Single-Phase Grid-Connected Battery-Supercapacitor Hybrid Energy Storage System

Abstract

Battery technology is popular in distributed energy storage systems (ESSs) due to its ease of implementation. However, batteries have limited power capabilities, and the lifetime of batteries deteriorates due to high and fluctuating battery currents. Battery-supercapacitor hybrid energy storage systems (HESSs) become a promising way of increasing the battery lifetime and system power capability. The HESSs can be divided into two groups, DC link based and direct AC line integrated HESSs. The direct AC line integrated HESSs have been proposed to eliminate multiple power processing stages associated with the DC link based HESSs. The objective of this research is to develop a direct grid-connected battery-supercapacitor HESS able to allocate fast power fluctuations to the supercapacitor while maintaining the state of charge of the ESSs within a safe operating region. Since the lifetime of ESSs deteriorates with ripple current components, ways of reducing the current ripple components of the ESSs are studied. In this thesis, a boost inverter based battery-supercapacitor HESS is proposed. A supercapacitor voltage controller and a filter based method is used to allocate the fast power fluctuations to the supercapacitor. Then, a supercapacitor energy controller (SCEC) based power allocation method facilitating the HESS dynamic analysis and precise supercapacitor sizing is proposed. Later, a sliding mode controlled HESS with a SCEC based power allocation method is proposed to achieve better output voltage reference following performance. To mitigate the second-order harmonic current components in the boost inverter based HESSs, a rule-based control method and a novel current feedback method are proposed. Both methods achieve a significant ripple current reduction without being affected by the output capacitor tolerances while the latter method mitigates the current ripple even during the output power transients. The proposed HESS is the first experimentally verified single-phase direct grid-connected HESS able to reduce the switching frequency and the second-order harmonic current ripples.