Virtual impedance-based active damping for LCL resonance in grid-connected voltage source inverters with grid current feedback

Abstract

Grid-connected Voltage Source Inverter (VSI) with LCL filters, controlled by proportional-resonant compensator in αβ frame, is very popular in utility application for its better power quality feature, while the inherent LCL resonance characteristic limits the control loop bandwidth and threatens the system stability. The active damping is essential to dampen the LCL resonance. This paper investigates the active damping method with only grid current feedback. Compared with the most widely used capacitor current feedback active damping, this novel method avoids the use of extra sensors to detect the capacitor current, which possesses the merits of lower cost and higher reliability. By performing the equivalent control block diagram transformation, it is revealed that a virtual impedance in series or parallel with the grid-side inductor can ideally be implemented with specific active damping controllers, which also comprehensively reflects the physical circuit property of grid current feedback active damping. However, the second-order derivative terms are needed in the ideal forms of the active damping controllers which can hardly be realized in practice. To deal with this issue, a second-order transfer function is proposed to approximate the s2 term in the required frequency range for realizing the similar LCL resonance damping performance. Further, a straightforward design method to determine the parameters of the active damping controller is introduced. Finally, the effectiveness of the proposed control method and relevant design strategies are verified both in time and frequency domain.