Stability improvement and control optimization of isolated two-stage AC-DC-DC converter systems

• Main Author: Feng, Fan
• Other Authors: Gooi Hoay Beng
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Electrical and electronic engineering::Power electronics
• Online Access: https://hdl.handle.net/10356/142948
• Institution: Nanyang Technological University

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. Subsequently, the optimization guideline of the DAB controller is proposed to improve the stability of the DAB-based two-stage converter. By comprehensively analyzing the characteristics of the DAB impedance, it has been found that the magnitude of the DAB impedance is reduced around the cut-off frequency due to the insufficient phase margin. This can lead to instability problems of the two-stage converter, especially when the two sub-converters present close impedance magnitudes. Under such cases, the proposed controller optimization method is effective to solve the instability issues because it can avoid the magnitude dip of the DAB impedance. Since no additional passive components (more power loss generated) or active control loops (need extra sensors) are required, the stability of the DAB-based two-stage converter with the optimized controller can be guaranteed with the minimum cost and volume compared to those using the conventional passive or active damping methods. A family of impedance shaping regulators (ISRs), namely bus-voltage ISR (BV-ISR) and bus-current ISR (BC-ISR), is proposed for improving the stability of the two-stage converter under the heavy load conditions. The proposed ISRs can modify the DAB impedance by adding a parallel or series connected virtual impedance. The design requirements of the BV-ISR and BC-ISR to solve the instability issues are fully presented. The comparisons between the BV-ISR and the BC-ISR have clarified that although they are both effective in stabilizing the two-stage converter, the BV-ISR is better than the BC-ISR in terms of the dynamic performance and costs. In general, this thesis focuses on the instability issues of the isolated two-stage AC-DC-DC converters. Several stability improvement methods have been explored, including the optimization of the circuit parameters during the design phase, modulation schemes and feedback controllers, and the ISRs. As the renewable energy integration trend continues, new challenges and opportunities will emerge. Although the proposed impedance shaping methods are necessary for the stability improvement of the two-stage converter, they also harm the dynamic performances of the converter. Advanced control methods should be investigated to solve this issue. For individual power converters with grid support functions, the grid support under weak grids may cause instability concerns, which should be addressed through further research efforts.