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Considering the shaft torsional vibration risk of the turbogenerator caused by the integration of large capacity wind farms in wind & thermal power transmission system, the impact of doubly fed induction generator ( DFIG) -based wind farms interconnected to power grid on subsynchronous oscillation of turbogenerator was investigated. Firstly, the detailed mathematical models for the small signal stability analysis with DFIG-based wind farms were established. By utilizing modal analysis method, the changes of the system subsynchronous oscillation modes with and without DFIG-based wind farms were investigated, respectively, and the presented models and modal analysis were demonstrated by comparison with the time-domain simulation results. Then, based on the concept of parameter sensitivity, the analytical sensitivity expression of subsynchronous oscillation damping ratio on wind farm parameters was deduced. Finally, Regarding for the different conditions of wind speed and reactive power of the wind farms, the variation of subsynchronous oscillation mode was discussed. The results show that the connected DFIG-based wind farm could reduce the subsynchronous oscillation damping in keeping the external condition. Furthermore, the parameters of active power and reactive power of the wind farms have an obvious effect on the system subsynchronous oscillation damping. [ABSTRACT FROM AUTHOR]

In order to explore the effect of biochar on the volatilization of ammonia in different surface soils, through the field test method, the responsiveness changes of ammonia volatilization, ammonium nitrogen, and urease were studied under the influence of biochar and crop cultivation. The study set six treatments:CK+, BC0.5+, and BC4.5+ (the biochar applied at 0, 0.5, 4.5 kg·(m 2 ·a) -1 , respectively, under the condition of crop cultivation), and CK-, BC0.5-, and BC4.5- (the biochar applied at 0, 0.5, 4.5 kg·(m 2 ·a) -1 , respectively, under the condition of bare land). The results showed that under the condition of crop cultivation, the ammonia volatilization of the BC4.5+ and BC0.5+ treatments increased in 4 days, then significantly decreased by 9.95%-61.80% and 7.97%-50.52% ( P <0.05), respectively, with respect to CK+. However, compared with CK-, the ammonia volatilization of the BC4.5- and BC0.5- treatments increased by 40.02%-93.15% and 28.09%-57.45% ( P <0.05), respectively. For the same amount of biochar application, the ammonia volatilization of the crop-planting soil was significantly lower than that of the bare land, and BC4.5+ and BC0.5+ declined by 27.10%-92.10% and 13.17%-83.45%( P <0.05), respectively, compared with the BC4.5- and BC0.5-, and there was no significant difference between CK+ and CK-. The above results indicated that biochar-mediated soil ammonia volatilization was inhibited by the surface cover. Moreover, with respect to CK+, the maximum increase rates of the contents of ammonium nitrogen and urease in the BC4.5+ and BC0.5+ treatments were orderly at 69.25% and 72.73% and 93.61% and 90.56% ( P <0.05), but the soil ammonia volatilization decreased in the same period. The biggest decline of the soil NH 4 + -N and urease content of BC4.5- and BC0.5- were 63.78% and 95.70% and 78.38% and 92.64% ( P <0.05), respectively. Simultaneously, the soil ammonia volatilization rose in the bare land compared with CK-. Therefore, the soil ammonia volatilization was inversely related to soil NH 4 + -N and urease under the influence of biochar, and the effect of crop planting was more profound.

