Review on Wayside Energy Storage Technology for Urban Rail Transit

The increasing demand for urban rail transit and its network size aggravates traction energy consumption and regenerative braking energy utilization. Train running chart optimization, inverter feedback, and energy storage are often used to minimize the use of braking resistors. At present, energy storage technology is widely used with different energy storage media and energy storage converters. The system can work in different states according to the train’s operating conditions and energy management strategies. The widely-used energy storage media include batteries, supercapacitors, and flywheels. Batteries have advantages in energy density for long-time energy storage, but the power density is relatively limited. Supercapacitors have power density advantages for high-power charging and discharging in a short period. In addition, supercapacitor plays the power supply function in urban rail transit to realize the regenerative braking energy absorption. Flywheels complete the conversion between electrical energy and mechanical energy through physical means, which have high power density. However, their costs are high. This paper compares the characteristics of the three energy storage media in terms of energy density, power density, and cost. According to the actual operation of urban rail, the energy storage converter needs to have bidirectional energy mobility. The bidirectional AC-DC converter and bidirectional DC-DC converter are the candidates. The basic topology of AC-DC converters is the two-level topology, but the power device cost and switching loss are high. This problem can be solved by applying multi-level technology. This paper takes three-level technology as an example. The three-level topologies for urban rail energy storage systems include diode neutral point clamped topology and active neutral point clamped topology. The basic current urban rail topology of DC-DC converters is the diode center-clamped topology based on the half-bridge structure. The module series-parallel and cascade technology is considered to meet high-voltage and high-power requirements. Due to the complexity of energy interaction among the traction power supply system, energy storage system, and train, formulating a reasonable energy management strategy is crucial to realize efficient charge/discharge control. Energy management strategies generally include threshold-based and power allocation-based strategies. The traditional threshold-based strategy is based on the fixed threshold, which is easy to realize. However, the coupling characteristics and coordinated control of the urban rail train and the whole traction power system need to be considered. Thus, strategies based on multiple fixed thresholds and dynamic threshold regulation are proposed. The power allocation strategy is mainly applied to the hybrid energy storage system to meet the energy and power allocation of different energy storage media. The energy storage system design re-optimization is put forward for energy storage medium selection and converter performance improvement, and the energy management re-optimization is proposed for source, network, storage, vehicle management and energy management system upgrades. It provides theoretical and methodological practice for the urban rail energy storage system. © 2024 China Machine Press. All rights reserved.