching system and the stability of the system with different synchronization control strategies.The relevant conclusions can provide research ideas and technical solutions for the researches on the safe and stable operation of microgrid under large disturbance.