Second-order generalized integrator (SOGI) based phase-locked loops (PLLs) are widely used for grid synchronization in single-phase grid-connected power converters. Previously, the estimated frequency of the PLL stage is fed back to the front-end SOGI block to make the conventional SOGI-PLL frequency-adaptive, which increases the implementation complexity, and makes the tuning sensitive, thus reducing stability margins. Alternatively, a frequency-fixed SOGI based PLL (briefly called FFSOGI-PLL) is proposed to ensure stability and simple implementation in this letter. It is commonly known that the in-phase and quadrature-phase signals generated by the frequency-fixed SOGI are of different amplitudes in the presence of frequency drifts, which causes second harmonic ripples in the estimated parameters of the PLL loop. To deal with this issue, a simple yet effective method is developed in FFSOGI-PLL. The standard SOGI-PLL is firstly introduced, followed by the working principle and small-signal model of FFSOGI-PLL. The FFSOGI-PLL is then compared with the SOGI-PLL in terms of stability and transient performance. Finally, experimental results are presented to demonstrate the effectiveness of FFSOGI-PLL.
The power grid containing wind farms is analyzed by AC probabilistic load flow. By using the method of the combination concept of Cumulants and Gram-Charlier series expansion theory, the DC probabilistic model is extended into AC probabilistic model, in which a three-parameter Weibull distribution is adopted to describe the random variation of wind speed, meanwhile the asynchronous wind turbine generator is treated as PQ node, then a probabilistic model of wind turbine generator is built. Based on the built model the calculation of IEEE 118-bus system is carried out, and then the probabilistic distributions of nodal voltages as well as the probabilistic distribution of branch load flow are obtained. The variation of nodal voltages and branch load flow before and after the connection of wind farms with power grid is compared.
Three novel homochiral helical metal-organic frameworks (MOFs), namely [Zn(LTP) 2 ] n (1), [Cd(LTP) 2 ] n (2), and [Ni(LTP) 2 ] n (3) (LTP = L-thioproline), have been synthesized and characterized. X-ray crystallographic analysis shows that all the three complexes consist of homochiral left-handed or right-handed helical chains. Complex 1 exhibits 1-D Zn-carboxyl right-handed helical chains constructed by pentacoordinated Zn 2þ ions. Complex 2 shows a 2-D homochiral coordination framework with (4, 4) topology based on Cd-carboxyl right-handed helical chains in which Cd 2þ ions are 6-fold coordinated. There are various hydrogen bonds between the adjacent helical chains in 1 and 2 which connect them into 3-D supramolecular networks. Complex 3 also exhibits a 2-D homochiral layer structure with (4, 4) topology containing Ni-carboxyl left-handed helical chains. The luminescence properties of 1 and 2 have been investigated briefly.
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