Evaluation of battery current harmonic suppression schemes for MMC-based energy storage systems through mission profile emulators

Abstract

In recent years, Modular Multilevel Converters (MMC) have been used in medium/high voltage applications and pointed out as a promising solution for Battery Energy Storage Systems (BESS). Especially, in the BESS application, the current and voltage conditions in the submodules (SM) are fundamental to evaluate the thermal dynamics, reliability, and lifetime of the batteries. For medium/high voltage applications, the MMC is based on tens or hundreds of SMs. Thus, the complete implementation of the MMC system can be expensive and complex for operational and reliability testing of batteries. In this sense, the so-called Mission Profile Emulators (MPE) were developed to emulate the typical voltage and current in an SM for low voltage and power, preserving the harmonic content. The MPE can reduce the need to implement the complete converter and contributes to more agile testing of the electrical characteristics of the MMC and especially the batteries, resulting in the design of more reliable converters. This work presents a detailed MPE design and implementation analysis for an MMC-based BESS. The MPE is validated in simulations and a small-scale prototype. The spectrum of arm current and battery current showed similar content comparing analytical equation, simulation, and experimental results. Furthermore, with the designed MPE, passive (inductive and capacitive filters) and active (dc/dc converter) filtering strategies are evaluated to eliminate harmonic components present in the battery current that affect its lifetime. The results showed that the maximum ripple of the arm current presents values lower than 10%, according to the limit for designing the MPE inductor. Furthermore, the arm and battery current spectrum showed that the harmonic contents of these signals are similar to the experimental and simulation results, in agreement with the typical current spectrum observed in MMC-BESS. The experimental results of the proposed system, MPE with filtering strategies in SMs, are presented in this work. The test bench with the main components is described and the conditioning, control, and steady-state operation results are presented. The passive filters and active filter topology design methodology proved effective in reducing the harmonic components present in the battery current, with the potential for reducing the fundamental harmonic component equal to 82.9 % in the passive filter and 97.1 % in the active filter topology. The potential for attenuation of these components would positively impact the reliability of the BESS under the direct impact on the temperature of the batteries.