New transformerless STATCOM topology for compensating unbalanced medium-voltage loads

Cascaded multi-level converters have some noticeable advantages such as high reliability, efficient fault management capability, simple structure, and adaptability with different power levels. Furthermore, these converters could be connected to medium-voltage networks without an isolating transformer. However, the voltage balancing of the DC capacitors is a highly challenging task particularly in harmonically polluted and unbalanced systems. To overcome this problem, a new cascaded converter based on two level half-bridge modules for shunt compensators is proposed in this paper. The control algorithm of the compensator and its configuration are designed and optimized in order to achieve an efficient performance in unbalanced systems. This makes it possible to compensate the currents produced by the electric traction system as a popular of medium voltage unbalanced load properly.