Study On The Active Control Strategy Of The Three-phase Grid-connected Converter

With large-scale distributed generation systems being connected to the power grid, more and more micro-grid characteristics appear in the power grid. Grid impedance characteristic becomes more obvious and the unbalance grid voltage or distortion of non-ideal operating state has occurred occasionally. At this time, the traditional synchronous coordinate system phase-locked loop(SRF-PLL) will not meet the requirements of the network synchronization for the grid converter. And the large grid impedance will greatly reduce the phase margin of the LCL grid converter, thus resulting in harmonic current increasing, and may lead to resonance in severe cases, which has great threat to the stable operation of the converter. Moreover, there may be asymmetries in grid impedance, the active impedance detection method with one single harmonic current injection will lead to erroneous result.Firstly, the LCL grid converter mathematical models,considering the grid impedance,in the stationary coordinate system, two-phase stationary coordinate system and synchronous rotating coordinate system are given out.Secondly, multiple frequency adaptive complex filters, harmonic decoupling network and phase-locked loop are adopted for grid voltage positive-negative sequence and harmonic separation. The method overcomes the disadvantages of the conventional phase-locked loop and can achieve fast and accurate grid synchronization under non-ideal grid conditions.Thirdly, the grid converter should have islanding detection function. In this paper, the active power grid impedance detection based on a single non-characteristic harmonic current injected is adopted. Since there may be asymmetry in the grid impedance, two methods that are both based on the non-characteristic harmonic current injection are proposed.The grid impedance has a greater impact on the stability of the converter. Grid-converter stability criterion based on impedance ratio is proposed in this paper. The output impedance of the inverter with different current controllers and the influence of phase-locked loop to the converter output impedance are analyzed. Active control strategy based on the impedance detection is adopted to overcome the small margin problem of the converter.Finally, in the MATLAB / Simulink, non-ideal grid synchronization, grid impedance detection and active control strategy are conducted comprehensively. And a three-phase inverter experimental platform is built for the experimental verification of the grid impedence detection and the active control strstegy. The simulation and experiment results prove the validity and feasibility of the proposed methods in this paper.