Stability Analysis of Three-Loop Control for Three-Phase Voltage Source Inverter Interfaced to the Grid Based on State Variable Estimation

Grid-tied inverters and associated control techniques have gained importance in the domain of distributed generation. Different methods of control have been compared in the literature depending upon the ability to meet total harmonic distortion limits, damping offered to resonant oscillations, and stable operation. Multiple-loop methods compared to their single-loop counterparts have proved to be highly effective for meeting the grid regulations. In the literature, while single-loop and two-loop control systems for LCL filter have been assessed, not much study on three-loop control (TLC) has been done, possibly due to the hardware complexity and cost imparted by the increase in the number of sensors. In this paper, a TLC structure for grid connected inverters is analyzed for stability and parametric variations. Robustness of TLC is compared with single-loop grid current feedback control and a two-loop control structure. An approach to estimate the state variables is proposed in this paper, which cuts down the number of sensors and cost of implementation. The performance of the system is analyzed through simulations and results are validated in a hardware setup, under dynamic and steady-state conditions, thereby showing that this is a promising approach.