Integrating Battery into MMC Submodule using Passive Filter and Control Technique
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The growing need and importance of Battery Energy Storage Systems (BESS) in grid support applications as well as in the transportation sector, has brought attention to the adaption of the Modular Multilevel Converter (MMC) to be used in BESS. The MMC is already known for its high efficiency, high voltage capability and good harmonic performance, but the modular arrangement of the converter also allows the battery to be implemented in a distributed manner. Known as a split battery topology or split storage. Connecting individual battery units directly into the submodules of the MMC. Opening the possibility of high AC-side voltage without a transformer even with low battery voltage. However, the current flowing through the submodules inherently consists of oscillating components on top of the DC-component. This thesis show that these are at fundamental and 2nd harmonic frequencies with amplitude of two and one times the DC-component respectively. It is assumed that this is unacceptable due its negative impact on battery performance and lifetime. In most literature this is solved by interfacing the batteries with the submodules using a DC-DC converter. But this thesis explores the possibility of mitigating these components using a control technique and an interface consisting of a filter based on passive components. Therefore, the proposed technique is two-legged. 1. injecting a circulating current that cancels the 2nd harmonic power fluctuation in the arms and hence submodules. Removing the need for 2nd harmonic filtration. 2. Employing a resonant filter to mitigate fundamental component together with a lowpass filter.The performance and feasibility of this technique is analyzed theoretically and through simulations. Resonant branch of the filter is analyzed in lab experiment. And results indicate that this could be an attractive solution, especially in applications where high reliability is of concern.