Broadband Methods in Dynamic Analysis and Control of Battery Energy Storage Systems

Battery energy storage systems have become essential in the operation of many modern power-distribution systems, such as dc microgrids, electric ships, and electric aircraft. Energy storage systems often rely on the operation of bidirectional converters to control the power flow. In modern power systems, these bidirectional converters are typically a part of an extensive converter system, a multi-converter system that consists of several electrical converter-based sources and loads. Even though each converter in a multi-converter system is standalone stable, adverse interactions between the interconnected converters can present issues to the system’s performance and stability.

Assessing the stability of multi-converter systems is usually challenging, given that the systems are complex, and the dynamics are affected by various operating modes and points. Recent studies have presented methods for assessing the stability of interconnected converters through impedance-based stability criterion. Impedance-based analysis is particularly advantageous for complex multi-converter systems as this method does not require the knowledge of intricate details of the system’s parameters. The method can also facilitate adaptive stabilizing control schemes using reliable and fast identification implementations. However, impedance identification of multi-converter systems is typically challenging due to the coupled nature of the interconnected converters and potential non-linear behavior. Moreover, the bidirectional power flow of battery energy storage systems further complicates the stability assessment.

This thesis presents small-signal modeling methods, online stability assessment methods, and adaptive stabilizing control strategies for multi-converter systems that have bidirectional converters. The accuracy of traditional, small-signal-model-based converter control design is enhanced with a procedure that extends a converter’s small-signal model with given load and source dynamics. In addition, frequency response identification methods are used to assess the system stability under varying operating conditions. The presented identification methods offer reliable and quick impedance measurements and stability assessment among several converters. The design aims to minimize the interference on the system, which allows the identification during the system’s regular operation. The stability assessment provides a platform for adaptive stabilizing control methods, and two such techniques are implemented on a bidirectional converter. Several experimental results confirm the effectiveness of the proposed methods.