Adaptive Control Strategy for Ultracapacitor Based Bidirectional DC–DC Converters

Ultracapacitor (UC) based energy storage systems (ESS) are often operated as boost converters during discharging mode of operation. The control of such a system is typically performed using an outer voltage loop and inner current loop structure. Since UC stacks undergo wide voltage variation, it is desirable to have a control structure that accommodates system characteristics' variations, which allow deep discharge of UC stack. This paper proposes an adaptive control method with reduced settling time where the control gain parameters for outer voltage loop are obtained online in a UC-based bidirectional dc-dc converter. For this, analytical expressions for controller gains for a required bandwidth (BW) and response characteristics are obtained. Using these, simplification in gain expressions that is required for adaptive implementation is achieved. In this regard, the design and drawbacks of using fixed controller gain parameters for voltage loop are presented. The proposed control incorporates the effect of right-hand plane (RHP) zero on the voltage loop design, which constraints the control BW especially during deep discharges. The adaptive control adjusts the BW of the voltage loop according to the RHP zero location, thereby ensuring best performance possible for wide range of operating UC stack voltages. Such a formulation of controller gains results in a voltage feedforward method, which improves the control performance of the ESS, with an improvement in the settling time ranging from 10% to 50% depending on the operating UC stack voltage level. The proposed control is also compatible with load feedforward method and is found to have better performance as compared to the case when fixed controller gains with load feedforward is utilized. As a proof of concept, the proposed control is verified on an experimental setup with power level Po = 62 W, voltage level Vg = 26 V with the dc-dc converter switching at fsw = 100 kHz.