Control of high-frequency-link inverter using optimal switching sequence

Abstract

This paper presents an Optimized Sequence-Based-Control (OSBC) scheme for an isolated multi-stage high-frequency-link (HFL) converter, which directly controls the switching sequences of the power devices without the need for an intermediate pulse-width modulator as in conventional power converter control schemes. OSBC is realized by solving an optimization problem, determining an optimal switching sequence (comprising a union of feasible sequences) and corresponding time horizon for each switching state of the sequence. The proposed model-predictive switching control scheme provides faster transient response compared to the conventional PI control scheme in the ac/ac converter. Another interesting feature of the OSBC is multi-objective control capability through joint-optimization of multiple control goals. In fact, in order to meet various control objectives, a compromise should be made between them so as to achieve the optimized performance and this is even more crucial in capacitor-less topologies (since there is no buffer stage to prevent cross-coupling between various control actions). For the capacitor-less HFL inverter in this paper, joint-optimization of the following control goals are implemented using the OSBC scheme: 1- output voltage regulation, 2- flux balance control (FBC), and 3- loss mitigation. A case study with a sudden change in the input dc voltage characteristics is considered to verify the effectiveness of the joint-optimization of the above control goals in improvement of the HFL inverter operation. The results are verified using simulation for a 4.5 kW, 40 V/208 V three-phase inverter.