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ABSTRACT High-standard farmland construction is an important process that can enhance food security and accelerate new-style modernization agriculture. Hyperspectral remote sensing can provide data and technical support for this type of construction to provide a reference when optimizing high-standard farmland construction areas. This study was performed in Xinzheng City, the primary grain-producing areas in Henan Province. Field sampling and indoor hyperspectral spectroscopy (350~2500 nm) were combined; spectral transformations such as continuum removal (CR) were performed after Savitzky‒Golay (SG) convolution smoothing; and the best hyperspectral bands were selected as the common index of the soil properties by correlation analysis and fuzzy clustering maximum tree. A hyperspectral inversion model was built for the panel data model of the fixed effect variable coefficient based on the ordinary least squares estimation method (OLS), including panel data describing pH, organic matter, nitrogen, phosphorus, potassium, iron, chromium, cadmium, zinc, copper, and lead of 116 samples in Xinzheng City. Results show that the panel data model is of good quality overall, and the goodness of fit is higher (R2 = 0.9991, F = 2195.67). The precision test results indicate that the models performed well at both description and prediction, including accurate quantification, with an RPD above 2.5. Thus, the proposed model provides an important basis for soil information management, resource evaluation, and a reference when optimizing high-standard farmland construction processes.

With the development of new energy, energy storage, and flexible DC technology, AC/DC hybrid distribution networks have gradually become an important development direction for future distribution networks. The access of a large number of power electronic devices has brought dynamic problems such as low frequency oscillations to the distribution network. In order to deeply study the low frequency oscillation problem on the DC side of AC/DC distribution networks, detailed modeling of key links is conducted such as H-bridge series and isolated bidirectional active bridge structures, three-phase inverters, and three-phase Buck converters in the main valve tower of power electronic transformer. Through time domain simulation, the 4 Hz low frequency oscillation phenomenon on the DC side of the AC/DC distribution network in the actual project is reproduced, and the correctness of the established mathematical model is verified. Based on the established mathematical model, qualitative analysis is conducted on key influencing factors such as power and controller parameters, and the final conclusions are as follows: power level and Buck link voltage loop control parameters affect system stability, the integral coefficient of the current loop in the Buck link affects the oscillation frequency.

Cultivated land is the foundation of food security, while coal guarantees the security of the energy. The coal-cropland overlapped area accounts for approximate 42.7% of the total cultivated land area in China, making it become a prominent contradiction between the extensive mining and cultivated land protection. How to realize the coordinated development of the coal mining and the cultivated land protection is an urgent problem in coal-cropland overlapped areas. This paper analyzes the research status and existing issues of the coordinated development between mining and cultivated land protection from the following three aspects: the damage mechanism of subsidence land in plain coal-cropland overlapped areas, the damage degree evaluation of subsidence land induced by mining, the control technology for the mining subsidence. Then, considering the characteristics of the cultivated land, such as the extensive protection-needed area and the high deformation tolerance, a new technical approach of controlling regional rock strata and surface deformation in green mining is proposed by combining source control and restoration. The corresponding prediction and control design methods of surface deformation are established. Additionally, the key research directions for the coordination of coal mining and cultivated land protection in the plain coal-cropland overlapped areas are pointed out, including the driving mechanism of cultivated land damage induced by mining in the plain area, protection thresholds for the quality of the cultivated land in subsidence areas, theories and methods of the subsidence coordinated control of the green mining facing the cultivated land protection. The main objective of this paper is to promote the coordinated development of the green mining and the cultivated land protection in the plain coal-cropland overlapped areas.

With the rapid development and wide range application of AC/DC distribution network,its stability has also become the focus. The low-frequency oscillation at the DC side in the commissioning stage of an AC/DC distribution network project is analyzed. Based on the small signal modeling,the eigenvalue analysis method is used to analyze the influence laws of short circuit ratio (SCR),power level and control parameters on low-frequency oscillation. Next,the measures of changing controller parameters and adding additional controllers to suppress oscillation are proposed. The theoretical analysis and simulation test show that weak grid affects the stability of voltage source converter (VSC). Low frequency oscillation occurs when the 375 V DC side power increases to exceed the rated power. The integral coefficient of the Buck converter current inner loop proportional integral (PI) controller does not affect the stability of the system,but it affects the oscillation frequency after the oscillation occurs. If the proportional coefficient of the voltage outer loop PI controller is too small or the integral coefficient is too large,the system is unstable. Changing voltage outer loop PI controller parameters of Buck converter and adding additional controller can effectively suppress low-frequency oscillations. The theory and simulation analysis of the actual AC/DC distribution network oscillation influence factors are of reference significance for the commissioning and operation analysis of other AC/DC distribution network projects.

To enhance the efficient utilization of bauxite tailings，the mullite was synthesized by bauxite tailings in molybdenum wire furnace at 1 550℃ and 1 650℃ for 4 h.The sintered samples were analyzed by X-ray diffraction(XRD)，scanning electron microscopy(SEM)and energy dispersive spectrometry(EDS).The content of mullite was determined by TOPAS.The results indicated that the mullite was in high purity and well growth.The mullite was long column in morphology，alternating each other.As the temperature increased，the crystal profile of mullite was much more clear and the content of mullite increased slightly.