Integrated current-energy modeling and nonlinear feedback control of modular multilevel STATCOM

STATic synchronous COMpensator (STATCOM) can be integrated into electric transmission systems to provide reactive power compensation and grid voltage support for achieving high-efficient and reliable Flexible AC Transmission Systems (FACTS). An emerging solution for STATCOM with modular structure and multilevel voltage output, named as modular multilevel STATCOM (mmSTATCOM) shows its great advantages of adaption to a wide voltage range, sinusoidal output voltages with less harmonics and less converter losses, compared with the traditional STATCOM. The cascaded Full-Bridge-based mmSTATCOM that is configured with three converter branches into a single delta connection can be classified into the modular multilevel cascade converter (MMCC) family and this configuration can be named as Single-Delta Full-Bridge (SDFB). This paper presents a novel integrated modeling method of SDFB-based STATCOM by taking the independent currents, the total energy and the internal energy balancing into consideration. Then a nonlinear multivariable state-space model is developed. Therefore, a nonlinear control method, named as nonlinear quadratic regulator (NQR), is accordingly proposed to guarantee a safe long-term operation of the SDFB-based STATCOM system both in the balanced and temporarily unbalanced grid conditions. The simulation results are followed to verify the system operations in the normal condition as well as during a temporary grid fault.