School of Electrical & Electronic Engineering -2020
A thesis submitted to the Nanyang Technological University
in partial fulfilment of the requirement for the degree of
Doctor of Philosophy
2020
Abstract: Driven by the ever-increasing energy demand and the desire for carbon footprint reduction, the power industry is under a wave of transformation from the current grid into the smart grid. As the trend of the grid transformation continues, a significantly high penetration level of renewable energy resources, in parallel with other emerging technologies such as the energy storage and electric vehicles, is to be expected. Since many types of renewables and energy storage devices, e.g., solar photovoltaic, batteries and supercapacitors, etc., are treated as DC sources, the multi-stage converter system is usually employed as the interface between the AC grid and DC networks. Specifically, the dual active bridge (DAB)-based two-stage AC-DC-DC converter is highly related to the distributed systems because of its advantages such as the high power density, soft switching properties, galvanic isolation and less passive components. Therefore, the stability and reliability of the two-stage AC-DC-DC converters are at the core of the distributed system operations. However, despite its control benefits, the two-stage AC-DC-DC converter system may suffer instability issues. This thesis aims to investigate and overcome the instability issues of the two-stage AC-DC-DC converter system. First, the existing primary stability criteria and stabilization methods for the non-isolated two-stage converter systems have been reviewed and summarized. The terminal impedances of the sub-converters are useful tools to determine the stability of the two-stage converter systems. The forbidden regions for the voltage-source and current-source systems have been discussed. To satisfy these stability criteria, the terminal impedances of the sub-converters should be modified via passive or active damping methods. The research gap for the isolated two-stage AC-DC-DC converter has been identified as well. To analyze the stability of the DAB-based two-stage AC-DC-DC converter, the full-order impedance model of the DAB converter is derived for the first time in this thesis. Since the high-frequency ac conversion stage of the DAB converter naturally violates the small ripple assumption of the traditional state-space modeling, the generalized averaging approach is applied for the DAB impedance derivation. The derived impedance model can provide fully continuous-time representations that are capable of describing the ac conversion stage of the DAB converter. Furthermore, based on the developed impedance model, the influences of the DAB circuit parameters on the stability of the two-stage converter are analyzed. The analysis results offer instructive implications to fine-tune the design rules of the DAB converters. Bearing the mind that the impedance model of the DAB converter is closely related to the modulation schemes, the impacts of three typical modulation methods on the DAB impedances are analyzed and compared. An interesting phenomenon is found that the open-loop impedances of Single Phase-Shift (SPS)-based and Dual Phase-Shift (DPS)-based DAB converters present the characteristics of the parallel-connected inductor and capacitor, while the open-loop impedance of Cooperative Triple Phase-Shift (CTPS)-based DAB converter presents the resistor characteristics. The optimal modulation scheme in terms of stability performance for the two-stage converter is pointed out. This can help engineers find a more stable modulation scheme of the DAB converter in practical cascaded applications.
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