A Real-Time Computation Method With Dual Sampling Mode to Improve the Current Control Performance of the $LCL$-Type Grid-Connected Inverter

Abstract

Due to the higher attenuation of switching frequency current harmonics, the LCL filter has been widely used in grid-connected inverters. To deal with the resonance of the LCL filter, the capacitor current is usually fed back to damp the resonance actively. However, the computation and pulsewidth modulation (PWM) delays in the digital control system have a significant influence on the active damping method, resulting in poor system robustness. Meanwhile, these delays also reduce the control bandwidth greatly and thus impose a severe limitation on the low-frequency gains. In this paper, a real-time computation method with dual sampling mode is proposed to remove the computation delay from the inner active damping loop and the outer grid-current control loop simultaneously; thus, the system robustness and the control performance can be greatly improved. Moreover, the time duration between the sampling instant and the switching transition of the inverter bridge is extended by the proposed method, which effectively prevents the switching noise distorting the sampled signals. Therefore, the noise immunity of the inverter is also improved greatly. Experimental results from a 6-kW LCL-type single-phase grid-connected inverter confirm the theoretical expectations and the effectiveness of the proposed method.