Distributed cooperative voltage equalization for series-connected super-capacitors

In this paper, we propose a distributed cooperative voltage equalization strategy in order to balance the voltages of the series-connected super-capacitors, which is the power supply of energy storage light rail vehicles. Taking each supercapacitor cell as an agent in multi-agent systems, the voltage equalization problem can be formulated as a cooperative tracking problem. The sparse communication network among the super-capacitors in the cyber layer can be modeled by a digraph. As the voltage changes within a limited range, a general saturation function is put forward to guarantee the boundedness of the control input. Based on the nearest neighborhood rule, a bounded voltage equalization controller with tunable control gains is designed accordingly. The proposed voltage equalization methodology is distributed and not using a central controller. Each super-capacitor cell only need its voltage information and its neighbors, the desired control objective can be accomplished by local interaction in a cooperative way. Under the assumption that the digraph has a spanning tree, the asymptotically stability of the overall close-loop system is rigorously proved with the aid of a novel Lyapunov function integrating Lasalle invariant principle. The voltage equalization has been achieved by the proposed cooperative control approach, which is verified by the simulations. under different initial conditions, cooperative objectives and different network sizes.