Control of a transformer-less cascaded H-bridge based STATCOM using low-frequency selective harmonic elimination technique

This paper presents a transformerless STATic synchronous COMpensator (STATCOM) based on seven-level cascaded H-bridge multilevel converter. The utilized converter is extremely modular and can be connected directly to medium voltage networks. Moreover, a new control strategy is proposed to control the flow of reactive power, as well as, the flow of active power to regulate the voltage of dc link capacitors. The proposed method employs the Selective Harmonic Elimination Pulse Width Modulation (SHE-PWM) with extra transition between two successive voltage levels to reduce the switching loss and simultaneously to eliminate more low order harmonics compared to the SHE modulation. The proposed control method is able to regulate the voltage of all dc link capacitors to the reference value, even the loss of H-bridge cells are different. The validity and effectiveness of the proposed method are confirmed by several simulations in PSCAD/EMTDC environment.