Research On The Control Strategy Of Active Power Filter In Microgrid

In recent years,the rapid development of power electronics has brought great convenience to human beings,but the introduction of a large number of nonlinear and unbalanced loads has also posed a great threat to the power grid,namely harmonic pollution.The traditional passive power filter(PPF)is simple in structure,but it can only eliminate the harmonic of a specific frequency,and it is easy to resonate with the power grid.In this paper,a shunt active power filter(SAPF)is chosen as the research object,aiming at mainly studying the nonlinear control strategy of its inner-loop current to simplify the design of controller and improve the effect of filtering,so as to lay the foundation for the practicality of SAPF.First,a most typical three-phase three-wire two-level SAPF is selected as the object of research in this chapter,and its mathematical model is established under static abc and rotating dq coordinate system respectively.Based on this model,the i_p-i_q harmonic detection method based on instantaneous reactive power theory is used for harmonic current detection,and the traditional double-loop PI control system of outer voltage loop and inner current loop is designed.The simulation results show that the traditional control strategy will be limited by the gain and bandwidth when tracking the harmonic current,and the effect of compensation is not ideal.Secondly,for the shortcomings of the traditional PI control strategy,the harmonic current must be detected and the instruction current calculated,the traditional one-cycle control strategy has been improved in this paper,and then an improved one-cycle vector control method is introduced into SAPF.The simulation results show that SAPF can effectively elimilate the harmonic and reactive current under the proposed method,and can also realize the control target under the unbalanced power grid.And then,in view of the shortcomings of three-phase three-wire SAPF ignoring the processing of zero-sequence components,the three-phase four-wire SAPF is taken as the research object,and its Euler-Lagrange mathematical model in dq0 coordinate system is established.At the same time,aiming at avoiding the weaknesses of the traditional double-loop PI control strategy,such as complex structure and many parameters need to be adjusted,the nonlinear passive control theory is introduced into the inner-loop current control of SAPF,and the damping injection method is used to simplify the design.The simulation results show that the SAPF can significantly improve the dynamic response speed of SAPF and realize the further sinusoidal current of the power grid.Finally,in order to solve the limitation of voltage level and voltage withstand value of switch devices of two-level SAPF,the NPC-type three-level SAPF is chosen as the object of study and the nonlinear control strategy based on Lyapunov function is derived in detail in this paper.And from the point of view of stability,the appropriate control parameters are selected to ensure the steady-state and dynamic performance of the system under balanced or unbalanced system.And then,for the problem of the unbalanced DC side voltage,a control method based on the principle of charge balance is used to change the time between positive and negative small vectors in SVPWM modulation algorithm.The simulation results show that the nonlinear control strategy needs fewer parameters to adjust,and can further reduce the harmonic content and maintain the DC-side voltage balance under the condition of grid balance/unbalance.