In this paper, the problem of controlling parallel charging system with supercapacitors for electric vehicle applications is considered. When the vehicle parks at the station, the charging process of supercapacitors needs to be completed in less than 30 seconds. The control objective is then to tightly regulate the supercapacitors state of charge (SOC) to a given reference constant and to ensure an adequate current sharing between different parallel chargers. Indeed, the current sharing is a critical issue for parallel charging system with supercapacitors, which is a nonlinear system with control inputs constraints.Besides, the SOC depends on the supercapacitors internal voltage, which is not accessible for measurement. Therefore, based on a large-signal model of the parallel-chargers-supercapacitors system, an output feedback controller (combining a state observer and a nonlinear control laws) is designed. The controller is formally shown to meet all objectives, namely, closed-loop stability, SOC reference tracking, and equal current sharing. The effectiveness of the proposed output feedback controller approach is verified both by simulation and by experimental tests.
KEYWORDSLyapunov stability, nonlinear control, nonlinear observer, parallel chargers, supercapacitors
INTRODUCTIONDesigning onboard energy storage systems for electrical vehicles has been an intensive research activity over the last two decades. First, only batteries have been used in the storage system, before supercapacitors where introduced. Recent solutions involved both batteries and supercapacitor in the storage systems. 1,2 Presently batteries are used as energy storage devices in most applications. Such hybridization topologies can result into enhancing the battery performances by increasing its life cycle, rated capacity, reducing the energy losses, and limiting the temperature rising inside the battery. Comparing the charger circuits of batteries and supercapacitors, there are similarities and differences. Concerning similarities, the constant current/constant voltage charging method is generally preferred in both circuits for safety of the storage system. One advantage with batteries is that the range of voltage between discharged and fully charged is not very wide, so it does not pose any issue for charger circuit, while with supercapacitors, it may be needed to reach the fully discharged state, which entails a zero voltage. On the other hand, the use of battery pack necessitates the monitoring and management of the state of charge (SOC) of each individual cell of the battery. This is not an issue with supercapacitors 1374