Holistic Control Approach for the Grid-Connected Converter of a Battery Energy Storage System

This paper introduces a holistic control approach applied to a grid-connected converter of a Battery Energy Storage System (BESS). The BESS is mainly used for power peak shaving, frequency supporting, and islanded operation mode which implies multiple control transitions depending on the operation mode. One existing challenge is related to the multiple tuning controllers for the grid-connected converter required for each operation mode, including the rectifier operation. Moreover, when the rectifier operation is running the system is naturally nonlinear which motivates the use of nonlinear controllers. From this standpoint, a strong motivation for this research is the seeking of a holistic control to face all the operation-mode changes seamlessly. Hence, the merit of the proposed holistic control approach consists of using a unique tuning process based on the linear technique of state feedback with integral action for all operation modes, which are: 1) grid-connected mode as an inverter, 2) grid-connected mode as a rectifier, and 3) islanded mode. Of special attention is the grid-connected mode as rectifier since the proposal avoids the introduction of the PWM rectifier model which increases the model order and is inherently nonlinear. A thorough analysis of eigenvalues in open loop and closed loop with the same tuning gains is presented to demonstrate the stability and feasibility of the proposal. An affordable tuning methodology is proposed considering the physical restrictions of the grid-connected converter related to the LCL-filter bandwidth and the switching frequency. To demonstrate the merits of the holistic control approach, several simulations are presented using PSCAD/EMTDC on a 100 kW, 480 V rated BESS using a delta-connected LCL filter for interconnecting with the grid. Simulation results show that the seamless capability with steady-state and transient state operation runs smoothly.