Main Circuit's Parametric Optimal Design Counting Inconsistent Battery Parameters and Control of 35 kV Large-Capacity Transformer-Less BESS

A large quantity of battery cells are required by each phase of the 35 kV large-capacity transformer-less battery energy storage system (LCTL-BESS) based on cascaded H-bridge converter (CHBC) and the parameters of battery cells are inconsistent. The inconsistencies of battery parameters should be fully considered in main circuit parameter design to improve the system's adaptability to them, thus delaying the aging rate of battery and ensuring the safety and high efficiency utilization during the whole life cycle of battery. The cascaded number of power modules in per phase is the most important main circuit parameter of system and has a significant influence on system performance, which should be optimized by considering system efficiency and safety comprehensively. Since the system efficiency and safety evaluation model are closely related to inconsistent battery parameters, it is impossible to establish them under different number of power modules based on the detailed parameters of each battery cell. Therefore, a method to establish system efficiency and safety evaluation model based on the capacity distribution of the battery cells is proposed. The massive battery cell parameter identifications and data processing are avoided. Our proposed method provides theoretical guidance for the optimization design of main circuit for LCTL-BESS. Additionally, the power and energy balancing control of 35 kV LCTL-BESS are investigated, and a novel energy balancing control strategy among modules considering the DC link voltage and state of charge (SOC) are proposed under the nearest level modulation (NLM), which can promise both the balancing effect and AC side output performance. Finally, the validity of the proposed parameter design method and the control strategy are proved through off-line simulation model.