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In northern Shaanxi province, the coal seam deposit conditions are generally characterized by large thickness and shallow depth of burial, the phenomenon of coal pillar sloughing is serious in the process of working face retrieval, which brings great potential danger to mine safety production. In this paper, by monitoring the stress and damage of the coal pillar in the field section, the stress changes and damage degree of the working face in different mine stages are determined. A new mechanical model of coal pillar in a thick coal seam is constructed and the stress changes and structural breakage law of each section are obtained, the stress transfers mechanism of overburden structure and the stress damage to coal pillar during the mining process of upper and lower section comprehensive mining working face is analyzed. The stress, microscopic damage, and the degree of sloughing in the coal pillar were significantly reduced by monitoring through the top plate hydraulic cracking pressure relief control program. The results of the study show that the extent of sloughing coal pillar and internal damage is closely related to the length of key block B of the masonry beam formed by the overlying rock layer and the position of the break line, by changing the orientation of the overlying masonry beam rock layer and the position and length of the tendency to break line, the sloughing coal pillar is effectively controlled and its stress and damage are reduced. We have achieved good results in control of the sloughing in the Hanjiawan coal mine and similar mines in northern Shaanxi. [ABSTRACT FROM AUTHOR]

Abstract This paper aims to study the stability region of hydropower plants (HPPs) at high‐voltage direct current (HVDC) sending terminal, to theoretically predict the frequency stability of isolated HPPs under small disturbances. The overall transfer function of the system is derived, and the Hurwitz stability criterion is applied to deduce the stability region. The theoretical analysis of the stability region and time‐domain simulation results of the system show good consistency, and this proves the accuracy of the model. The focus of this paper is mainly on the influence of the hydraulic‐mechanical aspects. Improperly tuned governor parameters and unfavourable design of the surge tank could lead to the frequency instability of HPPs, which appears as divergent oscillations of the frequency of units. A key suggestion of this paper is that the control strategy should consider the factors from not only the electrical side but also hydraulic‐mechanical aspects, for enhancing the stability of the HVDC sending terminal. Besides, the influence mechanism and parameter tuning of frequency limitation controller (FLC) are investigated and the results demonstrate its capability of quick compensation for power disturbances at the HVDC receiving terminal.

Abstract The mechanism and fracture aperture of thick hard roof failure were theoretically analyzed from the perspective of elastoplastic mechanics. A uniform load cantilever beam was established by considering the voussoir beam structure and stress condition. According to the stress inverse principle in solving the plane strain problem, the internal stress component analytic solution was deduced. Considering the lateral thrust and shear force and by applying the Mohr‐Coulomb failure criterion, the plastic zone boundary equation was obtained. Further analysis of the equivalent stress, equivalent strain, elastic energy density, distortion energy density, and strain energy density was studied with blocks of four different lengths, combined with the rotation angles after failure, to analyze the fissure aperture between straight and curved fracture traces. The theoretical analysis reveals that the deviatoric effect plays the lead role in rupture initiation and propagation; the cracks will deflect during failure, and the fracture trace presents a curved shape. The variation of fracture aperture exhibits a tendency to increase first, then decrease, and then increase along the fracture trace. The analytical approach presented in this paper provides a theoretical basis for determining the main seepage channel flow in underground mining.

This paper aims to study the stability region of hydropower plants (HPPs) at high‐voltage direct current (HVDC) sending terminal, to theoretically predict the frequency stability of isolated HPPs under small disturbances. The overall transfer function of the system is derived, and the Hurwitz stability criterion is applied to deduce the stability region. The theoretical analysis of the stability region and time‐domain simulation results of the system show good consistency, and this proves the accuracy of the model. The focus of this paper is mainly on the influence of the hydraulic‐mechanical aspects. Improperly tuned governor parameters and unfavourable design of the surge tank could lead to the frequency instability of HPPs, which appears as divergent oscillations of the frequency of units. A key suggestion of this paper is that the control strategy should consider the factors from not only the electrical side but also hydraulic‐mechanical aspects, for enhancing the stability of the HVDC sending terminal. Besides, the influence mechanism and parameter tuning of frequency limitation controller (FLC) are investigated and the results demonstrate its capability of quick compensation for power disturbances at the HVDC receiving terminal. [ABSTRACT FROM AUTHOR]

The mechanism and fracture aperture of thick hard roof failure were theoretically analyzed from the perspective of elastoplastic mechanics. A uniform load cantilever beam was established by considering the voussoir beam structure and stress condition. According to the stress inverse principle in solving the plane strain problem, the internal stress component analytic solution was deduced. Considering the lateral thrust and shear force and by applying the Mohr‐Coulomb failure criterion, the plastic zone boundary equation was obtained. Further analysis of the equivalent stress, equivalent strain, elastic energy density, distortion energy density, and strain energy density was studied with blocks of four different lengths, combined with the rotation angles after failure, to analyze the fissure aperture between straight and curved fracture traces. The theoretical analysis reveals that the deviatoric effect plays the lead role in rupture initiation and propagation; the cracks will deflect during failure, and the fracture trace presents a curved shape. The variation of fracture aperture exhibits a tendency to increase first, then decrease, and then increase along the fracture trace. The analytical approach presented in this paper provides a theoretical basis for determining the main seepage channel flow in underground mining. [ABSTRACT FROM AUTHOR]