Third phase solutions for compensation of second order harmonics in single-phase grid connected medium voltage converters

To eliminate bulky passive components used for the compensation of pulsating power flow in applications supplied from single phase AC grids, many solutions employing power electronics have been presented in the pertinent literature. One subclass of these solutions attaches an additional phase to an existing four-quadrant converter (4QC), to control a compensating power flow to a fully AC rated capacitor used as energy storage element. In this paper, it is shown that in medium voltage applications like railway traction, DC biased film capacitors reach significantly higher energy density compared to fully AC rated devices. For this reason, the existing topology is converted to a multilevel structure employing a DC biased capacitor as energy storage element, providing additional degrees of freedom for the current to be injected into the four-quadrant converter. By using an optimized current shape, the ratings for the passive components can be decreased at nearly constant semiconductor switch power compared to the original solution. Simulations results are shown to prove that the compensation works with the proposed solutions.