Characteristics Analysis And Enhanced Control Of Islanded Microgrid With Large Disturbance

The construction of islands and remote areas is a strategic requirement for national security and an important guarantee for social development.Independent power supply system is an important cornerstone for the construction of islands and remote areas.The traditional power supply system based on diesel generators has disadvantages such as poor power quality,low efficiency,high p ollution,and difficulty in fuel supply,and it is difficult to meet the urgent needs of safe and reliable power supply on islands and remote areas.Therefore,with full consideration of local energy characteristics,islanded microgrids with multiple renew able energy have become an important way to solve the problem of safe and reliable power supply on islands and remote areas.However,due to the weak supporting capacity from power grid,the variety of source and loads,and the harsh operating environment of the island microgrid,it is difficult to operate safely and reliably when subjected to large disturbances,which severely restricts the promotion and application of renewable energy.In general,there are four outstanding problems in the safe and reliable power supply of island microgrid: the high-frequency fluctuations in system voltage and frequency,the converter is easily burned due to fault impact,the long-term deviation of the bus voltage and frequency from the rated value,and the microgrid system Layer fault ride-through issues,etc.To this end,this thesis starts from these four problems,and has studied and achieved the following results:1)Aiming at the high-frequency fluctuations of the system bus voltage frequency,an inertia-enhanced virtual synchronous inverter is proposed.This thesis analyzes the active power oscillation mechanism of virtual synchronous generators in parallel operation encountering large load fluctuation,and concludes that the instantaneous active power incremental unequal distribution is the external excitation,and the virtual synchronous inverter itself has a weak damping ability is the internal causes of the oscillation.It is derived that the sufficient condition to eliminate the power oscillation excitation is that the inertia and the line impedance are distributed in inverse proportion.Therefore,an inertia-enhanced virtual synchronous inverter structure combining virtual damping and virtual impedance is proposed,which achieves a significant increase in inertia and significantly suppresses the problem of high-frequency fluctuations in the bus voltage of the traditional microgrid.2)Aiming at the problem that the virtual synchronous generator is easily burnt due to grid-side faults,this thesis focuses on the analysis of the generation mechanism,and influencing factors of the transient inrush current when the grid-side fault occurs,and proposes a fast fault ride-through control method of the VSG.Combining the fault current calculation of the traditional power system and the characteristics of micro-source,a high-precision instantaneous inrush current calculation method for virtual synchronous generators is proposed.This method realizes the accurate calculation of the inrush current at the time of the fault wit h the consideration of characteristics of the controller.On this basis,a method for fast suppression of inrush current of virtual synchronous generator based on switching idea is proposed.When the grid voltage drops,the virtual synchronous generator quickly switches to hysteresis control to achieve fast current limiting.After the voltage is restored,it switches back to the virtual synchronous control,thereby quickly reducing the instantaneous inrush current and achieving the effect of suppressing the inrush current.3)Aiming at the problem that the bus voltage and frequency deviates from the rated value for a long time when the load fluctuates,a dynamic reconfigurable master-slave control architecture of the island microgrid is proposed.The key idea of the proposed method is to automatically select the leader of the system through the proposed difference delay method,and then the lead er sends its own information to other units as a reference for the entire system,coordinating the output of other units to finally make the output of the micro-sources in the entire system equal.Since the proposed method has only one leader periodically and uniformly sends information at each moment,the basis of the judgment based on the stability of convergence can prove that the control structure has good stability and robustness to communication delay.4)Aiming at the problem of fault ride-through at the system level of island microgrid,this paper analyzes the fault characteristics of island microgrid and proposes a fault ride-through method for system-level power supply requirements.First,combined with the results of the analysis of the fault characteristics of the virtual synchronous generator,the transient characteristics of the islanded microgrid are analyzed,and then the amplitude,distribution and change characteristics of the three-phase symmetrical fault current when multiple virtual synchronous inverters are connected in parallel is further analyzed;secondly,based on the analysis of fault characteristics,a fault ride-through method based on reference current adjustment and inertia adjustment is further proposed,which solves the problem of ove rvoltage during fault clearing and power oscillation during reverse control switching.Based on the study of the converter and system-level operating characteristics of the islanded microgrid,this thesis has perfected the large disturbance analysis theory system of the islanded microgrid,which can provide support for the protection of the islanded microgrid and the development of safe and reliable operation.