Research On Enhanced Single Phase Locked Loop Based On Sliding Mode Control

Phase-locked loop(PLL)is one of the key technologies of the grid-connected new energy system.Its main function is to provide the phase and frequency of the base frequency component of grid voltage for grid-connected new energy system.The PLL of power system can be divided into single-phase PLL and three-phase PLL.Because the three-phase PLL has the natural three-phase signal,the controller can be designed in different coordinate systems,and the control method is flexible and changeable.However,the single-phase PLL has only one input signal,so it is more difficult to implement than the three-phase PLL.In addition,the theoretical analysis and control methods for the single-phase PLL can also be applied to the three-phase PLL.Therefore,this paper takes the single-phase PLL as the research object,and studies the problems of the DC offset suppression and insufficient dynamic response performance.The main work of this paper is as follows:(1)The DC offset suppression of single-phase PLLThe common single-phase PLL technology based on orthogonal signal generator usually uses a second-order generalized integrator to generate a twophase orthogonal signal.However,the traditional second-order generalized integrator structure cannot suppress the DC offset of the input signal.In this paper,the causes of DC offset and its adverse effects are analyzed.Aiming at the DC offset suppression,an all-pass filter is introduced to strengthen the traditional second-order generalized integrator structure,and a new enhanced structure is formed.By comparing with other common DC suppression methods,it is proved that the single-phase PLL based on the second-order generalized integrator combined with the all-pass filter has better suppression of DC bias and better dynamic response performance.In addition,the frequency adaptive design is introduced to strengthen the proposed single-phase PLL structure considering the fluctuation of voltage and frequency of the actual power network.(2)Dynamic performance improvement of the single-phase PLLThe suppression of the DC offset often causes the decrease of the dynamic response speed of the PLL.In addition,the requirements for the dynamic response speed of the PLL are often higher in specific occasions such as micro-grid and voltage sag.If the traditional PI control is used,the PLL dynamic performance may be insufficient.In order to solve the problem of insufficient dynamic response performance of the PLL,sliding mode variable structure control is introduced to replace the PI control that is widely used in the PLL.The switching function and the design of sliding mode variable structure control rate are design keys of sliding mode variable structure control.Aiming at the inherent chattering phenomenon of sliding mode variable structure control,this study combines fuzzy control to suppress it.The MATLAB/Simulink simulation proves that the buffeting of the system can be effectively suppressed by the combination of fuzzy control,and the dynamic response speed of the single-phase PLL based on fuzzy sliding mode control is faster than that of the traditional PI control.Finally,an experimental platform is built to verify the effectiveness and feasibility of the proposed DC suppression method and the dynamic response performance improvement method.