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Abstract In this paper, an integral‐type half‐tangent phase‐locked loop (IHTan‐PLL) is proposed for the varying‐frequency AC grids of more electric aircraft (MEA). First, the performance of the synchronous reference frame phase‐locked loop (SRF‐PLL) and half‐tangent phase‐locked loop (HTan‐PLL) is discussed from a large‐signal viewpoint. When the frequency changes largely, the SRF‐PLL has many oscillations and endures a rather long transient process. The HTan‐PLL can address the large frequency jumps. However, for the varying‐frequency AC grids with harmonic disturbances, the HTan‐PLL still has oscillations in estimating the frequency when the frequency jumps largely. To enhance the performance of the SRF‐PLL and HTan‐PLL, the IHTan‐PLL is developed for the varying‐frequency AC grids with harmonic disturbances. The IHTan‐PLL performs better than the SRF‐PLL and HTan‐PLL in addressing the large frequency jumps. Specifically, under any large frequency changes, the IHTan‐PLL still converges to the equilibrium point (0,0), and no oscillations emerge. In addition, the IHTan‐PLL has better steady‐state accuracy in estimating the frequency than the SRF‐PLL and HTan‐PLL. The experimental results also demonstrate the superior performance of the proposed IHTan‐PLL.

Abstract Grouting is an effective method to improve the integrity and stability of fractured rocks that surround deep roadways. After years of research and practice, various theories and a complete set of grouting technologies for deep roadways with fractured rocks have been developed and are widely applied in Chinese coal mining production. This paper systematically summarizes and analyzes the research results concerning the theory, design, materials, processes, and equipment for the grouting and reinforcement of fractured rocks surrounding deep roadways. Specifically, in terms of grouting methods, pregrouting, grouting‐while‐excavation, and postgrouting methods are explored; in terms of grouting theory, backfill grouting, compaction grouting, infiltration grouting, and fracture grouting theories are studied. In addition, this paper also studies grouting borehole arrangement, water‐cement ratio, grouting pressure, grouting volume, grout diffusion radius, and other grouting parameters and their determination methods. On this basis, this paper explores the physical and mechanical properties of organic and organic‐inorganic composite grouting materials, and assess grouting reinforcement quality testing methods and instruments. Taken as the field cases, the application of pregrouting in front of heading faces, grouting‐while‐excavation, and postgrouting in the Kouzidong coal mine are then introduced, and the effects of the grouting reinforcements are evaluated. This paper proposes a development direction for grouting technology based on problems existing in the grouting reinforcement of fractured rocks surrounding deep roadways.

Abstract In wind farms, the uncertainty of wind power and the incoordination of multiple time scales increase the difficulty of voltage/var control (VVC), and result in the reactive power output of VVC devices becoming unreasonable. This will decrease the power quality and even cause the voltage collapse of wind farms at large voltage deviations. For these problems, hierarchical control of VVC devices is a promising solution, as VVC devices can adjust the reactive power output in different control strategies. Therefore, this paper proposes a three‐stage robust voltage/var optimal control strategy for wind farms. The robust optimization model is introduced to solve the VVC problem caused by the uncertainty of wind power in this strategy. Besides, compared with traditional VVC strategies, the proposed strategy can coordinate different control stages and VVC devices at multiple time scales by combining centralized and local VVC strategies, which achieves voltage/var optimal control objectives. Simulation results demonstrate the good performance of the proposed strategy in mitigating system voltage violation rate and power loss of the wind farm.

Abstract This paper proposes a novel distributed bilayer coordinated control scheme (BCCS) to realize the autonomous economic operation of islanded hybrid AC/DC microgrids, which only requires a sparse communication link. In the lower layer, the optimal power reference iterative algorithm (OPRIA)‐based droop control is proposed to realize the economic dispatch of each subgrid individually, while the frequency of AC subgrid and the bus voltage of DC subgrid are regulated to their rated values. Then, the upper‐layer control for the interlinking converter is designed to achieve the global economic operation of the hybrid AC/DC microgrid through optimizing the exchanging power between AC and DC subgrids. The proposed BCCS has the advantage of reaching the economic operation and frequency/voltage regulation simultaneously for the hybrid AC/DC microgrid. Besides, the proposed BCCS realizes the stable and economic operation with less communication, flexible controllability, and good compatibility of renewable energy sources. Numerical simulation results based on Matlab/Simulink verify the effectiveness and excellent performance of the proposed BCCS even with the integration of renewable energy sources, and the test based on RT‐LAB illustrates the good real‐time performance of the proposed control scheme.