| Micro-grid system containing different energy forms and power source capacities has become a hot international research topic currently, which includes micro-sources with rotating machine interface and micro-sources with inverter interface. The technology of the former type such as micro turbine, diesel generator units and small hydroelectric power plant is already mature. However, the latter type is not equipped with excellent outside characteristics as synchronous generators in large power system. Thus, how to control the coordination mechanism between multi micro source inverters in micro-grid system, how to realize accurate power distribution between micro sources with different capacity as well as circulation control and how to improve power supply quality are the key problems to realize micro-grid operation that need to be solved urgently.Due to the operating and control as well as some special power quality problems of micro-grid with inverters, meanwhile, under the support of the following projects: the sub-item " Power Quality Analysis and Control of Micro-grid and distribution system with micro-grid"(2009CB219706) of The National Basic Research Program Basic Science Research of Distributed Power Generation Systems"(973Program) Hunan Province Graduate Student Scientific Research Innovation Project(CX2011B130)"Power Quality Research on Large Photovoltaic Station(Gird-connected state, Islanded state) with Multi Inverters Environment", Hunan university Ph.D. thesis selection aid scheme in2009, the paper illustrates efficient power quality control of distribution system with micro-grid, discusses the comprehensive control strategies of micro-grid, compound control methods of micro-sources with inverters, power quality integrated management platform of distribution system with micro-grid not only from inner parts of micro-grid with inverters to external distribution system, but also from the whole control strategies to unit control methods, thus forms comparatively perfect basic research and technical methods of efficient use of energy on micro-grid. The key points and achievements are showed as follows:1. Research status of distributed generation and micro-grid has been described. The paper expounds the subject significance and summarizes micro-grid structure and definition, system characteristics, micro source varieties,the principal control method and primary problems existing in control process. The existing micro-grid control strategies and their respective advantages have been contrasted. Then, the main subject research content, micro-grid operation and control, special power quality problems, compound control method and group control platform research have been summed up.2The integrated control scheme of multi-inverter interface micro-grid based on the combination of peer-to-peer and master-slave control is raised. In this control strategy, improved drooping controller with constant voltage constant frequency function has been designed to supply voltage and frequency standard for micro-grid in island operation mode and to share load power variation. The paper takes inductive equivalent output impedance as an example, analyzing its influence mechanism to power distribution and the limitations of traditional drooping control method. Directing at the multi-loop feedback controller structure containing voltage and current loop, by constituting integrators, redesigning equivalent output impedance, removing the balance relationship between reactive power control and equivalent output impedance, the improved drooping controller possesses strong robustness ability in application of micro source parallel system with different capacities, which increased load power distribution precision as well as reduced system circulation effectively. PQ controller which is suitable for voltage source inverter has been devised, ensuring that micro source output power is equal to the reference value. Multi-inverter interface micro-grid simulation platform adopted the above improved drooping controller and PQ controller is structured for subsequent chapters analysis.3.According to topology structure, output characteristics, network structure and load characteristics of micro-sources with inverters, comprehensive power quality control measures haves been presented in active and passive control aspects directing at special power quality problems in micro-grid. In the active control direction, inverter control method with harmonic suppression and reactive power compensation function has been put forward. Active power can be sent out while harmonic generated by nonlinear load and reactive power can compensated according to scheduling instruction. Meanwhile, power quality of grid-connected node can be guaranteed at high level. Following operation characteristic of synchronous generator in power grid, micro-grid inverter controller based on a virtual synchronous generator technology has been described. Moreover, contrasting with traditional power drooping control method, the paper analyzes its advantages in the way of suppressing circulation. Output voltage waveform of three-phase four-wire micro-grid inverter is affected by grid distortion voltage, harmonic current of load and neutral-point voltage balance at dc side jointly. In order to maintain neutral-point voltage stable and to make output voltage waveform tracking reference voltage, the paper has set up model according to a four-bridge arm inverter structure and proposed four-bridge arm inverter high quality grid-connected H-oocontrol method for three-phase four wire micro-grid. High bandwidth, wide robust controller has been built to control neutral bridge arm and three-phase bridge arm unified. The four-bridge arm inverter could overcome various kinds of disturbance factors under the large neutral current or grid voltage distortion conditions, which could improve micro-grid power quality. Improved PQ control methods under voltage unbalance conditions of power grid have been proposed. Feed-forward decoupling PQ control based on balance assumption is a typical micro-grid control strategy. By carrying out power analysis on the above control strategy and taking advantage of switch function method to analyze output performance of grid-connected inverter, the output of inverter would contain3th order harmonic and negative sequence component based on balance assumption control strategy under voltage unbalance condition. On the basis of the original PQ control structure, negative sequence voltage control loop has been imposed to offset negative sequence components in power grid voltage. Thereby, three-phase balance can be maintained. Furthermore,3th order harmonic component output could be reduced by the modified switch function modulation method.4、Based on the proposed micro-grid control structure, transient characteristics simulation analysis under various working conditions has been carried out. The paper analyzes and simulates the transient stability influence caused by typical induction motor load, power supply quality level at special moment when micro-grid change the operation mode, the impacts on micro-grid power supply when remove load or micro source purposely, micro-grid transient response process when change micro source output power and special power quality problem induced by unbalanced load or nonlinear load. The paper analyzes the dynamic process of micro-grid connecting to distribution grid in detail, compares the role of inverter interface micro source with rotating machine interface micro source in supporting distribution grid power and assisting passing through voltage breakdown process.5、An integrated management platform including micro-grid, distributed generation and power quality control equipments has been presented. Grid structure as well as communication mode with its constraint condition based on multi-agent system have been introduced. The paper raised the network topology consisting of core layer, convergence layer, access layer and foreign service layer. In the examples, micro-grid combined with DSTATCOM operation system as well as inverter interface micro-source combined with static var compensator (SVC) operation system have been proposed. Besides, the mutual interaction mechanism and coupling relation in control field between power quality management equipment equipped with the same topological structure as inverters and micro-sources, as well as the effect that power quality control equipment performed in the aspect of suppressing micro-grid voltage fluctuation, assisting low-voltage crossing and compensating reactive power have all been studied in detail.The paper discusses multi-inverter interface micro-grid operation and control methods as well as management methods on some special power quality problems. A new micro-grid control structure based on improved drooping control methods and PQ control has been presented chiefly. Besides, micro-grid power quality management scheme combined with active and passive control has been put forward. According to compound control methods on special power quality problems of inverter interface micro-grid, transient simulations of multi-inverter interface micro-grid under various working conditions have been completed. Furthermore, the paper set up integrated management platform including distributed generation, micro-grid and power quality control equipments from theory to method, then to simulation. Coordination of centralized control platform could strengthen the control ability of micro-grid power and promote the level of power quality. The research results can be applied in such occasions as large photovoltaic power station, roof power station, intelligent distribution network, agricultural distribution network, new energy and especially multi-group parallel inverters. |
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