Hopf Bifurcation and Parameter Sensitivity Analysis of a Doubly-Fed Variable-Speed Pumped Storage Unit

The doubly-fed variable speed pumped storage unit is a storage system suitable for joint operation with renewable energy sources to smooth the imbalance between renewable energy supply and electricity demand. However, its working principle and operation control are more complex than those of constant speed pumped storage. In this study, a nonlinear model of doubly-fed variable speed pumped storage units (VSPSUs) considering nonlinear characteristics of the head loss is established. The study finds that a supercritical Hopf bifurcation occurs in the system, and the area enclosed by the lower side of the bifurcation line and the coordinate axis is the stability domain of the system. The active power step perturbation from −0.3 to 0.3 will gradually reduce the area of the stability domain and narrow the adjustable range of the control parameters. In addition, the sensitivity of the model full state variables and the primary and secondary relationships to the changes of subsystem parameters is analyzed systematically using the trajectory sensitivity. It is found that there is a large difference in the sensitivity of different state variables to the parameters. The state variables are much more sensitive to the transfer coefficient of hydraulic turbine torque to guide vane opening, the unit inertia time constant, and the controller proportional gain change than other parameters, which are defined as highly sensitive parameters. The receiver response time constant and the turbine flow-to-head transfer coefficient are the corresponding low-sensitivity parameters

Hopf Bifurcation and Parameter Sensitivity Analysis of a Doubly-Fed Variable-Speed Pumped Storage Unit

The doubly-fed variable speed pumped storage unit is a storage system suitable for joint operation with renewable energy sources to smooth the imbalance between renewable energy supply and electricity demand. However, its working principle and operation control are more complex than those of constant speed pumped storage. In this study, a nonlinear model of doubly-fed variable speed pumped storage units (VSPSUs) considering nonlinear characteristics of the head loss is established. The study finds that a supercritical Hopf bifurcation occurs in the system, and the area enclosed by the lower side of the bifurcation line and the coordinate axis is the stability domain of the system. The active power step perturbation from −0.3 to 0.3 will gradually reduce the area of the stability domain and narrow the adjustable range of the control parameters. In addition, the sensitivity of the model full state variables and the primary and secondary relationships to the changes of subsystem parameters is analyzed systematically using the trajectory sensitivity. It is found that there is a large difference in the sensitivity of different state variables to the parameters. The state variables are much more sensitive to the transfer coefficient of hydraulic turbine torque to guide vane opening, the unit inertia time constant, and the controller proportional gain change than other parameters, which are defined as highly sensitive parameters. The receiver response time constant and the turbine flow-to-head transfer coefficient are the corresponding low-sensitivity parameters

An overview of deterministic and probabilistic forecasting methods of wind energy

Summary: In recent years, a variety of wind forecasting models have been developed, prompting necessity to review the abundant methods to gain insights of the state-of-the-art development status. However, existing literature reviews only focus on a subclass of methods, such as multi-objective optimization and machine learning methods while lacking the full particulars of wind forecasting field. Furthermore, the classification of wind forecasting methods is unclear and incomplete, especially considering the rapid development of this field. Therefore, this article aims to provide a systematic review of the existing deterministic and probabilistic wind forecasting methods, from the perspectives of data source, model evaluation framework, technical background, theoretical basis, and model performance. It is expected that this work will provide junior researchers with broad and detailed information on wind forecasting for their future development of more accurate and practical wind forecasting models

Assessment of desertification in the agro-pastoral transitional zone in Northern China (1982-2006) using GIMMS NDVI data

The Agro-Pastoral Transitional Zone in Northern China (hereafter APTZNC) is situated in an arid/semi-arid area, and is one of the most vulnerable areas in the world subject to climate change. Annual integrated the NASA Global Inventory Modeling and Mapping Studies (hereafter GIMMS) Normalized Difference Vegetation Index (hereafter ΣNDVI) and annual rainfall were used in this study. Meanwhile, the dynamics of ΣNDVI and rain-use efficiency (hereafter RUE) were predicted during the period, through the use of the Mann-Kendall nonparametric test and linear regression temporal trend analysis. The tendency of desertification under different precipitation scenarios was also analyzed. The results showed that annual ΣNDVI and rainfall were not significantly correlated in most sections of the study area, yet opposite results were observed for a smaller percentage of the study area (p<0.01). Changes in vegetation productivity may increase, whereas a significant decrease in a small pixel proportion was observed. The northeast and central sections of the study area are characterized by positive trends in RUE slope values, contrary to what was observed in the southwestern sections of the study area. The results fit well with the findings through ΣNDVI and RUE. Rainfall in the range of 200-500 mm can be seen as a threshold value as the desertification trend decreases and vegetation restoration capacity is enhanced with increasing rainfall. © Copyright 2013 SPIE