Delta-CHB STATCOM with reduced energy storage requirements based on third harmonic injection

The delta connected cascade H-Bridge converter (Delta-HUB) has emerged as an interesting topology for STATCOM applications. A drawback of this topology is the low-frequency ripple in the sub module voltage that leads to high energy storage requirements. Due to the inherent characteristic of Delta-HUB topology, the zero sequence components present in the circulating current do not flow into the grid. This work aims to explore the third harmonic circulating current injection to minimize energy storage requirements, since this method changes the shape of the capacitor voltage waveform. Initially, the expression of the sum of the capacitor voltages is analytically derived. Then, the third harmonic injection is evaluated based on 17 MVA/13.8 kV Delta-HUB case study with 24 cells per cluster. Also, the methodology was evaluated with an experimental setup based on 500 VA/220 V submodule. The results indicate a reduction in the capacitor voltage ripple of 26.64% and 62.10%, when the converter is injecting and absorbing nominal reactive power from the grid, respectively.