| As a smart integration mechanism of distributed energy sources (DER), microgrid is apromising direction of DER applications due to its outstanding performance on eliminating thenegative impact caused by the large scale penetration of DERs into the distribution network. In amicrogrid, the voltage source inverter equipped with energy storage unit (microgrid inverter, MGI) isa critical element that guarantees reliable and stable control and operation of microgrid, therefore,the study on MGI is an important research area. This dissertation focuses on the control strategiesand performance of MGI, especially the voltage and frequency controllers of MGI for islandedmicrogrids, and the unified controller for both islanded and grid-connected microgrids. Thisdissertation is organized as follows:(1) A detailed overview of existed research on microgrids is presented with the perspectives of theresearch background, the definition of microgrids, the essential characteristic of microgrids and thecorresponding demonstration projects at home and abroad. In the presented literature review, thecontrol strategies of power converters applied in microgrids especially the control strategies of MGIare classified, analyzed and summarized.(2) A mode-adaptive active power-frequency (P-ω) controller for MGI, named as voltage-sourcevirtual synchronous generator (VVSG), is proposed. The controller allows MGI to mimic theoperation of a synchronous generator (SG) by implementing the swing equation of SG with aprimary frequency controller. In addition, a generalized model of the active power generationdynamics is developed to compare the proposed controller with the conventional droop controller. Toachieve the optimal design of the proposed controller, a methodology of the parameters selecting ispresented in which decoupling, stability, steady and dynamic performances of a unified small signalmodel operating in both islanded and grid-connected modes of microgrid are used as indices.(3) An algorithm of virtual impedance is proposed in order to obtain the accurate designed outputimpedance and the switchless controller for both islanded and grid-connected operation. Theaccuracy of output impedance, the seamless transfer performance and the improved power qualityare achieved by adding an extra PI control loop of reactive power-voltage (Q-U) droop characteristicand adapting the droop coefficient. Moreover, an optimal design for the main control parameters isproposed to assure the seamless transfer without any chance of control structures and parameters. Asanother research result in this part, an algorithm of synchronous-frame-based virtual impedance isproposed to eliminate the side effect on the virtual impedance brought from the low pass filter in abcframe.(4) The relations between distribution line impedance, output impedance and output power sharing are revealed in the conditions of various power levels that are measured in the conventional droopcontroller. Several scenarios of various impedance mismatching are analyzed for both inductive andcapacitive reactive power. After that, an improved droop controller is proposed to reduce theunexpected dynamic and steady power sharing errors. With this proposed combined compensation,the power quality and the dynamic and steady performance are benefited. The design principle of theparameters in the proposed controller is also presented. The simulation and experimental results areprovided to validate the proposed controller and the parameters design methodology.(5) A hierarchy microgrid in lab is introduced and it includes an islanded microgrid formed by threeMGIs, a proposed virtual lab for microgrid center controller (MGCC) with a synchronizedsimulation clock and a physical MGCC platform in testing. With this three-MGI-formed islandedmicrogrid, the above controllers proposed in this dissertation are verified and analyzed in detail. |
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