Research On Control Strategy Of Static Synchronous Compensator Under Frequency Fluctuation Of Power Grid

With the development of science and industrialization,the problem of power quality is becoming more and more serious because of the reactive power and harmonic sources.Static synchronous compensator(STATCOM)is widely used as a device which can dynamically compensate the reactive power and harmonics of power grid.With the wide range of new energy equipment connected to the power grid,the randomness and intermittence of its output power will lead to the fluctuation of power grid frequency,which will reduce the compensation accuracy of STATCOM.In this paper,the three-phase two-level voltage STATCOM is taken as the research object,and the frequency fluctuation of power grid is taken as the research background.Aiming at the problem that the traditional repetitive control has poor adaptability to the frequency fluctuation of power grid,an improved composite control strategy is designed,and the theoretical analysis and simulation experiment are carried out.This paper is mainly divided into the following points:(1)Basic theory and control structure analysis of three-phase voltage type STATCOM.The working principle of three-phase STATCOM is introduced,and the mathematical models of the system in three-phase stationary coordinate system and two-phase rotating coordinate system are derived.The d-q command compensation current detection method based on Park transformation and the principle of software phase-locked loop based on biquad generalized integration are briefly described.The current decoupling control method in two-phase rotating coordinate system is analyzed,and a voltage and current double closed-loop control strategy is designed based on this.(2)Analysis of traditional compound control strategy.Firstly,the DC voltage outer loop controller is designed by the typical ? system correction method,and the parameters of the voltage loop controller are adjusted by the"oscillation index method";the current inner loop controller is designed by the repetitive control method,aiming at the problem that the response speed of the traditional repetitive control is slow,the composite proportional control method is adopted to improve the response speed of the current loop controller.(3)A compound control strategy against grid frequency is designed.Aiming at the problem of poor adaptability of traditional repetitive control to power grid frequency fluctuation,a FIR digital filter based on Lagrange interpolation polynomial is used to approach the fractional order phase lag link.Combining with the real-time power grid frequency obtained by the phase-locked loop,an adaptive fractional order repetitive control internal model is obtained to improve the anti power grid frequency fluctuation ability of the controller.To solve the problem of large amount of computation in traditional repetitive control,the idea of multi rate repetitive control is introduced to improve the stability margin of the composite controller and reduce the occupation of digital system resources by the algorithm.The controller characteristics of the improved composite control strategy are analyzed,and the stability judgment conditions and the controller parameter design method are given.(4)The feasibility of the design is verified by simulation.Firstly,the parameter characteristics of the composite control strategy are simulated and analyzed,and then the performance of the traditional composite control strategy and the improved composite control strategy are compared under different grid frequencies.(5)A prototype experimental platforam with DSP TMS320F28335 as the control core is set up and physical experiments are carried out.On the hardware side,the design of sampling circuit and driving circuit are introduced.On the software side,the code generation method based on model design of MATLAB is used,and the core control part code is designed from top to bottom with the thought of three-segment finite state machine.The program flow chart of the project code is also introduced.Experimental results show that the compound control strategy based on adaptive fractional order repetitive control in this paper can improve the system's adaptability to grid frequency fluctuations,but the algorithm has a large amount of calculation;while the control strategy has adaptability to grid frequency fluctuations,it can also reduce the calculation amount of the algorithm and improve the system stability margin,but the compensation accuracy of the algorithm is reduced.The experiment proves the validity and feasibility of the designed scheme.