Dynamic Stabilization of DC Traction Systems Using a Supercapacitor-Based Active Stabilizer with Model Predictive Control

The increasing urbanization, population, and travel demand lead to a higher density of trains in subway or tramway lines, which weakens the energy-feeding dc substations of these city transportation means. Under such circumstances, voltage sags in the upstream ac grid, resistive voltage drops in dc feeders, and constant power load (CPL) nature of subway and tramway traction units can lead to momentous fluctuations in the dc-link voltage. To rectify this problem, a supercapacitor-based active stabilizer (SBAS) with a two-mode control strategy (TMCS) is proposed in this article. It is shown that the proposed SBAS exhibits the behavior of a varying capacitance at the bus side. This feature is used in conjunction with the proposed TMCS based on model predictive control (MPC) to improve the stability of the system and to mitigate the variations of the dc-link voltage. The effectiveness of the proposed method is demonstrated by simulation of a typical subway system as well as by a series of experiments on a scaled prototype. The results show that the dc-link voltage drop never exceeds 2% when the proposed SBAS is used. In addition, the real-time feasibility of the proposed MPC-based TMCS is demonstrated by real-time hardware-in-the-loop (HIL) experiments.