The modular multilevel converter has gained popularity in various applications, including photovoltaic (PV) solar energy conversion. Its modular structure allows for the transformation of an MMC into an MMC-based photovoltaic system, sharing key operational characteristics such as modularity, flexibility, redundancy, increased efficiency, and fault tolerance. To ensure the reliability and uninterrupted operation of the modified MMC, even in the event of potential failures in the photovoltaic submodules (PVSMs), a fault-tolerant strategy is developed in this study. It assumes that the Maximum Power Point Tracking (MPPT) of the PVSMs is already guaranteed. Redundant submodules (rSM) are utilized to maintain power balance between the converter arms through voltage control, while reserve submodules (RSMs) are in place to rescue the converter in case of a failure. The detection and localization of faults in the PVSMs/rSMs are achieved through sliding mode observers (SMOs), and the converter reconfiguration is carried out using the proposed permutation algorithms for switching signals and SMs voltages. For precise control of the output current and electrical grid connection, the dq -reference frame control method is employed. To validate these proposed algorithms, time-domain simulations are conducted using the Simulink/Matlab software.

Oversizing is widely adopted in photovoltaic systems to increase inverter utilization and improve energy yield. However, higher DC/AC ratios also intensify electrothermal stress, accelerating inverter wear-out and potentially shortening its service life. In this context, this work evaluates DC/AC ratio selection by jointly considering reliability and levelized cost of energy (LCOE). The proposed framework is applied to a 75-kVA three-level neutral-point-clamped (3L-NPC) photovoltaic inverter and integrates lifetime models, Monte Carlo analysis, annual energy estimation, and LCOE evaluation over 25 years, considering panel degradation. In addition, the annual energy estimates are cross-checked against PVsyst simulations using a user-defined inverter with comparable rated power and oversizing capability. Seven DC/AC ratios, ranging from 1.008 to 2.016, are assessed under three Brazilian cities. The results show that the inverter B10 lifetime decreases monotonically with oversizing, whereas the minimum LCOE is site-dependent. The unconstrained LCOE optimum does not meet the reliability targets in two of three scenarios, requiring lower DC/AC ratios, resulting in LCOE differences of 1.39% and 0.31%. These findings show that reliability-constrained oversizing avoids reliability-infeasible designs while incurring only a limited increase in LCOE.

In this article the improved sampled average modulation technique is proposed, this technique has 2N + 1 levels in the output voltage waveform of MMC and it is considered as of low complexity of implementation for any number of submodules per arm. For that, characteristics such as dynamic response, implementation complexity, inverter output voltage waveform levels, and switching frequency are considered to evaluate and validate the proposed modulation technique. The proposed technique is compared with other three previously proposed techniques, its considering parameters such as the THD and the fundamental value of the output voltage, and also the peak-to-peak variation of the submodule capacitor voltage. Several simulations were performed in the Matlab/Simulink software and with these results, it was validated the proposed modulation technique, and also it is verified that the proposed technique is computationally more efficient. This last one shows its potential for multiphase multilevel applications.