Research On Enhancing Grid Stability With High Penetration Of Renewable Energy By Renewable Energy Synchronous Generator

It is a trend for power system to have more and more renewable energy resources.This can not only make full use of some advantages of power electronics devices,for example,fast and flexible control ability,but also can bring challenges for stable operation of the grid.The challenges mainly focus on lack of reliable inertia response,insufficient abilities for supporting current,voltage and frequency and lack of reliable damping to restrain oscillation.Traditional synchronous generator has advantages over renewable energy converter.For this reason,most of the current solutions based on control aim to give some characteristics of synchronous machine to converter.Though these solutions gained certain achievements,the limitation comes down to power electronics itself.Based on the fundamental conditions and challenges,this paper propose Motor-generator Pair(MGP)as a new grid-connection way for renewable energy resources.This grid-connected system is called the Renewable Energy Synchronous Generator(ReSG).By this way,some good characteristics of synchronous machine,for example,reliable inertia response,abilities of overcurrent and supporting reactive power and damping oscillation by excitation control,can be used as a supplement to power electronics to better serve the stable operation of power grid with high penetration of renewable energy resources.The main research contents and innovative achievements are as follows.(1)A novel grid-integration method——MGP based on synchronous machines for high-penetration renewable energy power grid is proposed,and its mathematical model and power angle characteristics are theoretically and experimentally studied.First,the structure and working principle of MGP are introduced in detail,the mathematical model of MGP is established and the power angle characteristics are studied.On this basis,the dual-machine model of MGP is proposed.Then,the MGP prototype system is established.Load mutation experiment and Power angle characteristics experiments are carried out.The results of load experiments show that MGP can be the power source of load and verify the reliability of MGP mathematical model.The simulation results of dual-machine model show that this model can effectively improve the simulation speed of MGP and lay a foundation for the follow?up research of MGP.The results of the power angle characteristic experiments show that the frequency difference between the two AC systems connected by MGP will cause the voltage phase difference between the two sides to change,which will cause the change of the transmission power.Such power angle characteristics are of great significance for the study of MGP power control methods.(2)Based on the power angle characteristics of MGP,the power feedback control method suitable for MGP grid-connected is proposed.First,based on the power angle characteristics of MGP,the source network phase difference control method is proposed,that is,the phase difference of the AC system at both ends of the MGP is adjusted to control the power transmission under the condition that the frequencies of the two AC systems are consistent.Considering the fluctuation of the grid frequency,the power feedback control method is proposed,and the stability of the control system and the control parameter selection method are analyzed.Then,The source network phase difference control method was first tested on the 2kW prototype.The experiment result shows that this control method is too sensitive to the frequency fluctuation of the power grid.Therefore,the power feedback control method was proposed.The feasibility of the method was verified by experiments.The results show that the proposed power feedback control method can achieve the MGP's grid-connected control,which lays a foundation for the renewable energy to connect to the network through MGP.(3)The DC voltage feedback control method for renewable energy integrating by MGP is proposed,and the self-synchronization characteristic of renewable energy integrating by MGP is analyzed.First,the DC voltage feedback control of the photovoltaic panel and the Direct drive wind turbine integrated by MGP is proposed based on the DC voltage fluctuation characteristics of the renewable energy grid-integrated inverter,the PV operating characteristics of the photovoltaic panel stable operation range,and the power angle characteristics of the MGP.The self-synchronization characteristic of renewable energy integrating by MGP is analyzed,so that the renewable energy can run synchronously without collecting the grid frequency signal.Then,the grid-connected experiment of photovoltaic simulator integrating by MGP is carried out on the 5kW-MGP grid-connected prototype system.The power following characteristic of the MGP system under the conditions of the photovoltaic simulator's light intensity and ambient temperature changes is studied.The results show that MGP can effectively follow the power fluctuation of renewable energy with DC voltage feedback control method,and the renewable energy can get self-synchronization characteristic by MGP.The renewable energy does not need to rely on grid voltage for reference to run,which offers the possibility for renewable energy sources to actively maintain grid stability.(4)Analysis of the inertia lifting effect of MGPFirst,the experiment of load change for inverter and MGP is compared.The frequency fluctuation characteristics of the two operating modes are compared when the load is abrupt.Then,the unique damping characteristic of the MGP's two synchronous machine excitation systems is analyzed.The damping effects of MGP and simulated thermal power units are compared by experiment.The results show that MGP can improve the inertia of renewable energy,reduce the frequency fluctuation under power imbalance,and provide sufficient damping to suppress the frequency fluctuation of the grid.(5)Analysis of the voltage isolation of MGPFirst,the isolation effect of MGP on voltage surge and sag on the grid side is studied by transient simulation.The voltage and current waveforms of transient overvoltage and voltage sag from the case of PV integrated to the grid through inverter and MGP are compared by prototype experiment.The results show that MGP can isolate the adverse effects between the renewable energy electric field and the grid,and prevent the renewable energy from being disconnected due to grid voltage fluctuations.