Dr. Espinosa-Neira, Eduardo

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.

The use of advanced modulation and control schemes for power converters, such as a Feedback Quantizer and Predictive Control, is widely studied in the literature. This work focuses on improving the closed-loop modulation scheme called Feedback Quantizer, which is applied to a three-phase voltage source inverter. This scheme has the natural behavior of mitigating harmonics at low frequencies, which are detrimental to electrical equipment such as transformers. This modulation scheme also provides good tracking for the voltage reference at the fundamental frequency. On the other hand, the disadvantage of this scheme is that it has a variable switching frequency, creating a harmonic spectrum in frequency dispersion, and it also needs a small sampling time to obtain good results. The proposed scheme to improve the modulation scheme is based on a Discrete Space Vector with virtual vectors to obtain a better approximation of the optimal vectors for use in the algorithm. The proposal improves the conventional scheme at a high sampling time (200 μs), obtaining a THD less than 2% in the load current, decreases the noise created by the conventional scheme, and provides a fixed switching frequency. Experimental tests demonstrate the correct operation of the proposed scheme.