Research On Regenerative Braking Energy Utilization Technique Based On Stationary Supercapacitors In Metro-Transit Systems

The metro-transit system provides two railway lines supplied at750V DC or1.5kV DC. In orderto utilize the regenerative braking energy of Metro vehicles, it is necessary to save this energy in thestorage systems based on supercapacitors, which can reduce the energy consumption and stabilize theline voltage. Regarding the supercapacitors energy storage systems (ESSs) implementation, there aretwo options. One is mobile ESS installed onboard of the vehicle, the other one is stationary ESSinstalled at the substation level. Several studies about supercapacitors ESSs have accessed at750VDC power supply system. In this paper, the utilization technique for recovery regenerative energybased on stationary supercapacitors ESSs at1.5kV DC has been researched.The ESSs includes supercapacitor modules and the high power bi-directional DC-DC converter.Concerns on the demands of the stationary ESSs with the aim of utilizing braking energy are taken inthe paper by comparison with several ESSs.With the aim of sharing high voltage of the overhead catenary and the high power, twomulti-phase-parallel Buck/Boost converters are connected in series on the high voltage side andindependent on the supercapacitors side. Comparing the interleaved control scheme and thesynchronous one, both advantages and disadvantages have been shown in this study by means oftheoretical calculations and experiments. A balance control scheme for balancing the voltage ofin-series converters on high voltage side is emphasized by system small signal analysis.Actual traffic scenarios and vehicles’ braking process are considered for the study, which willeventually determine the size and structure of supercapacitors banks. Taking the supercapacitors cellequalization into account, a voltage equalization scheme is necessary for supercapacitors banks. A fastcharger circuit for precharge of the supercapacitors is also presented in detail.Finally, the energy management strategy for balancing the network voltage at weak points of theline is proposed, based on the voltage of overhead catenary and supercapacitors state of charge (SOC).The energy management strategy adjusts the operation mode of the ESS to meet the regenerativebraking energy for energy saving purpose. Simulations and experiments results show that the energymanagement strategy is validity.