B. LI-ION BATTERY INTEGRATION CHALLENGES Railway systems have very demanding operating conditions including vibrations, conducted and radiated noise, as well as extreme temperature conditions. These make designing the battery system and its protections a challenging task. Specific challenges related to the integration of battery onboard railway vehicles include [12], [112], [119]: 1) safety issues, protections, and the consequent high number of electronic components and 2) recharging infrastructure and scheduling. Li-ion batteries can be dangerous if they undergo a thermal runway also called thermal avalanche, which is an irreversible process that eventually causes the battery cells to catch fire or even explode depending on the battery type [120]. Thermal runaway is triggered if Li-ion cell safety is compromised when any of its internal components are misused or undergo instability due to extreme operation. The latter situation would eventually lead to the release of stored energy in an uncontrolled manner increasing rapidly the cell temperature and leading to the thermal runaway phenomena, most probably propagating nearby cells and jeopardizing the complete battery system. Some of the conditions that could lead to thermal runaway include overvoltage between battery terminals, overtemperature, overcurrent, isolation failure (i.e., overvoltage between any terminal and ground), etc. In order to survive the harsh railway conditions without compromising cell safety, the battery system needs to be very carefully designed and monitored [120], [121]. For this reason, battery packs are necessarily equipped with a BMS and a battery thermal management system [122]; these have direct control over the precharge circuit and the power box containing the main circuit breakers/contactors and fuses. The presence of these electronic units often adds complexity to the system and increases the number of components as well as making the system more susceptible to noise interference. Consequently, filters are needed. Differentialand common-mode filters are typically included to attenuate differential- and common-mode noise. Common-mode filter still represents an open issue for battery system design [79]. It requires battery manufacturers to have very precise knowledge of their battery pack immunity limits regarding tolerable dv/dt, common-mode voltage, EMI, etc., and define acceptable noise levels imposed by other system components especially switching converters.
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