Resonant Current Regulator Implemented in the Discrete-Time Domain for Grid-Connected Converters

Effective current control is an aspect of paramount importance to achieve a high level of performance for grid-connected converters (GCCs). A common practice is to first develop a continuous-time controller and then convert it to a discrete equivalent using one of the discretization techniques available. However, determining the optimum choice is elaborating, and the performance of the discretized controller has been shown degraded performance compared to that of continuous version, especially for cases in which switching frequency is relatively low with respect to the excitation frequency. The principle of the current controller design in the discrete-time domain has been comprehensively addressed in this work, which can be summarized as three major steps. The first step is to develop the controller in the positive-sequence synchronous reference frame (PS-SRF), by canceling the weakly damped pole of the plant with a matching zero and providing infinite open-loop gain for the dc current in the PS-SRF. Then, the corresponding controller for the negative-sequence current regulation can be derived, following a similar procedure. Based on the double SRF control (DSRFC), the resonant current controller implemented in the stationary reference frame can be finally developed. In this manner, the discretization is avoided, and the one-sampling delay and the zero-order holder characteristic of the converter can be considered in the developed controller, which means that the additional delay compensation strategy can be avoided. Experimental results have verified the effectiveness of the proposed controller for the current regulation of GCCs under different grid conditions. It should be noted that the proposed method can also be applied to other applications, such as
$LCL$
-filtered grid-connected or
$LC$
-filtered grid-forming converters with proper active damping although only the simpler
$L$
-filtered GCC is analyzed as an example in this work.