Impact of Inverter Distribution and Redundancy on Reliability of Battery-Integrated-Converter Systems

The concept of modularizing battery storage systems by distributing DC-DC or DC-AC converters among battery module in the battery string brings several benefits. This paper compares the reliability of a DC-AC battery-integrated-converter system to a conventional single inverter battery energy storage system. To provide a comparative evaluation, an existing real 100 kWh energy storage system with a single-phase 25 kVA DC-AC inverter has been considered as a case-study for modularisation. The system is modularized by distributing and integrating individual DC-AC inverters into the associated extra low voltage (ELV) battery modules. Two different battery structures are proposed; one based on individual 24 V modules, a second on 48 V battery module pairs. The reliability of the original system and proposed systems are evaluated and further compared for different redundancy levels. The results show that for low failure rates of battery cells and inverters, the redundancy provided by multiple modules ensures a high reliability battery system. Furthermore, when designing the battery system with battery cells and inverters of higher failure rates, other approaches such as scheduled preventive maintenance is suggested as a means to maintain acceptable reliability. However, cost effectiveness of designing such systems should be taken into account as compared to the battery systems with low failure rates of components.