Study On Voltage Stability Control Renewable Energy Grid-connected System Based On Electric Spring

Today,with the depletion of fossil energy,vigorous development of renewable energy can reduce the dependence of the grid on fossil energy.On the other hand,the increasing penetration of renewable energy will bring new challenges to grid system.Studies have shown that when the proportion of renewable energy in the power grid reaches a certain level,it will have a serious impact on the stability of the power system.Maintaining the balance between power supply and demand is the basic principle for ensuring grid stability.But this task will become increasingly difficult as the share of renewable energy in grid continues to increase.At present,supply side management is the main means to deal with the intermittent and unstable renewable energy.Limiting power and droop control are typical methods of providing voltage and frequency support to the grid.In addition,the use of expensive energy storage equipment to maintain grid supply-to-demand balance is also an alternative approach.With the continuous upgrading of the power grid system,the traditional control paradigm of “supply following demand” in the grid can no longer meet the demand,and it is gradually being replaced by “demand following supply”.The “Demand-side management(DSM)” techniques that have been proposed currently includes load scheduling,direct load control,real-time pricing,and demand bidding.Although these DMS methods provide power supply companies with power generation prediction information,they can not guarantee the supply and demand side power can be balanced in real time.Electric Spring(ES)is a fast,automatic demand side response technology with a response time in the order of milliseconds.It can be used to stabilize grid voltage,adjust system frequency,improve power quality,and balance three-phase power in the grid.This thesis focuses on the control algorithm of electric springs to improve the stability of renewable energy grid-connected systems.The main research contents of this thesis are as follows:1 The basic principle,typical topology and working mode of ES are introduced,and the active and reactive capacity of ES-? and ES-B2 B intelligent loads are compared and analyzed.2 The existing control algorithm of ES-? is complex,which is not conducive to the direct tracking of AC signals on critical loads.An ES control strategy based on single-cycle control technology is proposed.The system collects the voltage,transmission line impedance,critical load and non-critical load resistance parameters of the grid system in real time to calculate the phase angle of the given voltage reference signal of the one-week controller lags behind the grid voltage signal,and control the ES to exchange reactive power with the grid,as well as transfer the power fluctuation of the grid to the non-critical load.The rationality of the algorithm is verified by building a simulation model in Matlab/Simulink.3 Based on ES-?,a simpler control algorithm is proposed.The control algorithm only needs to obtain the voltage RMS value on the critical load and the current phase flowing through the non-critical load to realize real-time control of the feeder voltage,which remains stable in real time.The rationality of the simple algorithm is proved by building a simulation model in Matlab/Simulink.4 The back-to-back electric spring(B2B-ES)topology and its control algorithm are studied.Aiming at the shortcomings of the control algorithm of the existing ES-B2 B,such as complex structure,not easy to debug and practical application.This paper proposes a multi-loop control algorithm based on PR controller to control the improved B2B-ES,which enables independent control of series ES and parallel ES,making debugging easier and increasing the practicality of B2B-ES.The simulation results show that the proposed improved ES-B2 B topology and its control algorithm can stabilize the voltage on the critical load perfectly.