Research On Grid Stability With High Penetration Of Renewable Energy Adopting A New Way Of Grid-connection

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.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)Preliminary demonstration of feasibility for a new way of grid-connection by theoretical analysis and experimentsFirst,the structure and working mechanism of MGP is introduced and the working efficiency is discussed.Second,mathematical models of MGP is established and the rotor angle characteristic is studied.Then,a active power control method is proposed.On this basis,experimental systems are established and some experiments are done,including load experiment,rotor angle experiment and active power control experiment.The results show that MGP can operate with load,be connected to the grid and control its active power.The voltage phase difference in each side of MGP is due to rotor angle caused by change of active power,which is significant to power control and stability modeling.(2)Establishment of comprehensive small signal models and transient simulation model of MGP and analysis of its basic characteristicsFirst,the models of classical synchronous machine,constant and automatic excitation,renewable energy resources using MGP for grid-connection and phase locked loop are established based on rotor angle characteristic.The transient simulation model of MGP is established based on swing equations.Then,the inertia level of MGP is evaluated and damping characteristic is analyzed by solving the output damping torque component of MGP.The results show that the proposed modeling methods can reflect the whole output characteristics of MGP and MGP can provide higher inertia and damping due to two synchronous machines and their exciters.(3)Analysis of small signal stability by calculation and experimentsFirst,testing system of single machine infinite bus and three machine nine bus systems are established.Second,the small signal stability is calculated by using the models in(2)and the effect MGP has on power grid is analyzed.Then,load disturbance experiments are done considering different loads and excitation ways.The results show that MGP can isolate and weaken adverse oscillation modes and improve original modes.(4)Analysis of transient rotor angle stability and isolation effects of MGPFirst,the dynamic process of rotor angle of MGP is analyzed by equal area criterion,numeric calculation and transient simulation,considering fault occurs in the side of grid and fluctuation of renewable energy.Then,the isolation effects for voltage,current and frequency disturbance in grid side is analyzed by transient simulation.The voltage of different point of MGP systems is measured and analyzed on the experimental system.On this basis,the attenuation effect MGP has on non-power frequency components is studied.The results show that MGP can improve rotor angle stability during grid fault,cope with fluctuation of renewable energy and isolate adverse influence between renewable energy resources and the grid.