An online state-of-charge balancing strategy for energy storage STATCOM based on modular multilevel converters

Abstract

The increasing integration of renewable energy sources into power systems has introduced significant challenges related to intermittency and grid stability. In this context, the Energy Storage Static Synchronous Compensator (ES-STATCOM), based on the Modular Multilevel Converter topology, emerges as an effective solution to enhance the flexibility and reliability of power systems. However, the configuration in which battery racks are connected in these systems, combined with varying operating conditions, can cause State of Charge (SoC) imbalances among battery modules, reducing both the system’s efficiency and its lifetime if operated under such conditions. To address this issue, this dissertation proposes an online SoC balancing strategy employing an algorithm integrated into the ES-STATCOM control structure. In addition, control loops for output current regulation and circulating current suppression were designed in the synchronous dq reference frame. The developed approach enables the SoC balancing process while simultaneously maintaining the converter’s capability to inject and absorb active and reactive instantaneous power. This strategy not only improves the utilization of the Battery Energy Storage System (BESS) but also enhances the operational reliability of the ES-STATCOM under different grid conditions. The proposed methodology was verified through detailed simulations using the PSCAD/EMTDC software, where an ES-STATCOM with 18 sub-modules per arm, connected to a BESS in a single-stage distributed configuration with first-life lithium-ion battery cells installed in the racks, was modeled and connected to a balanced three-phase system with a voltage level of 13.8 kV. The ES-STATCOM dynamics during the SoC balancing process were analyzed in different scenarios, injecting and absorbing active and reactive power at its terminals. Subsequently, the ES-STATCOM was tested in a modified IEEE 14-bus system to evaluate its performance during frequency and voltage support, respectively, after the connection of a large load and the occurrence of a symmetrical fault. The simulation results confirmed the effectiveness of the proposed control strategy and its ability to maintain SoC balancing while providing ancillary services to the power grid.