Optimizing Train Speed Profiles to Improve Regeneration Efficiency of Transit Operations

With reduced environmental impact becoming an increasingly important benefit of the rail transportation mode, continual improvement in efficiency and reduced energy consumption has become a major concern to rail transit operators. For electrified rail transit operations, regenerative braking is one practical way for saving energy because it enables the kinetic energy of a train to be transmitted via the overhead catenary wire or third rail for use by adjacent trains. Although various regeneration technologies have been introduced, work is still needed to improve energy recovery efficiency. This paper focuses on energy recovery efficiency from an operational point of view. In a mass transit system, there are two operating modes for two consecutive trains: the following train either systematically applies the same speed profile as the leading train, or the following train adjusts its speed to a different speed profile according to the position, speed and regeneration potential of the leading train. With operations synchronized to reuse energy, the latter mode achieves better energy recovery efficiency than the former one. Based on the above understanding, the objective of this paper is to develop the optimal speed profile for a following train in order to minimize pantograph voltage fluctuations and improve energy recovery efficiency. Dynamic programming is applied to this problem in order to optimize the speed profile for a set of given infrastructure and train characteristics. Simulation results with Visual C++ demonstrate that the algorithm can provide an optimal operational strategy with better energy performance while satisfying safety constraints and comfort criteria. Based on this work, energy optimization potentials with different headways are discussed in the case study. This research will facilitate development of on-board train control system logic or system energy analysis that will reduce energy consumption and provide rail transit operators with operational cost savings. INTRODUCTION Today, as environmental impact and the cost of energy have become a significant concern for public transportation managers, more and more metropolitan areas in the United States have been investing heavily in constructing or extending urban rail transit systems to mitigate traffic congestion and air pollution caused by excessive use of automobiles [1][2]. Urban rail transit (light rail, subway and tramway) has generally been considered more energy efficient than highway vehicles in terms of unit fuel cost and greenhouse gas emissions. Regenerative braking is one of the technologies applied on board electrified rail equipment to achieve this efficiency and lower operating cost. During the braking process, traction motors on the electric locomotive or multiple-unit railcars are converted into generators, and electricity generated from kinetic energy of the train is available for use by auxiliary components on the train, with any excess transmitted back through the catenary to the power grid. This electrical power can be used by other trains in the same power section for propulsion [4]. However, in DC distribution networks, which are common on urban rail transit systems, regenerative energy cannot always be fully absorbed. When there is insufficient power demand from adjacent trains, the excess energy must be dissipated by the resistors on-board the train where electrical power is converted into heat and wasted into the air [5]. The traditional method of transit system energy calculation has been based on the single-train point of view. The total energy cost is regarded as the sum of the traction energy consumed by a single train with a credit for regenerative energy returned to the catenary. In practice, this requires on-board installation of an energy meter. This is common on systems with multiple operators or open access, such as in many European railroad systems, where it provides a convenient