Minimum Cell Operation Control for Power Loss Reduction in MMC-Based STATCOM

This article proposes a minimum cell operation control to reduce power losses in static synchronous compensator (STATCOM) based on the modular multilevel converter (MMC). The main principle is to bypass cells from the MMC, according to the reactive power reference. For such, analytical expressions for the minimum dc-link voltage to keep the converter operation in the modulator linear region are derived. These expressions are used to compute the number of cells, which can be bypassed for given operation conditions. Moreover, a simplified analytical modeling of conduction losses in MMC is presented. The limits of the proposed strategy are also investigated. Simulation studies are performed during cell bypass conditions to analyze its impact on the converter dynamics. In addition, the output current distortion and power losses under different operating conditions are evaluated. Finally, a one-year energy-saving study was conducted for a real reactive power profile, which presented a 7.37% reduction in the yearly energy losses. The results indicated that this methodology is a breakthrough solution to reduce power losses and operational costs. The proposed technique can be applied in MMC-based STATCOMs with more than ten cells per arm and does not require additional hardware.