Second Order Sliding Mode control for three-level NPC converters via extended state observer

In this paper, a model based robust control for three-phase three-level Neutral Point Clamped (NPC) power converters is studied. Based on the continuous averaged model of the system, a Second Order Sliding Mode (SOSM) technique is employed to the control design. The control objectives are to achieve desired dc-link capacitor voltage regulation, voltage balance in the two dc-link capacitors and the instantaneous active and reactive power tracking. In order to achieve fast dynamic response of the proposed controller in the presence of external disturbances, an Extended State Observer (ESO) is employed to asymptotically reject the disturbances which are integrated in the controller design. The proposed control strategy has a cascaded structure which consists of power tracking (inner loop) ESO-based dc-link voltage regulation and capacitors voltage balance (outer loop). Multi-rate simulation illustrates the effectiveness and robustness of the proposed controller under parametric uncertainties and load variations.