Research On Control Strategy Of Grid-Connected Inverter Considering Phase-Locked Loop In Weak Grid

With the increasing scarcity of traditional fossil energy and the increasing environmental pollution,the development and utilization of new energy has become very important.Among them,the solar energy industry has been strongly supported because of its advantages of green and environmental protection.For LCL type inverters,phase-locked loop(PLL)is usually required to detect phase information of common coupling point voltage to ensure synchronization of grid-connected current and grid voltage.However,in the weak grid,there is a strong coupling between PLL and power grid impedance.The resulting negative phase shift will have a great impact on the output impedance of the inverter,making the grid connected inverter control system less robust and even difficult to operate stably.In view of this,this paper takes the three-phase grid connected inverter with phase-locked loop structure as the research object.From the perspective of introducing additional loop filter and optimizing the inherent structure of phase-locked loop,two optimization and improvement strategies are proposed to solve the instability problem caused by PLL.Firstly,the mathematical modeling of grid connected inverter and the parameter design of LCL filter and controller are carried out under the ideal grid condition.The transfer function of PLL control loop is given,and the equivalent impedance model of grid connected inverter with PLL structure under weak grid is deduced.The impedance stability criterion is used to analyze the stability of the control system for subsequent research.Then,from the perspective of additional filter,the instability mechanism of the system is deduced.It is analyzed that the equivalent output impedance of grid connected inverter will introduce an additional term of phase lag when considering PLL.Therefore,a novel PLL compensation control strategy for first-order complex vector filter is proposed in this paper.The new first-order complex filter is connected in series in front of the PLL and the common coupling point voltage feedforward channel to weaken the influence of the additional term,and the stability is analyzed by using the inverter equivalent impedance model.Theoretical research shows that this method can make the system still have sufficient phase margin when the grid impedance is large,and improve the robustness of grid connected inverter.Secondly,from the perspective of optimizing the structure of PLL.In order to make the performance of PLL more excellent in weak current network,a new first-order PLL is presented.The PLL has additional inertia and damping links,so it can well adapt to the sudden change of system operation state.In order to further broaden the adaptive range of grid connected inverter to grid impedance,a complex filter link is added before the new phase-locked loop.The proposed comprehensive improvement method can greatly improve the stability of the inverter system on the premise of ensuring the basic functions of the phase-locked loop.Finally,it is verified by MATLAB/Simulink simulation platform and real-time digital controller rtu-box204 experimental platform.The analysis results are consistent with the theoretical derivation,which verifies the effectiveness and rationality of the two improved methods.