Compound Feedback for Current-Controlled Grid-Side Inverters With LCL Filters

Abstract

Мost grid-forming and grid-following inverters contain an LCL output filter and an internal current controller. The resonant nature of the filter interferes with the injection of high-quality grid currents, degrades disturbance rejection, and compromises stability. Although the subject is quite mature with many innovative control solutions already available, the character of the step response of the grid current and the closed-loop bandwidth obtained with the LCL filters remain far below the performance obtained with the L filter. Most solutions essentially depend on the mutual ratios of resonant frequency, sampling rate, and target bandwidth. In contemporary grids, grid-side inverters should meet conflicting requirements of significantly faster grid-current response with significantly less injection of pulse width modulation (PWM) ripple. To overcome the shortcomings and meet these objectives, a novel control solution is proposed with analytically derived control actions, which ensure internal and external stability, provide a robust and well-damped step response of the grid current with an unprecedented bandwidth, and ensure a stable, resonance-free operation with uncertain grids. The achievement of the stated objectives is confirmed experimentally by carrying out a series of tests in an installation with industrial grid inverters.