Disturbance observer-based composite voltage synchronisation control of three-phase four-leg inverter under load variation

The three-phase four-leg inverter is widely used in microgrid recently. The control strategy is critical for the performance of nonlinear four-leg inverter system owing to the problems of coupling, disturbances of nonlinear/unbalanced loads, variable operating condition, etc. In view of the above problems, a nonlinear composite controller is proposed for four-leg inverter system under dq0 reference frame by integrating a higher-order sliding mode observer (HOSMO) and finite-time feedback control technique. The HOSMO can observe the uncertainties and disturbances by means of the soft sensing manner for feedforward compensation, which economises some sensors for their states being inconvenient to measure. In addition, it is a novel way for the HOSMO to realise the decoupling of the inverter system under dq0 frame. The finite-time feedback controller works in coordination with the HOSMO which provides more rapid convergence rate and further improves anti-disturbance capacity of the system. Finally, the simulation results with PI comparison illustrate the feasibility of the proposed method for stabilizing system.