Research On Autonomous Control And Optimal Sizing Of Low-voltage DC Microgrids

With the penetration of renewable energy sources(RESs) continuously increasing among modern power systems, the concept of microgrid(MG) is getting more and more attention since it can integrate distributed generations and local loads. Due to the existing of a large number of DC micro-sources such as photovoltaic(PV) cells and energy storage units(ESUs), and the increasing number of DC coupling devices, DC MGs which can remove the AC bus show their advantages on shortening the energy conversion chain and improving the system efficiency. Meanwhile, DC MGs also present their benefits like simple structure, high controllability, etc. Taking single-bus low-voltage DC MGs as the research object, and surrounding some parts of the key issues in DC MGs, the researches about autonomous control among each unit, improved DC droop control method, hybrid energy storage(HES) converter and its control strategy, and the optimal sizing of DC MGs are conducted in this dissertation. The research findings have important impacts on operating control and optimal sizing of DC MGs.Firstly, based on the detailed analysis of the architecture of DC MGs and the working status of each unit, the research on operating control method for the internal component units is carried out. In order to achieve peer-to-peer control of DC MGs, DC droop control is adopted to solve the power sharing issue of parallel units under constant voltage mode, and DC bus signaling is used as the unique criterion for the working status switching of each unit. Aiming to maximize the use of renewable energy and extend the stability region of system operating, a unified autonomous control strategy allowing DC MGs operating both at grid-connected and islanded modes is proposed. Operating modes of DC MG can be transformed according to the DC bus voltage levels, and failure mode can be skipped autonomously. Meanwhile, considering the extreme operating condition, converters for the non-critical loads are also included to participate in the DC bus voltage regulation. Furthermore, on the basis of achieving autonomous control in DC MG, a seamless switching control strategy of PV units by shifting the droop curve is proposed, which allows PV units seamless switching between constant voltage mode and maximum power point tracking mode.The DC droop control principle and the effects of droop parameters are analyzed, which reveal the reasons for the deviation of DC bus voltage and the decrease of current sharing accuracy caused by droop control. The dynamic current sharing accuracy of the droop-shifting secondary control method is quantitatively analyzed based on the comparison of the existing secondary control schemes. Furthermore, an improved adaptive impedance secondary control scheme with enhanced steady and dynamic current sharing performance of droop-controlled DC MGs is proposed. The proposed control approach adopts distributed secondary control scheme, and makes reasonable slope-adjusting by using the designed dual-closed-loop regulator and voltage-shifting equalization method when it regulates droop-shifting amount simultaneously. The impedance characteristics and system stability analyses are carried out by establishing the equivalent circuit model and small-signal model of the proposed secondary control. The corresponding experiment prototype is built to investigate the validity of the proposed secondary control scheme under the conditions of different communication delays, line impedances, number of converters, etc. Furthermore, test processes with converter failure and recovery, local loads connected and disconnected are set up to verify the the resiliency of the proposed control scheme under complicated scenarios.In order to improve transient performance and output power quality of DC MGs, and extend the life span of energy storage unit, the application of HES system in DC MGs is studied. With the comparison and analysis of the existing HES connection schemes, phase-shifting full-bridge three-port converter with high power density, high efficiency and electrical isolation is employed as the interface converter among the batteries, ultra capacitors and DC bus. A novel power allocation scheme for HES based on virtual inductance and virtual capacitance is proposed, in which low frequency power fluctuation in the system is responded by batteries, and high frequency power fluctuation is responded by ultra capacitors respectively when the HES unit is operated under constant voltage mode. Furthermore, based on the power transmission mechanism and power decoupling control of the three-port converter and by integrating the proposed control method, HES control strategy of the three-port converter is designed. The proposed HES control strategy and its application on three-port converter in DC MG are verified by empirical studies.At last, the optimal sizing method of low-voltage DC MGs is studied. Due to the feature of DC MGs that demands grid-connected converter to be linked with AC grid, the operating status of grid-connected converter and on-grid electricity price are taken into account. Meanwhile, with constraints including battery state of charge(So C), loss of power supply probability(LPSP), renewable energy efficiency(REE), etc., particle swarm optimization(PSO) algorithm is adopted to calculate the optimal sizing results targeting at the lowest annualized capital cost and operation & maintenance costs. Furthermore, based on the analysis of the operating schemes of DC MGs and considering the life span estimated results of batteries, an optimal sizing approach with comprehensive evaluation of operating modes and operating targets of DC MGs is proposed. The variation trends of annualized capital and maintenance costs are investigated under different LPSP and REE. Meanwhile, the system operating schemes are validated by using the obtained results, and comparison and analysis of the detailed results are conducted.