It was aimed to investigate the response to biochar addition on N 2 O gas production and emission in different surface conditions. To study the dynamic changes of soil N 2 O release, soil nitrate(NO 3 - -N) and ammonium(NH 4 + -N), a field trials experiment was conducted from 2014 to 2015 in wheat and corn season, which contained three treatments[the blank control group (CK), biochar applied at 5 t·(hm 2 ·a) -1 (BC5) and 45 t·(hm 2 ·a) -1 (BC45), respectively] under crop cultivation(+) and non-cultivation(-) condition. The results indicated that:1 During the season of wheat growth, the soil N 2 O emissions of CK+, BC5+, BC45+ were 21.70-88.91, 21.42-130.09, 64.44-179.58 μg·(m 2 ·h) -1 respectively, and that of BC45+ possessed a higher value than those of the other treatments( P <0.05). Compared with wheat winter period, the soil N 2 O emissions of the three treatments decreased evidently in wheat peak stage(returning green and jointing stage, booting and heading stage) ( P <0.05), and the amplification of BC45+ reduced by 18.43% and 14.62% in comparison with CK+ and BC5+ in wheat booting and heading stages. In the early stage of maize growth, the soil N 2 O emissions of BC45+ were significantly increased compared with CK+ and BC5+( P <0.05). However, there were no significant differences among treatments of maize heading stage and mature stage. It showed that the biochar-mediated promotion effect of soil N 2 O emissions was effectively inhibited by crop growth and the increase of surface mulch. Besides, the result of soil N 2 O release in the same treatment had also confirmed this conclusion in bare land. 2 Under the conditions of wheat cultivation and homochronous non-cultivation, the soil NO 3 - -N and NH 4 + -N contents of BC5+ and BC45+ treatments were raised with respect to CK+, but the values dropped significantly in wheat peak stage, especially for BC45+ treatment, with 96.44% and 69.40% decrease respectively. The soil inorganic nitrogen content of maize growth season had a similar trend in wheat season. Parallel to this result of the apparently falling soil NO 3 - -N and NH 4 + -N concentrations, the soil N 2 O emissions of BC45+ also declined remarkably in peak stage. The decline in respiratory substrate caused by the increase of nitrogen uptake by crop growth, may be one of the reasons for the decrease of N 2 O emission. 3 In wheat growth season, the soil pH values of the biochar treatments were improved from 4.62 to 5.18. In maize season, the soil pH values ranged from 4.42 to 5.02. When the soil pH value was relatively low, the soil N 2 O emission was high, and vice versa. The soil N 2 O emission was partly influenced by the soil pH value.

Biochar application to soils is currently considered as a means of mitigating climate change by sequestering C, which withdraws CO 2 from the atmosphere and consequently influences the trend of global climate change. However, only a few studies have been done on surface albedo variations on farmland, it might be true that the application of biochar to soil could induce a radiative forcing by changing the surface albedo. Based on this background, this study aiming at the characterization of seasonal changes in albedo on the farmland both with plants and without plants, and its effect on soil temperature, soil moisture, soil respiration and soil organic carbon fractions. There were 3 major treatments in the experiment with 3 repetition of each major treatment, namely, CK (the control treatment), BC0.5 (biochar applied at 0.5 kg·(m 2 ·a) -1 ) and BC4.5 (biochar applied at 4.5 kg·(m 2 ·a) -1 ). Each major treatment contained 2 sub-treatments, which were crop cultivation (+) and non-cultivation (-). The experiment results showed that in the early stage of crop growth (maize seedling stage to jointing stage, wheat seedling stage to winter period), the surface albedo of BC4.5+and BC0.5+significantly decreased compared with CK +( P <0.05). The biggest surface albedo decline rates of BC4.5 + and BC0.5 + in winter wheat season were 23.7%, 17.9% and 44.5%, 44.9% in summer maize season. The leaf area index of each treatment was remarkably correlated with the surface albedo. The shadow produced by the development of crop canopy structure would cover the change of surface color induced by the input of biochar. Under the condition of bare land, the surface albedo of biochar treatment was significantly decreased compared with the control treatment( P <0.05). Compared with the control treatments, the soil CO 2 flux of the biochar treatments was significantly increased( P <0.05). With the extension of time, the growth rate of soil CO 2 flux of biochar treatment was decreased gradually. The increase from BC4.5 + to CK + was gradually reduced from 276.7% to 36.1%, and the increase of BC4.5-from to CK-was significantly reduced from 163.5% to 39.8%. The increase of soil CO 2 flux could be derived from the mineralization of the readily decomposed carbon fractions in the biochar-soil system. The decrease of surface albedo caused by the input of biochar had no direct effect on soil respiration, and the input of biochar could reduce the temperature sensitivity of soil respiration. This result could provide a foundation for the verification of the chemical and biological stability of biochar.