| Micro-grid is an independent control system in order to solve the problem of distributed generation in power generation cost,control complexity and coordination with the utility grid.With the rapid development of power electronic technology and the emergence of a large number of DC power supply and DC load,DC power distribution technology has attached more and more attention and the concept of the DC micro-grid emerge to meet the times requirement.Compared to conventional AC micro-grid,control of DC micro-grid does not need to consider the frequency and phase,it is also not subject to the factors of three-phase unbalance,harmonics,flicker which are existed in utility grid,which will greatly simplify the design of the control system,and has a high reliability of power supply.In this paper,the DC micro-grid with single ended power supply and single bus structure,which contains renewable energy and hybrid energy storage unit,is used to study.The fault recognition ability of the structure is high,and protection system is easy to design.As well known,the power generated by renewable energy such as photovoltaic panels is intermittent and random.Therefore,sustained increase penetration of renewable energy will increase challenges of the reliable and safe operation of micro-grid.A new control strategy which is called power tracking control for DC micro-grid has been studied in this paper.The power tracking control strategy has been analyzed and designed based on frequency domain method,the characteristics of different frequency response characteristics of the distributed power supply are proposed.According to this response characteristic,each energy storage unit is used as the controller of the micro grid,and the controller model is designed to restrain the power fluctuation of the PCC and the tie line of the micro-grid.The system uses the coordinated operation of storage device to stabilize the random fluctuation of PV and load in the transient state of the system,the power balance of each part in the system.Based on the control strategy above,a central energy management method is designed for DC micro-grid control system.The method can be implemented to set the pre-determined amount of power that the micro-grid absorbs/delivers from/to the utility grid in the grid-connected mode and zero power in islanding mode.This design makes the power exchange between the micro-grid and the utility grid becomes aconstant value,which can effectively achieve the smooth switching between grid connected mode and islanding mode and the power balance of the micro grid system.Finally,the energy management method for DC micro-grid system is designed with CompactRIO from NI company as the main hardware for distributed control and data acquisition systems.And the graphical programming software Labview is used to realize the collection and analysis of the power parameters and the system control function of photovoltaic DC micro-grid power.Users can easily access the underlying hardware through graphical development environment and quickly establish embedded system control and data acquisition applications,which greatly shorten the time of system design and reduce the cost of development. |
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