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Analysis And Stability Control Of A Series Microgrid In An Island Mode

Posted on:2016-01-25Degree:DoctorType:Dissertation
Country:ChinaCandidate:W M YangFull Text:PDF
GTID:1222330503954109Subject:Control theory and control engineering
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Micro-grid(MG) proposed in the background of environmental pollution and energy shortage is a new type of power generation and distribution system, which has the features including high permeability of renewable clean energy, flexible operation, and high power quality. MG can realize the unified management and sharing of energy producting by large amount of distributed generators, even can be used as the underlying structure of smart grid advocated in recent years.MG has developed more than ten years ago, its structure adopted by scholars and engineers mainly include three types as AC-type, DC-type and hybrid AC/DC-type. They almost follow the traditional power network structure, which result in the problems of circulation, harmonic, complicated control and low safety, so far these problems has not been well solved. A series micro-grid(SMG) studied in this dissertation can completely solve these main problems existing in the common MG or make certain problems easier to solve from the series structure. However, there are some new contents need to be researched, and some problems need to be resolved in this particular system.In order to make the series micro-grid capable for islanded operation, and popularized in the engineering, some contents including control of DC-link voltage based on hybrid energy storage system(HESS) in the renewable energy source, traditional control and advanced control methods of the series inverter- link, power sharing, splitting / recovery of one confined energy source, combination of a three-phase system have been studied in the dissertation. The content are in detail summarized as follows:1) Analysis of the series micro-grid characteristics during islanded mode, that mainly including output voltage and its harmonic characteristics, output current and frequency characteristic, local dynamic characteristics of the series inverter-link, DC-side equivalent load model of H-bridge inverters connected in series and their relationship each other.2) In view of stability control of DC-link voltage in random energy sources(RES), firstly, the influence of RES on output voltage of the SMG is analyzed.Secondly, small signal model of HESS composed of ultra-capacitor, battery and buck/boost bidirectional DC/DC converter is established in this study. Main causes of DC voltage fluctuation are analyzed based on the small signal model, and multi-loop composite control method may be formed by adopting the disturbances.3) Series inverter-link is the last link of output power conversion of each egergy source, the effectiveness of one control method is the key to determine waveform quality of the output voltage. Therefore, control of series inverter-link is seen as a key content in this research, and application of linear control and nonlinear control methods are employed respectively. The linear methods includes traditional PID double-loop control, composite double-closed loop control and double loop control based on quasi proportional resonant(QPR) voltage regulator. The nonlinear methods contain two advanced nonlinear control methods, i.e. input/output feedback linearization control and active disturbance rejection control.4) So as to realize optimal operation and reduce the operation cost of one SMG during islanded mode, power sharing method in optimal category is studied, which mainly involves two aspects.On the one hand, coordination control is completed between battery and ultra-capacitor in one HESS; on the other hand, the control is used among energy source. Coordination control of the HESS is completed by secondary adjusting reference current of the battery, and the control of each other RES is realized by directly fine-tuning the ratio of modular index. Double closed-loop method based on QPR voltage outer-loop and proportion incurrent inner-loop is responsible for ensuring system stable output.In addition, the small signal model of series inverter-link is considered to determine the constraints that coordinated control avoids affecting the main function.5) In order to meet the demand of three-phase load and realize balance gridconnection, the combined three-phase system with unified control and independentphase control is respectively studied. Series inverter-link adopts the same control method with double closed-loop, but the difference is that the former is realized in the single-phase natural axis, the latter is directly finishied in the two-phase static axis. Moreover, the demand of special splitting / recovery for a confined energy source in SMG is realized by coordinated regulations among DC-link voltages and closed-loop control of the series inverter-link.
Keywords/Search Tags:micro-grid, islanded mode, hybrid energy storage system, series inverterlink, splitting / recovery
PDF Full Text Request
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