Voltage balancing in transformer-less MMCC-SSBC based STATCOM

With massive penetration of renewable energy sources into power distribution systems, the static synchronous compensator (STATCOM) has been expected to play an important role in taking charge of reactive-power control and/or voltage regulation. The modular multilevel cascade converter based on single-star bridge-cells (MMCC-SSBC) has been considered as one of the promising circuit configurations for the STATCOM. The MMCC-SSBC based STATCOM can be connected to a power distribution system without bulky line-frequency transformer. The main problem with MMCC-SSBC based STATCOM is that it suffers from a voltage imbalance between the multiple floating dc capacitors. A possible control algorithm that devotes itself not only to meeting the demand of reactive power but also to voltage balancing of multiple galvanically isolated and floating dc capacitors for 6.6-kV 1-MVA MMCC-SSBC-based STATCOM, both in balanced and unbalanced system, is studied and simulated in Simulink. The whole control is divided mainly into two controls: "Reactive-power control and Overall voltage control" and "Voltage balancing control". Voltage balancing control is further divided into "cluster balancing control" and "individual balancing control. The simulation results show that the control algorithm performs good in both stable and unstable system. Moreover, zero sequence voltage method is applied in asymmetric/unbalanced system as well as to check the Low-Voltage-Ride-Through (LVRT) capability of the SSBC-based STATCOM under the single-phase, two-phase, and three-phase voltage sags and results are simulated and compared in Simulink.