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Stable Operation And Control Of A Hybrid AC/DC Microgrid

Posted on:2015-04-01Degree:DoctorType:Dissertation
Country:ChinaCandidate:X L LiFull Text:PDF
GTID:1222330452970623Subject:Power system and its automation
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Via ac and dc buses, a hybrid ac/dc microgrid can take advantages of differenttypes of distributed generation (DG) units and storage systems to meet ac and dc loadsdemand requirements repectively with less energy conversion stages. So the systemcan operate with improved energy conversion efficiency, economy and reliability.Researches on the microgrid stable operation and control have been paid much moreattention for a long time. In this dissertation, some control methods to improve thehybrid ac/dc microgrid stability are systematically and thoroughly studied, and themajor work can be summarized as follows:(1) For the DGs based power supply system with a common dc bus, two basic dcbus voltage control strategies based on the DC-AC converter and DC-DC converterare studied respectively. To suppress the transient impacts and fluctuations of dc busvoltage and maintain the power balancing of the power supply system, methods suchas the improved feedforward method for the dc bus voltage control loop with thecombination of power balancing and a first derivative element, the PWM dead-timecompensation and the steady state duty cycle compensation for the current controlloops of the DC-AC converter and DC-DC converter respectively are proposed, whichare verified through both theoretical analysis and experimental results.(2) A nonlinear disturbance observer (NDO) based dc bus voltage control isproposed for the considered hybrid ac/dc microgrid system. Besides, influences of thedc line impedeance, dc bus capacitance variation and DC-DC converter controlperformance on the presented dc bus voltage control have been analyzed. Theoreticalanalysis and experimental results indicate that the proposed method can minimize thedc bus voltage fluctuations effectively. Furthermore, the high bandwidth communicat-ion between the dc subgrid and the DC-AC converter can be avoided, which isbeneficial for the scalability and plug-play of the distributed generators (DGs) withinthe dc subgrid.(3) To reduce the impact on the DG units and critical loads within the microgridcaused by microgrid operation mode transitions between the grid-connected mode andislanding mode effectively, a control strategy that can realize seamless microgrid ope- ration mode transfer between the above two modes is developed, which contains thecontrol state/reference compensation algorithm and separation switch control logic ofthe master DG unit. The proposed control strategy can eliminate inrush current ortransient over/under voltage during operation mode transition, and ensure a smoothtransition for the microgrid operation from grid-connected mode to islanded mode andvice versa.(4) The last of the dissertation is focused on the eigenvalues analysis of thestability issues in a master-slave control based microgrid under islanded mode. Thecomparison of the accuracy of the modeling methods based on continuous-time anddiscrete-time state equations respectively for the microgrid stability analysis isderived firstly; the main influence factors that lead to the microgrid instability causedby the unstable high-frequency oscillation modes are revealed after that; finally,band-pass filter based on a second-order generalized integrator (SOGI) has beenadopted for the feedback loop of the master converter voltage control, in order toimprove the system stability and ensure the stable operation of the microgrid.
Keywords/Search Tags:Hybrid AC/DC Microgrid, DC Bus Voltage Control, Nonlinear Disturbance Observer(NDO), Seamless Transfer, Small Signal Stability, Second-order Generalized Integrator (SOGI)
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