Indirect minimization of common-mode voltage with finite control-set model predictive control in a five-level inverter

Abstract

Multilevel inverters (MLIs) are counted as the crucial part of the electric drive systems (EDSs) primarily due to their ability to handle higher operating powers and voltages, to provide high-quality output waveform, and to reduce dv/dt. Nevertheless, MLIs require a complex control system to handle multiple control objectives such as load currents, floating capacitors (FCs) voltage, current tracking errors, and common-mode voltage (CMV) minimization. Among them, the load current and FC voltage objectives are associated with the basic operation of the MLI, whereas the CMV minimization is associated with the safety and reliability of the MLI-fed EDSs. Hence, the CMV minimization becomes obligatory to ensure the safety of the system. To achieve multiple objectives of MLIs, finite control-set model predictive control (FCSMPC) methods became promising solutions due to the following features: (i) ease of implementation, (ii) intuitive philosophy, (iii) fast transient response, (iv) ability to handle multiple objectives with a single cost function, (v) easy to compensate control delay, and (vi) flexible to include system constraints and nonlinearities. However, FCS-MPC methods use a cost function with weighting factor to directly minimize the CMV of MLIs. The improper selection of weighting factors directly affects the current harmonic distortion of MLIs. They also need higher execution time to implement in real-time control platforms. To address the challenges associated with the conventional FCS-MPC methods, a new per-phase FCS-MPC philosophy is proposed in this thesis. The proposed philosophy minimizes the CMV without using a cost function, thereby eliminating the need of weighting factors and their impact on current harmonic distortion. Also, the proposed FCS-MPC is designed to achieve the control objectives of each phase by using an independent cost function, resulting in a shorter execution time. The proposed philosophy is applied to a five-level inverter (FLI). The continuous-time and discrete-time models of FLI are developed to implement the proposed FCS-MPC. Finally, the steady-state and transient performances are verified on dSPACE-DS1103 controlled FLI laboratory prototype. Also, a comparative analysis of the proposed and conventional FCS-MPC is presented.