Multi-objective control strategy for multilevel converter based battery D-STATCOM with power quality improvement

Differences in battery state-of-charge (SOC) in a cascaded H-bridge multilevel converter (CHB-MLC) based distribution static synchronous compensator (D-STATCOM) in battery storage system (BSS) applications could result in undesirable efficiency and performance reductions, and even system failure. Moreover, faults occurring on batteries and/or H-bridges (modules) of CHB-MLC severely increase the risk of failure. Power quality at the output of CHB-MLC is reduced as results of those faults. Therefore, active power differences that are introduced by SOC balancing among modules and bypassing the faulty modules yield to undesired harmonics at the output of CHB-MLC. This paper proposes a multi-objective control strategy for CHB-MLC based BSS D-STATCOM. The proposed control strategy includes a bidirectional power flow controller with unity power factor operation (UPF) feature, a fault-tolerant (FT) controller, a SOC balancing scheme that is designed to work on fault conditions, and a harmonic minimization strategy to enhance the power quality at the output of the CHB-MLC. Performance of the proposed method is validated through different operating conditions including faults, unbalanced SOCs and dynamic load changes. Results indicate that charging and discharging of the batteries with 5 kW rated power are held successfully along with maintaining UPF operation under both normal and fault conditions. Moreover, grid currents are balanced with the help of the FT controller and SOC balancing is properly operated even in fault conditions. Negative impacts of faults and unbalanced operating conditions on the power quality are also mitigated with the help of the harmonic reduction controller with significant decreases on total harmonic distortion reaching up to 47% and 69% during normal operation and faulty operations, respectively.