Short-term solar irradiance forecasting (STSIF) is of great significance for the optimal operation and power predication of grid-connected photovoltaic (PV) plants. However, STSIF is very complex to handle due to the random and nonlinear characteristics of solar irradiance under changeable weather conditions. Artificial Neural Network (ANN) is suitable for STSIF modeling and many research works on this topic are presented, but the conciseness and robustness of the existing models still need to be improved. After discussing the relation between weather variations and irradiance, the characteristics of the statistical feature parameters of irradiance under different weather conditions are figured out. A novel ANN model using statistical feature parameters (ANN-SFP) for STSIF is proposed in this paper. The input vector is reconstructed with several statistical feature parameters of irradiance and ambient temperature. Thus sufficient information can be effectively extracted from relatively few inputs and the model complexity is reduced. The model structure is determined by cross-validation (CV), and the Levenberg-Marquardt algorithm (LMA) is used for the network training. Simulations are carried out to validate and compare the proposed model with the conventional ANN model using historical data series (ANN-HDS), and the results indicated that the forecast accuracy is obviously improved under variable weather conditions. OPEN ACCESS
Abstract:Energy security and sustainability are crucial factors for the development of China. The creation of an evaluation theoretical system of the energy has theoretical and practical significance that is important for ensuring the safe and sustainable development of energy security that matches the national development phase and reflects the sustainable development of national energy. Sustainable energy security must not only take into account the security of energy supply-demand in the long-term and short-term, it must also focus on the coordinated development between energy, the environment, and the economy in China. This paper proposes five dimensions of energy security (availability, accessibility, affordability, acceptability, and develop-ability) to construct China's Sustainable Energy Security (CSES) evaluation index model. Based on the model, an empirical study of China's energy security is carried out with data from 2005 to 2015, and dynamic changing trends are analyzed accordingly. The results indicate that availability and develop-ability are the most important weights in China's Sustainable Energy Security index system, where availability shows a general downward trend, and develop-ability presents an inverted U-type trend, with its lowest point in 2011. From 2008 to 2012, China's sustainable energy security had been at risk. Taking the year 2010 as the demarcation, two phases were obtained: before and after 2010, during which the level of China's sustainable energy security first dropped, and then rose. However, compared with 2005, CSES level decreased by 28% in 2015 due to the decline of availability and accessibility. During 2005-2015, China's energy security system had relative high scores in acceptability and develop-ability, while the sustained downward trend of availability is in need of more regulation.
High-voltage direct current (HVDC) transmission line protection is becoming increasingly desirable with the expanding worldwide popularity of HVDC technologies in recent years. This paper proposes a transmission line backup protection scheme based on the integral of reactive power for HVDC systems. The directional characteristics of reactive power flow are theoretically analyzed for internal and external faults, and these characteristics are used to construct a directional protection scheme. The Hilbert transform is adopted to calculate the reactive power, which ensures a continuous output of calculation results and improves the reliability of the protection. A bipolar 12-pulse HVDC test system based on the CIGRE benchmark is modeled using PSCAD/EMTDC, and extensive simulations of various fault situations are conducted to test the effectiveness of the proposed scheme. The simulation results show that the proposed protection scheme correctly identifies internal and external faults and performs well with different fault distances and fault resistances. Furthermore, the proposed protection is insensitive to the sampling frequency, making it practical for future applications.Index Terms-directional protection, Hilbert transform, HVDC system, power system protection, reactive energy.
Differential protection is widely used for transmission lines, but its performance is affected by distributed capacitance current and current transformer (CT) saturation. Travelling wave (TW) differential protection is immune to these factors and can achieve ultra-high-speed operation. These characteristics are essential for long-distance extra/ultra-high-voltage (EHV/UHV) transmission line protection. However, the contradiction between sampling frequency and communication traffic makes it hard to ensure high sensitivity and high reliability at the same time. To solve this problem, a new TW differential protection based on equivalent travelling wave (ETW) is proposed in this paper. Wavelet transform is adopted to extract wavelet transform modulus maxima (WTMM) of TW, which are used to reconstruct ETW. Only several WTMMs need to be exchanged with the opposite terminal, so the amount of communication is largely reduced. Current energy ratio is defined and used for operation criterion to enhance the sensitivity during faults with large fault resistance and small fault inception angle. Detailed discussions on the selection of setting value ensure the reliability of the proposed protection scheme. Extensive simulations are conducted to test the performance of the protection. Simulation results verify that the protection can correctly discriminate between internal and external faults with high sensitivity and reliability.
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