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In order to investigate the effect of different substrate surface roughness on the friction and wear properties of amorphous carbon films, and improve the tribological properties of the 2Cr13 sample surface under 601EF lubricating grease lubrication, the methods such as machining, polishing and other techniques were employed to prepare different surface roughness on the surface of 2Cr13 samples. Subsequently, amorphous carbon thin films were prepared on the surface of samples with different roughness by magnetron sputtering. The friction and wear properties of amorphous carbon films and silicon nitride balls under 601EF lubricating grease lubrication were investigated through ball-disc contact friction and wear tests, and the wear places were analyzed by optical microscopy. Results showed that the Si3N4 balls that rubbed against the substrate surface all presented a clear platform, while the substrate surface did not display any obvious grinding marks. The samples with the Ra range of 0.20～0.75 μm on the coupling ball had the smaller wear amount, the wear spot area range was 0.152～0.165 mm2, and the wear form was adhesive wear. By contrast, the samples with the Ra range of 0.95～3.19 μm had more wear amount on the coupling ball, the wear spot area ranging from 0.228～0.275 mm2, and there were furrow-like wear marks, which indicated more serious wear. Under the same operating conditions, as the surface roughness of the substrate increased, the starting and average friction coefficients between the two friction pairs, as well as the wear amount of Si3N4 balls, all showed a trend of first decreasing, then increasing, and then decreasing. When the Ra was 0.56 μm, the tribological performance of the amorphous carbon film was the best, and the friction curve was relatively stable, with an average friction coefficient of 0.162, and the wear amount of its coupling ball was the smallest, 0.152 mm2. In general, the surface roughness of the substrate affected the tribological performance of amorphous carbon films, and the surface with good anti friction properties could be obtained by preparing appropriate roughness on the substrate surface via the means of machining, polishing, etc.

The MMC is composed of a large number of sub-modules. The operating conditions of each sub-module are different, which will inevitably affect the capacitor voltage of each sub-module, resulting in the difference of the capacitor voltage of each sub-module. The problem of uneven voltage of the sub-module capacitor voltage of the MMC system is unavoidable, and the uneven voltage of the capacitor will greatly affect the stable operation of the MMC system. This paper studies the multi-terminal flexible DC transmission system (MMC-MTDC) based on modular multi-level converters. The multi-terminal MMC-MTDC system model is established through the MMC-MTDC converter station level and the MMC-MTDC system level controller, and then according to the shutdown process of MMC-MTDC system, several optimal control methods are proposed to realize energy feedback and sub-module discharge to achieve capacitor voltage balance. Finally, the effectiveness of the proposed method is verified by building a simulation model.

In the H-bridge inverter, the control and circuit topology parameters are the key factors affecting its dynamic stability, and the changes of each parameter have different effects on the stability of the system. This paper takes the current-controlled first-order H-bridge passive inverter based on PI control as the research object. Firstly, based on the different modal of the circuit, an accurate discrete iterative mapping model of the H-bridge inverter is derived according to the stroboscopic mapping principle. Then iterate through the above iterative formula, after retaining the data of the current peak value, the dynamic bifurcation diagram on the control parameters of the system is obtained, with the bifurcation behavior and stable working area of the system being analysed when the control parameters change, and verified by the current folding diagram the correctness of the analysis. Finally, according to the same method, the influence of other parameters on the system stability is studied. The research results show that when different parameters change, there are critical maximum or minimum values to keep the system running stably.

Gas drilling technology not only has the advantages of avoiding well leakage, shale hydration expansion,reservoir pollution and other issues, but also can greatly improve the drilling speed 4 to 8 times, it is helpful to timely discover and effectively protect reservoir and improve recovery efficiency and single well production of oil and gas reservoir, especially has important significance for hard strata and dense reservoir exploration. However, there are some problems hindered the promotion of this technology, such as gas is hard to carries cuttings and gas hole blocking. One of the key technologies to solve these problems is the determination of gas volume. Most of today’s literature use formation temperature instead of annulus temperature to calculate minimum gas volume. Owing to gas’ PVT effect, temperature is dramatically influenced by pressure flow velocity, there will be big error if we use formation temperature instead of annulus temperature. Meanwhile, most of traditional minimum gas volume methods use top of the drill collar as key point. In this study, we found that on the top of the drill collar, the carrying kinetic energy of gas is not is not necessarily the minimum, and change of the key position is strongly influenced by wellbore structure and make-up of drill tool. Minimum gas volume determined by applying temperature - pressure coupling calculation is more close to site construction, under the given conditions, error of the minimum gas volume is about 0.4%.