Voltage source inverters (VSIs) are usually used for all kinds of distributed generation interfaces in a microgrid. It is the microgrid's superiority to power the local loads continuously when the utility fails. When in islanded mode, the voltage and frequency of the microgrid are determined by the VSIs; therefore the power quality can be deteriorated under unbalanced and non-linear loads. A voltage unbalance and harmonics compensation strategy for the VSIs in islanded microgrid is proposed in this study. This method is implemented in a single synchronous reference frame (SRF) and is responsible for both the voltage unbalance and harmonic compensation. Furthermore, the virtual impedance loop is modified to improve the compensation effect. The impedance model of the VSI is built to explain the compensation ability of the proposed strategy. The whole control system mainly includes power droop controllers, a modified virtual impedance loop and inner SRF-based voltage unbalance and harmonics compensators. The proposed strategy is demonstrated in detail and validated with simulations and experiments.
Biodegradable poly(tert-butyl acrylate)-poly[(R)-3-hydroxybutyrate]-poly (tert-butyl acrylate) triblock copolymers based on bacterial poly[(R)-3-hydroxybutyrate] (PHB) were synthesized by atom transfer radical polymerization. The chain architectures of the triblock copolymers were confirmed by 1 H NMR and 13 C NMR spectra. Gel permeation chromatography analysis was used to estimate the molecular weight characteristics and lengths of the PHB and poly(tert-butyl acrylate) blocks of the copolymers. The thermal properties of the copolymers were studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA showed that the triblock copolymers underwent stepwise thermal degradation and had better thermal stability than their respective homopolymers, whereas DSC analyses showed that a microphase-separation structure was formed only in the triblock copolymers with the longer PHB block. As a similar result, from wide-angle X-ray diffraction experimentation, the crystalline phase of PHB could not be seen evidently in the triblock copolymers with the shorter PHB block. The enzymatic hydrolysis of the copolymer films was carried at 37 8C and pH 7.4 in a potassium phosphate buffer with an extracellular PHB depolymerase from Penicillum sp. The biodegradability of the triblock copolymers increased with an increase in the PHB block content. V V C 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4857-4869, 2005
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