Advanced Controllers of Power Electronic Converters in DC Microgrids

To cope with the pressure of climate change and depletion of fossil fuels, distributed power generation based on sustainable and green resources, such as photovoltaic and wind, have been exploited over the past decades. High penetration of renewable energy sources challenges the normal operation of traditional power grids, due to their characteristics of intermittence and uncertainty. To address this issue, an effective way is to aggregate distributed generators, energy storage systems, and customer loads together, as a single entity, that is, the so-called microgrids. Every microgrid is a fully dispatchable unit for grid operators, relieving the strains brought by renewable energy sources. Also, microgrids are able to provide reliable power for customer loads by supporting autonomous operation. Distributed energy resources are linked to microgrids by means of power electronic converters. As most of resources and future appliances are DC in nature, DC microgrids are more appealing than their AC counterparts. They can potentially achieve higher energy conversion efficiency and lower system costs, mainly by minimizing the number of DC-AC and AC-DC power conversion stages. Droop control is a common decentralized solution to implement primary level control. With the droop control method, DC bus voltage is employed to convey the loading condition of DC microgrids, and load power can be automatically allocated among parallel resource converters. This dissertation focuses on performance improvement of droop-controlled converters, mainly in the following three aspects: i) reduction of DC bus capacitance while maintaining tight DC bus voltage regulation; ii) suppression of second-order harmonic current flowing into distributed energy resources; iii) smooth transfer from power flow control to droop control, allowing DC microgrids to seamlessly disconnect from upstream grids. The first aspect: one of the constrains to reduce DC bus capacitance is the voltage surges and sags during load changes. From this point of view, resistive output impedance is a better design option than non-resistive output impedance for resource converters. This is because, given a certain output voltage tolerance band, resistive output impedance allows larger voltage dynamic variations, so that smaller output capacitance can be used. A systematical design approach, including the selection criteria of output capacitance and the design of droop coefficient, is proposed, covering both non-isolated (buck, boost, etc.) and isolated (dual active bridge) DC-DC converters. Following this design method, resistive output impedance can be effectively obtained. On the other hand, hysteresis control is another way to further reduce output capacitance, since it features faster dynamic response than classical PID control. Herein, hysteresis controller is implemented on digital signal processors instead of field programmable gate arrays. The implementation details, including the generation of driving signals for power switches and the effect of non-negligible computation time, are presented. The second aspect: second-order harmonic power is an unavoidable issue in DC microgrids with single-phase inverters/rectifiers. Since droop-controlled converters usually show low output impedance at twice the line frequency, second-order harmonic power can flow into resource sides of converters. In some application like fuel cells, such harmonic current ripples can shorten device lifetime. To prevent the diffusion of second-order harmonic power, this dissertation studies the adoption of notch filter and resonant regulator in control loops. Although these two methods could mitigate second-order harmonic current, they deteriorate the stability performance of converters. In such a case, modified notch filter and modified resonant regulator are proposed to overcome the shortcoming of the traditional schemes. A comparative study is carried out to highlight the advantages of the proposed filter and regulator. The third aspect: there are two limitations of the traditional droop control: one limitation is that the output power of droop-controlled converters is determined by load condition, and the other one is that the power sharing performance of droop control degrades with the presence of interconnecting cable impedance. To enhance the power flexibility and accuracy, a power-based droop controller, which unifies power flow control and droop control, is proposed for resource converters. When grid-interfacing converters impose the DC bus voltage, resource converters could operate with power flow control. When grid-interfacing converters fail, resource converters could work with droop control to stabilize the system. Importantly, the switch from power flow control to droop control can be automatically accomplished without communication or detection schemes. The operation principle, the design criteria, and the power sharing performance of the proposed controller are analyzed comprehensively. All of the above-mentioned proposals are verified by relevant experimental results performing on different laboratory-scale DC microgrid prototypes.