Hierarchical control of vehicular fuel cell / battery hybrid powertrain

In a proton exchange membrane (PEM) fuel cell/battery hybrid vehicle, a fuel cell system fulfills the stationary power demand, and a traction battery provides the accelerating power and recycles braking energy. The entire system is coordinated by a distributed control system, incorporating three key strategies: 1) vehicle control, 2) fuel cell control and 3) battery management. They make up a hierarchical control system. This paper introduces a hierarchical control strategy for a fuel cell / battery hybrid powertrain applied in a city bus. The vehicle control strategy comprises three parts: an energy management strategy, a brake regeneration strategy and an active tolerant control strategy. The entire powertrain power is split between the fuel cell system and the battery in such a way that the fuel cell system works in a quasi-steady state and the battery can keep charge sustaining. The braking energy is recycled on the basis of the pedal position and the rotational speed of the electric motor. An active tolerance control strategy is developed to guarantee the work of the entire system in case of malfunctions. A fuel cell system consists of a water and heat management system, an air system, a humidifier system and a hydrogen in-out let system. Control strategies are designed to regulate air supply and water cooling temperature. Primary tasks of a battery management system are to estimate the state of charge (SOC), and to keep the temperature in safety range. A real-time applicable strategy for SOC is developed. The temperature of the battery is controlled by some air fans with on-off strategy. The hierarchical control system was applied on a fuel cell city bus. Experimental results show the effectiveness of the proposed strategy.