Low voltage ride through (LVRT) capability is required by new power system operating codes, then voltage sag generator (VSG) is generally used to simulate different voltage sag faults. The work principles of transformer and bidirectional switches based VSG and full-power converter based VSG are explained and discussed, the former can choose relay, thyristor or IGBT as bidirectional switches, and the comparison research of several VSG solutions is carried out by analysis and experiment. The experiment results make out that the full-power converter based VSG is powerful but its control is complex; the transformer and bidirectional switches based VSG has simple structure and high reliability, can simulate various types of voltage sag faults, and the voltage transition between normal and fault operation is smooth, then provide favorable conditions to verify the LVRT capability of wind power system.
Keywords-wind power system; low voltage ride through (LVRT); voltage sags; voltage sag generator (VSG); autotransformer; bidirectional switches; full-power converterI.
Urban shrinkage has become a topic of major concern to scholars of geography and urban science. However, the methods of identifying urban shrinkage and growth have mostly focused on traditional statistical methods, and studies based on nighttime light (NTL) data are rare. Here, we use the NTL data for 56 months from 2012 to 2019 obtained by the Visible Infrared Imaging Radiometer Suite (VIIRS) on board the Suomi National Polar Orbiting Partnership (NPP) to identify the shrinkage and growth patterns of Yichun in China, by calculating the slope of the NTL radiance value after denoising. At the same time, by combining high-resolution Google satellite images and traditional demographic data, we analyzed the shrinkage characteristics of Yichun. The results of the study confirmed the characteristics of partial shrinkage in China’s shrinking cities. In addition, the use of NPP-VIIRS NTL data was able to more accurately identify the urban shrinkage and growth patterns, and may also be seen to present a more objective picture of reality, thus providing a new perspective for studies of urban shrinkage.
Pitch control system is the most important part of wind turbine to keep the rotor speed within its design limit. Large scale wind turbines usually adopt PI controller to adjust rotor speed above the rated wind speed. Since wind turbine system is a high-order nonlinear system, one set of PI parameters can't achieve excellent control performance, especially when wind turbulence is very strong. In view of the shortcoming of conventional PI controller and nonlinear characteristics of wind turbines, an expert PID controller based on a trackingdifferentiator is researched in this paper. The trackingdifferentiator is used to arrange transitions of the control variables, such as the speed, and extract the one-order differential signal reasonably, which has the potential to cope with the nonlinear characteristics of wind turbines. The simulation results show that proposed controller can effectively restrain the overshoot of machine's rotor speed and has better control performances than conventional PI controller.
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