Submodule Capacitor Voltage Stability Improvement with Virtual DC Machine Control for Delta Connected Cascaded H-Bridge Converter Based ESS

This paper introduces a Cascade H-Bridge (CHB) Converter based Energy Storage System (ESS) which has series connected submodules in each arm. Each submodule consists of an H-bridge circuit, bi-directional DC-DC converter, and a battery. Maintaining a constant DC submodule capacitor voltage on the DC side is crucial during voltage control implementation in the DC-DC converter. This constant voltage is essential for achieving smooth voltage synthesis in the H-Bridge converter. However, existing control methods might need further enhancement to more effectively regulate DC capacitor voltage variations, particularly during abrupt transients. To address this issue, this paper aims to enhance submodule capacitor voltage stability by introducing Virtual DC Machine (VDCM) control strategy, which mimics the dynamic characteristics of a DC machine. Validation of the proposed approach involved analyzing and comparing the characteristics of DC capacitor voltage with and without VDCM control through offline simulations and experiments using a 1.5kW submodule prototype.