Research On Control Scheme Of Gridconnected Inverter Under Unbalanced Grid Voltage In Stationary Frame

For the wind turbine can be decoupled to the grid in the directly-driven with permanent magnet synchronous generator(PMSG) system, the stability and efficiency of the PMSG system are improved compared with the doubly-fed induction generator(DFIG) system. However, due to the grid side inverter is directely connected to the grid, the negative sequence component of the unbalanced grid voltage will lead to harmonics in the output power and currents. This paper studied about the grid-connected inverter under unbalanced grid voltage in stationary frame, proposed an adaptive law for separatiing positive and negative sequence components, and then designed the corresponding contorller to the inverter, finally adressed several coordinated control strategies of different targets.How to separate the positive and negtive sequence of the unblanced grid voltage rapidly and accurately is the fundamental work for controlling the inverter. This paper analyzes the unbalanced grid voltage based on the symmetrical component method. Then an improved adaptive observer is proposed to seperate the positive and negetive sequences according to the character of the mathematical model of the traditional adaptive observer, meanwhile the stability of the designed observer is proved. At last, a simulation is bulit in the PLECS software, and the simulation results verify that the method proposed in this paper can separate the positive and negtive sequence under the unbalanced condition, and it can still work even under the frequency distortion and phase mutation caused by grid voltage.According to the character of the mathematical model in stationary frame, a full-order sliding mode controller of the grid-connected inverter is proposed for the inner current loop. The aim of designing the full-order sliding mode controller is to trace the current references rapidly without singularaty and chattering phenomenon. The other advantage of the designed controller is its simple structure, because Phase Locked Loop(PLL), feedforward decoupling and rotating coordinate transformation can all be eliminated. Then considering the power relationship between the turbine side and grid side, the DC voltage outer loop adopted a Proportional Integral(PI) regulator and the power of the turbine side is adopted as feedforword which can improve the dynamic response speed of the outer loop. And the correctness and effectiveness of the proposed control strategy is validated by simulation results.The negative sequence component of the unbalanced grid voltage will have a bad effect on the quality of the output power of the inverter. In order to solve the above problem, four different current references are caculated to achieve different targets. The four different current references can achieve the purposes of balanced instantaneous power, sinusoidal currents without harmonic components, balanced active power and balanced reactive power, seperately. For the reason that the different targets can not be achieved at the same time and they affect each other normally, the coordinated control schem is adopted in this paper. Simulation results show that different current references can achieve the desired purposes effectly, and the coordinated control can improve the quality of the output power reasonable and flexible.A 3k W experimental platform of the PMSG system is set up to validate the practical feasibility of the theoretical analysis. The components of the system are designed as well as the software program based on TMS320F2812. A series of experiments are implemented, containing separation scheme of positive and negative sequence, control stratagy of the inverter and the different targets including the coordinated control. The experimental results and analysis are showed to verify the correctness and practical feasibility of the theoretical analysis.