A Robust Grid-Voltage-Modulated Direct Power Control Strategy for Three-Phase Four-Leg Rectifiers Under Uncertainties and Disturbances Using Super-Twisting and Nonlinear Varying-Gain Observer

This paper presents an enhanced control strategy for a three-phase four-wire four-leg rectifier (3P4W4LR), aimed at improving dynamic and steady-state performances, disturbance rejection, robustness, and implementation efficiency. A nonlinear varying-gain observer (NVGO) integrated with a simple PI controller is proposed for the outer loop to regulate the DC-link voltage under different internal and external disturbances. Unlike traditional disturbance observers, which are highly sensitive to measurement noise and compromise transient response when gains are reduced, the proposed NVGO employs a variable-gain mechanism. High gain value is used during transients to ensure rapid disturbance estimation and rejection, while low gain value during steady-state minimizes noise sensitivity and estimation error. This results in superior disturbance rejection, improved dynamic response, reduced DC link voltage steady-state error, enhanced noise immunity, increased reliability, and reduced system size and cost. For the inner loop, a Grid-Voltage-Modulated Direct Power Control method based on super-twisting sliding mode control is introduced. Operating directly in the synchronous reference frame without relying on phase-locked loop or Park transformations, this approach simplifies implementation while delivering faster transient response, improved steady-state accuracy, and greater resilience to grid current system uncertainties and disturbances. To ensure robust performance, the closed-loop regional stability of the proposed NVGO with PI controller is rigorously analyzed using a piecewise quadratic Lyapunov function. The effectiveness and superiority of the proposed method are validated through both simulations and real-time hardware-in-the-loop testing. Extensive testing under dynamic load conditions, different grid voltage conditions, and DC-link capacitance uncertainty demonstrates the controller’s robust performance and practical viability.