Research On Instability Mechanism And Suppression Of Large Scale Grid Connected Photovoltaic (PV) System Under Reactive Power Compensation

Large-scale grid-connected PV system is mainly in remote areas due to its large scale,and its power needs to be transmitted by the high-voltage transmission system to loads center.Due to the existence of transformers and high-voltage transmission lines,the equivalent grid impedance of the system can not be ignored.Given the large capacity and the large output power of the PV system,the existence of the equivalent grid impedance is prone to lead to voltage instability.Reactive power compensation(RPC)is commonly used in solving the voltage instability problem.However,it is found out that RPC can change system operation characteristics,and lead to the risk of system instability.As PV system is mainly constituted by inverters,the stability of inverter is crucial to system overall stability.For the instability caused by RPC,this thesis takes the inverter as the main research topic,establishes the equivalent model of the PV system,analyzes the instability caused by RPC,and puts forward corresponding solutions for stability improvement.The work in this thesis is the extension of the project “Resonance Mechanism and Suppression Strategy Research of the Large-scale Grid-connected Photovoltaic System” based on the national natural foundation(gratanted unmber51477021),and can be concluded as follows:Firstly,the voltage instability in large PV plants caused by grid impedance is analyzed and RPC such as series compensation(SC)and parallel compensation(PC)are introduced to solve this instability.The typical structure of the large-scale grid-connected PV system and the control mode of the inverter are reviewed.The grid impedance is modeled into the grid-connected system with one inverter.The equivalent grid impdedance considering the system capacity parameter and grid impedance is established,and the related effect of the impedance on the voltage stability in inverter output side and dc side is analyzed and RPC is utilized in solving the voltage problems.The theories in this section are the fundation for further researches in this thesis.Then,based on the RPC in PV system,the instability mechanism of the large-scale PV system with SC is revealed.The mechanism of the instability dominated by current inner loop and the phase-locked loop(PLL)in inverter control system is investigated.In the analysis of the current inner loop,the inverter current control model considering SC is established,and the effect of it on inverter current control is analyzed by Bode diagram.It is found out that SC has little influence on the current control loop,and the stability is only affected by system equivalent grid inductor.In the analysis of PLL,the nonlinear model of PLL is established and the SC is considered.The system stability curve is presented,and the mechanism of how SC affects PLL's control is revealed.It is found that SC can effectively guarantee the system stable operation point at fundamental frequency point,however,it will lead to frequency oscillation and the reduction of system stability range in sub-synchronous frequency range,which will lead to the PLL dominated system instability.Next,this thesis revealed the mechanism of system instability caused by PC,where the parallel capacitor and parallel static-var-generator(SVG)are investigated.It is found out that SVG has little influence on system control stability,while parallel capacitor has big effect on both inverter current control and PLL's control.On the one hand,parallel capacitor introduces a notch peak in inverter current control loop reducing system phase margin and making current control system unstable.On the other hand,parallel capacitor affects the stability curve in super-synchronous frequency range,reduces system stability range and causes PLL to work on super-synchronous frequency point,resulting in PLL dominated system instability.The research results have revealed the mechanism of system instability caused by RPC,and can provide theoretical support for further suppressing of the instability.Finally,for the inverter control instability caused by RPC,this thesis puts forward the corresponding stability improvement methods for inverter current control loop and PLL to improve the stability of whole PV system.For the instability dominated by the current loop,by designing the lead compensation,this thesis effectively suppresses the influence of the resonance notch introduced by parallel capacitor.For the instability dominated by PLL,this thesis proposes the methods of utilizing band-pass filter and reducing PLL controller parameters to limit the frequency oscillation and then guarantee system stability.The theories concluded above are finally verified by simulation and experiment results.The researches in this thesis can provide theoretical support for further development of the large-scale grid-connected PV system and the application of RPC.