Stability Analysis Of LCL-type Grid-connected Converters

As an important device in power electronics, three-phase voltage-source converters(VSCs) are widely used in many applications, such as grid-integration of renewable energy, energy storage, high-voltage dc transmission and flexible ac transmission systems, which are combined together to form the converter-grid systems. However, with the existence of weak grid especially in remote areas and the complexity of grid because of the paralleled inverters, VSCs can be bothered by the stability issues, especially the resonances caused by the interactions of impedances. Therefore it’s necessary to analyze the resonant problems and improve the stability margin from the point of the output impedances of VSCs.To the VSC with LCL filter, according to different sampling currents and different control frame of axis, there mainly exist four control schemes, which are phase-domain grid-side current(GC) feedback control, dq-domain GC feedback control, phase-domain inverter-side current(CC) feedback control and dq-domain CC feedback control. The limited impedances of VSCs can be influenced by filters and control loops. Different control schemes present some differences in their corresponding impedances. So this paper analyzes the stability problems existing in inverter-grid systems in the full frequency range by modeling the output impedances of the VSC, the analysis is verified by the simulation and experiments.Firstly, to eliminate the high-frequency resonance, this paper adopts an active damping method basing on the feedback of the filter capacitor current. In the view of transport delays in a digital system, this paper calculate the feedback coefficient out of the consideration of the damping effects and the system stability, which can ensure that the resonance peak will not appear in the damped LCL-filter with the variation of the grid impedance. After that, this paper models the output positive- and negative-sequence impedance of the VSC with different control schemes directly in the frequency domain using the harmonic linearization method to avoid the coupling and nonlinearities in three-phase systems. The modeled impedances are verified by a frequency sweep method in Plecs. Then this paper takes the positive-sequence for example to characterize the impedance in high, medium and low frequency range and analyzes the dominant factor in different frequency range, which gives an indispensible reference to the stability analysis.This paper applies the impedance-based stability analysis approach to analyze the interaction between the output impedances of the inverter and the grid. By comparing the stability the VSC with different control schemes with the increase of the grid impedance, this paper derives the the best control scheme to the LCL-type inverters to adapt to the varying grid. On the basis of the above analysis, this paper make further analysis of control parameters contributing to the improvement of stability in weak grid. The simulation results not only validate the accuracy of the impedance models as well as their effectiveness applied to the comparative analysis of stability margin quantitatively, but also verify the theoretical analysis relating to the stability issues in grid-converter systems. Finally, this paper establishes the corresponding experimental platform to verify the study in this paper.