Control of a Modular Multilevel Converter STATCOM under internal and external unbalances

This paper develops a complete control scheme for the Modular Multilevel Converter (MMC) in a STATCOM application. The focus is laid on the internal balancing of the converter and its reliable operation under internal mismatches as well as under external unbalanced conditions. The leg energy controller ensures that the sum of the capacitor voltages of each phase are equal. An intuitive arm energy controller is also implemented which allows the decoupled control of the energy levels of every arm. The control performance of the STATCOM was evaluated under asymmetrical grid faults and the simulation results verified the capability to provide maximum reactive power to the AC grid while maintaining internal balance through the individual control of each arm energy level.