Using variable DC sources in order to improve the voltage quality of a multilevel STATCOM with low frequency modulation

In this paper, a Selective Harmonic Elimination Pulse Width Modulation (SHE-PWM) method, which utilizes adjustable dc links, is presented for a Static Synchronous Compensator (STATCOM) based on Cascaded H-Bridge (CHB) multilevel converter. The proposed switching algorithm is able to fix the capacitors voltages at optimum and unequal values, while injecting the desired reactive power, in order to eliminate more low order harmonics. An indirect current control method is combined with a loss estimation block to control the dc link voltages that might have different losses. Moreover, by using unequal dc link voltages more levels at the output is achievable compared to the symmetrical multilevel converters that could be used in enhancing the power quality, while keeping the switching frequency near the fundamental (the maximum switching frequency can be kept below 150 Hz). The validity of the proposed method is verified by simulations in Matlab/Simulink environment.