Control & Energy Management of Battery/Ultracapacitor Based Hybrid Energy Storage System

Abstract

The present research work deals with the control & energy management of battery & Ultracapacitor (UC) based Hybrid Energy Storage System (HESS). The proposed con-trol and energy management algorithms improve the system response and the HESS utilization. The efficiency, durability, and health of the HESS depend on control-ling the power flow of the HESS components subjected to their specified operational constraints. It is achieved by the Energy Management Algorithm (EMA). Power allo-cation is done by frequency sharing algorithm by allotting high-frequency components of load demand to UC and low-frequency components to the battery. An improved frequency sharing algorithm for battery/UC HESS in the presence of delay has been proposed in this work. The presence of the delay due to the digital implementation has been included in inner current control transfer functions for designing the PI con-troller. Initially, the PI controller parameters have been designed with phase margin and bandwidth specifications. The total time delay in the system varies and depends on the controller speed, and computational complexity and also, due to the presence of the unmodeled dynamics of the system, the stability and the performance of the system might be affected. Hence, to ensure system stability in the presence of de-lay, initially, the complete stabilizing set for inner PI controllers have been derived. Furthermore, the stabilizing set satisfying the specified frequency domain specifica-tions, i.e., phase margin and gain margin, have also been analytically derived and experimentally verified. As stated above, apart from frequency share-based power allocation, the EMA should also ensure proper operational constraints of the HESS components. In conventional EMAs, UC voltage is maintained at a nominal value by the UC voltage regulation loop, which reduces energy supplied by UC to the load during transients. This research proposes a filter based UC voltage control loop which eliminates the high-frequency components of UC charging current and hence reduces the conflict with DC-link voltage regulation loop. However, in UC voltage control loop based EMA’s, the UC operation remains restricted to a reference voltage to prevent it from overcharging/undercharging. This leads to a very narrow utilization of the UC voltage range. Since UC voltage can safely be varied from zero to its maximum rated voltage, therefore, rather than employing a UC voltage control loop, a time-share based approach has been proposed for UC charging/discharging. The proposed EMA operation depends on the defined UC voltage band instead of a UC reference voltage, which increases its power delivery capacity by approximately 2-4 times than the conventional EMAs. An experimental prototype of the system is designed, and the proposed control algorithm and EMA have been tested in the different operating regions for validation. The major contributions of the thesis can be summarized as: 1. Improved frequency sharing algorithm in the presence of delay.
2. The complete stabilizing set satisfying the desired phase margin and gain margin has been derived.
3. Filter based UC voltage control has been proposed.
4. Time-share based EMA has been proposed that improves UC utilization.