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This paper presents a hybrid modulation technique for the three-level neutral-point-clamped converter. A modulation strategy, based on two modulation signals per phase, was presented previously. This strategy completely removes the low-frequency voltage oscillations that appear at the neutral point (NP) in some operation conditions. However, it also has a major drawback: it significantly increases the switching losses of the converter. The proposal in this paper combines such a modulation strategy with sinusoidal pulsewidth modulation (SPWM). The main characteristic of this hybrid modulation is the reduction in switching losses at the cost of some low-frequency voltage oscillations at the NP. The amplitude of these oscillations can be controlled by varying the combination of the two strategies. The performance of the hybrid modulation is analyzed and compared with the original strategies. Power losses and oscillation amplitudes on the dc-link capacitors are evaluated. Experimental results show how the hybrid modulation performs by balancing the dc-link capacitors. Index Terms--Carrier-based modulation, neutral-point--clamped (NPC) converter, optimal compensator, three-level converter, voltage balance.

This paper presents some modified topologies of the neutral-point-clamped converter. In all of them, the main change consists of adding a fourth leg, which is based on the flying-capacitor converter structure. The aim of this additional leg is to provide the converter with fault tolerance. Furthermore, during normal operation mode, this leg is able to provide a stiff neutral voltage. Consequently, the low-frequency voltage oscillations that appear at the neutral point of the standard three-level topology in some operating conditions no longer exist. As a result, the modulation strategy of the three main legs of the converter does not have to take care of voltage balance, and it can be designed to either achieve optimal output voltage spectra or improve the efficiency of the converter. Simulation and experimental results are presented to show the viability of this approach both under normal operation mode and in the event of faults. Index Terms--Fault tolerance, multilevel converter, neutral-point-clamped (NPC) converter, postfault-handling reconfiguration, voltage oscillation.

Ocean energy is regarded as an important future source of energy generation in many countries for transition to a low-carbon future. While commercial interest in ocean energy is growing significantly at a global level, there are considerable investment costs and bottlenecks that will need to be overcome. Research and funds are spread over many different wave and marine current energy concepts under development, and there is still no technology convergence, in contrast to what happened in wind energy. Although an important marine energy resource, discussion of offshore wind energy is not included in this manuscript. This article focuses on the latest developments in ocean energy--in particular, open-sea test-ing facilities set up by several countries as a measure to encourage deployment and streamlining procedures--and gives an overview of projects going into the water this past year. In addition, the article highlights the importance of collabo-rative research and development on ocean energy projects and the unique role of the Ocean Energy Systems Implementing Agreement as an intergovernmental or-ganization promoting the use of ocean energy (wave, marine currents, tidal, ocean thermal gradients and salinity gradients) for energy extraction. [ABSTRACT FROM AUTHOR]