Delta-CHB Statcom with Reduced Capacitance Through Third Harmonic Injection

Abstract

This work investigates the third harmonic circulating current injection in delta-connected Cascaded H-Bridge (Delta-CHB) STATCOM, aiming the cell capacitance volume reduction. Firstly, analytical expressions for capacitor voltage ripple are derived without and with third harmonic injection. These expressions are used to obtain the third harmonic current waveform which minimizes the sum of capacitor voltages ripple in the whole operation range. These expressions are validated in an experimental setup based on 620 VA/220 V cell. Although this approach minimizes the ripple, an increase in the semiconductors current effort is expected. This paper quantifies the increase through analytical expressions and proposes an improved third-harmonic injection scheme which injects the third harmonic in a range of operating points. The performance of the proposed strategy is evaluated for a 17 MVA/13.8 kV Delta-CHB STATCOM with 24 cells per cluster. The results indicate a reduction of 17.2% in the theoretical cell capacitance. A physical realization of cell capacitor reveals that the proposed scheme, in the best scenario, reduces the volume in 25%. Finally, if the same power module and heatsink are employed, the proposed scheme leads to an increase in junction temperature of 13◦C for IGBT and diode.