Abstract
Multicell converters, based on power cells that use low-voltage semiconductors, implement AC motor drives for medium-and high-voltage applications. These converters feature an input multipulse transformer, which performs low-frequency harmonics cancelation generated by three-phase diode rectifiers in the power cells. Despite this advantage, the multipulse transformer is bulky, heavy, expensive, and must be designed according to the number of power cells required by a specific case, limiting the modularity of the topology. This work proposes a multicell converter based on power cells that requires a standard input transformer and uses active front-end rectifiers controlled by employing a finite control set-model predictive control algorithm. The proposed approach emulates the multipulse transformer harmonic cancelation owing to the predictive algorithm operation combined with input current references that are phase-shifted for each active front-end rectifier. Simultaneously, the DC voltages of the power cells are regulated and equalized among the cells using PI regulators. Experimental results confirm the feasibility of the proposed system as input currents in each Multicell AFE rectifier with a unitary displacement factor, and a low THD of 1.87% was obtained. It is then possible to replace the input multipulse transformer with standard ones while reducing the copper losses, reducing the K factor, and extending the modularity of the power cell to the input transformer.
Highlights
Medium AC drives based on multilevel converters have been increasingly applied in recent years [1]
It is appreciated that these input currents have a distortion, as shown in FIGURE 7(a), owing to the harmonics imposed by the current references
This is consistent with the mathematical modeling of the input current waveform in the active front end (AFE) rectifiers (1)–(3)
Summary
Medium AC drives based on multilevel converters have been increasingly applied in recent years [1]. Cascaded H-bridge-based converters with symmetrical DC voltages are widely used in industrial applications [4] This power topology is based on an input multipulse transformer, which feeds several three-phase to single-phase power cells. Other alternatives consider the use of modulating techniques developed especially for this type of topology using active front end (AFE) rectifiers [10]- [12] All these approaches require the use of a multipulse transformer, which must be designed according to the number of power cells used in each case, leading to a bulky and expensive transformer. This paper is organized as follows: Section II presents the mathematical development to determine the input current references of the power cells, concentrate the harmonic spectrum around 6 nc ± 1, and obtain a reduced THD in the input current of the AFE multicell rectifier.
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