Stabilization and Dynamic Performance Enhancement of DC Distributed Power Systems
Open AccessThis dissertation proposes methods to stabilize and enhance the dynamic performance of DC distributed power systems (DPSs). Various applications are based on the DPS configuration such as electric vehicles, and DC microgrids. Regardless of how geographically-spread the DPSs are, they operate similarly. A DPS can be readily synthesized by connecting two switched-mode regulators in series. Such a configuration constitutes a highly reliable DC power system, which is efficient and cost-effective. Switched-mode regulators are tightly regulated devices, so cascading two of them would adversely affect their relative stability margins. As a consequence, the voltage at the point of interfacing the converters might be unstable. The instability originates from an impedance interaction between the output impedance of the source converter ($Z_o(s)$) and the input impedance of the load converter ($Z_{in}(s)$). Therefore, these impedances are crucial for distributing DC power stably. Many criteria were proposed in order to build a deterioration-free DPS. The criteria define permissible areas in the complex plane for the Nyquist plot of $Z_o(s)/Z_{in}(s)$ in order to prevent any impedance interaction. Designing a DPS according to any of the criteria is technically infeasible, and leads to a long manufacturing time. Therefore, passive damping methods (PDMs) were introduced in order to integrate regulators that are supplied by different vendors, so the manufacturing time is substantially reduced. Yet, PDMs jeopardize the efficiency and power-density of DPSs due to the incurred power-loss, and the added weight and size; PDMs are not applicable to size and weight limited applications such as satellites.Active damping methods (ADMs) were introduced in order to improve the dynamic performance of DPSs using novel control techniques. Hence, the efficiency, size, and weight of DPSs are not affected. The proposed ADMs in the literature have limitations that can be classified as compatibility with a specific switched-mode regulator (such as buck converters), ability to stabilize a system without improving much the dynamic performance, and incapability of handling multi-converter load subsystems.This dissertation proposed three control techniques that address the drawbacks of the presented ADMs in the literature. All of the three controllers stabilize and considerably improve the dynamic performance of DPSs. The first controlling method aims to diminish the magnitude of the source converter output impedance such that the source converter will act as an ideal voltage source. As a consequence, the dynamic performance of the source and load converters will be intact, thereby ensuring the bus voltage stability. The second controller injects the bus-voltage oscillations into the control loop of the source converter using a high-pass filter. The filter suppresses the DC quantity of the bus voltage, while passing the high-frequency oscillations. Thus, the operating point of the source converter does not change, while the dynamic performance is improved. The third controller improves the dynamic performance of DPSs that are under the Average-Current-Mode (ACM) control technique by adding an extra negative feedback to the original ACM controller. Therefore, the impedance interaction between the source and load converters is eradicated. Each proposed controller was verified using small-signal analyses. In addition, numerous time-based simulations and experiments were conducted, under rigorous testing conditions, in order to validate the controllers' effectiveness. Collectively, the outcomes of the mathematical analyses, simulations, and experiments of each proposed controller were in agreement of validating its veracity.
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Aldhaheri_gwu_0075A_14040.pdf | 2018-03-26 | Open Access |
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