Charging and discharging methods for modularized battery energy storage systems (MBESS)

Abstract

Electric vehicles (EV) promise to be a panacea to address the pressing problem of global warming from greenhouse gas emissions. EV possesses the potential to radically transform the personal transportation sector in the near future. The research studies related to EV are multi-disciplinary, comprising of material science, electro-chemistry, power electronics, mechanical, embedded systems etc. This research primarily focuses on the study of operation and control of power converters that are suitable for EV application. Modularization and swapping of battery system are taken as major approaches in this research study to improve the reliability of EVs from the energy source standpoint. Conventionally, the battery pack and hence the power converters employed for charging and driving the motor in an EV are monolithically constructed. Consequently, the operation of EV completely depends on a single energy source. Alternatively, modularizing the energy source could greatly increase the operational reliability of an EV and thus the necessity of modularizing the power converters arises. The charging time required by the EVs is significantly longer when compared with the time required to refuel a gasoline vehicle. This is considered as the debilitating disadvantage for the electric vehicle commercialization. Hence, instead of charging the battery in the EV, swapping the depleted battery with another charged battery could ameliorate the afore-mentioned disadvantage. The novel inductive power transfer (IPT) concept is considered for implementation to improve the performance of this battery swapping process. Two systems incorporating IPT are proposed and analysed as a part of this research study. The Cascaded H-bridge (CHB) multi-level converter is generally considered an apt choice when multiple DC sources are available as in the case of modularized battery energy storage system (MBESS). This work investigates a battery module/micro-pack level SOC balancing control in a CHB based MBESS using Multi-dimensional pulse width modulation (MD-PWM). MD-PWM is a generalized modulation strategy for CHB converters, and a review of which is conducted. Modular battery pack system or micro-pack concept for CHB is introduced to increase the availability of the EV in the event of fault. A control strategy is proposed for the SOC balancing between the battery micro-packs in the CHB. This proposed control strategy aims to deliver a semblance of uniform operation of the whole battery system irrespective of the current SOC of each micro-pack. The realization of fault tolerant operation using MD-PWM is also proposed. The advantage of the proposed methods is that it can be conveniently integrated with the MD-PWM algorithm, eliminating any external circuit. The results obtained from the laboratory setup of a five-level three-phase CHB with six battery modules of 15 V 6.2 Ah is presented for RL load. The results obtained from the laboratory setup of a five-level three-phase CHB driving an induction motor with six battery modules of 52.8 V 60 Ah is…