Backstepping Control for Multilevel STATCOM

Modular multilevel cascaded converters prove to be well-suitable for high and medium voltage systems due to their lower cost and high-redundance design. For STATCOM, SingleStar Bridge-Cell (SSBC) topology is well applied for its satisfactory performance and superior component count. Multilevel SSBC-STATCOM control is composed of three layers which are output voltage control, internal current control, and capacitor voltage balancing. Although PI regulators are commonly used for its simplicity and ease in implementation, STATCOM system is an essentially nonlinear system, therefore, a nonlinear controller can effectively improve performance and robustness. In this work, a backstepping nonlinear control based on Lyapunov function design is proposed to regulate the overall capacitor voltage. Besides, detailed control design and implementation of the proposed control of SSBC-STATCOM using FPGA is discussed. Experimental set-up was designed to verify the results practically which confirmed the robustness and stability of the proposed control approach. In addition, the performance of the proposed method under Vdc step change and variation of system impedance has been analyzed and results were compared with the traditional PI controller.