Fault tolerant cost-effective carrierless stochastic synthesis of voltages and currents in multi-cell multilevel converters via the central limit theorem

The paper presents - with experimental evidence - a highly fault tolerant, cost-effective and carrierless solution to synthesize voltages (or currents) across the series (or parallel) connection of several voltage-source (or current-source) cells. Hundreds of voltage-source cells already characterize modern multilevel HVDC converters and structures based on current-source cells could become industrially meaningful too, if sufficient advancements in semiconductor and magnetic materials will occur. Exactly by relying on a large number of cells, the key approach is characterized by an unconventional absence of any determinism. The voltages (or currents) of all cells are generated as discrete independent random variables whose distribution is parameterized by the desired reference waveform to be synthesized. This is a radical difference from the already existing random PWM modulations, where only the switching instants are aleatory. The essence of the proposed method lies in observing that multilevel converters, which synthesize the desired waveforms through additive linear combinations (e.g. the simplest sum) of several elementary contributions, become naturally ruled by the Central Limit Theorem of the theory of probability. Such a fundamental law of nature assures also that the greater the number of cells, the more the waveform synthesis becomes inherently robust, fault-tolerant and accurate. This property strikingly differs from those of deterministic centrally ruled modulations and it is also achieved with reduced hardware complexity and cost. The proposed method benefits from emerging technologies employing smaller mass-produced cells based on devices capable of improved switching characteristics, like SiC or GaN, thereby suiting design philosophies characterized by a high number of cost-optimized standard elementary units. Equally important, the proposed synthesis eliminates most physical communication channels (e.g. optical fibers) among the controller and the switches, thereby greatly improving the system reliability. The paper introduces also a hybrid variant of the method conceived for fewer cells.