This paper presents a new control method for cascaded connected H-bridge converter-based static compensators. These converters have classically been commutated at fundamental line frequencies, but the evolution of power semiconductors has allowed the increase of switching frequencies and power ratings of these devices, permitting the use of pulsewidth modulation techniques. This paper mainly focuses on dc-bus voltage balancing problems and proposes a new control technique (individual voltage balancing strategy), which solves these balancing problems, maintaining the delivered reactive power equally distributed among all the H-bridges of the converter.

This paper presents a modulation strategy for the modular multilevel converter (MMC) which provides the voltage balancing of the capacitors of different submodules comprising the converter. Not only is this modulation applicable to MMCs constructed with classic two level (2L) submodules, but it is also effective for arrangements with different submodule concepts, for instance, topologies like neutral point clamped (NPC), flying capacitors (FCs), neutral point piloted (NPP), etc. Therefore, firstly, the general modulation philosophy is explained applied to the two level (2L) submodule concept, and secondly, the extension to 3L-NPC and 3L-FC submodule topologies is analyzed. After that, the validation of the studied modulation strategy is carried out by means of successful simulation results, at different switching frequencies and number of submodules. The corresponding experimental results are shown in Part II of this paper after having implemented the aforementioned modulation strategy in a real test bench. In addition to this, a comparison based on thermal analysis and sizing of elements among the four studied submodule topologies is also included.

This paper is the continuation of part I in which a modulation strategy for the modular multilevel converter, which provides voltage balancing of the capacitors of different submodules comprising the converter, is presented. Here, the validation of the studied modulation strategy is carried out by means of successful experimental results in a downscaled real test bench. First, the converter submodules are 2L topologies, and then the connections are changed to configure 3L-FC submodules showing the effectiveness and implementability of the proposed modulation in different submodule concepts. Finally, a comparison based on thermal analysis and sizing of elements among 2L, 2L serializing several insulated gate bipolar transistors per valve, 3L-FC, and 3L-NPC submodule concepts is made.