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This paper combines real-time temperature detection of MOSFETs inside solid-state switches with a temperature field model, and proposes a current limiting protection control strategy based on real-time heat management of MOSFETs. This strategy can avoid the unreasonable setting of current limiting protection time in solid-state switches during short circuit faults, leading to overheating and failure of MOSFET, and thus causing faults in HVDC power distribution system. Firstly, we analyzed the protection characteristics and shortcomings of solid-state switches such as solid-state power controller(SSPC) and solid-state circuit breakers (SSCB). Secondly, we optimized the circuit design of the solid-state power controller and the conversion relationship under different operating conditions, and elaborate on the implementation method of this control strategy in detail. Thirdly, we established a heat dissipation model for the MOSFET of SSPC, conduct theoretical and simulation analysis, and obtain the temperature field distribution of the MOSFET; Finally, the temperature of the MOSFET core is calculated by collecting the temperature of the position through the solid-state power controller temperature sensor, achieving accurate measurement. This paper also developed a prototype of a solid-state power controller with a rated current of 20A at 270V DC, and conducted short-circuit current limiting protection tests at 5 times the rated current; Experiments have shown that this control strategy can effectively prevent the overheating failure of the MOSFET.

In engineering practice, a linear poroelasticity stress model in combination with a rock strength criterion is commonly used to determine the minimum and pressure required for ensuring wellbore stability. Therefore, a main aspect of well bore stability analysis is the selection of an appropriate rock strength criterion. The Mohr-Coulomb criterion is the most widely used strength criterion in rock engineering problems. However, the nonlinear strength characteristics of rocks under high in-situ stress can no longer be accurately described and characterized by traditional strength criteria. Bottom-hole rock strength assessment of deep hole is important for optimization of drilling method, drilling parameters and design of bit-tool. In this paper, the distribution characteristics of rock true triaxial strength test data in the main stress space are studied. Then the exponential strength is used to predict the collapse pressure of wellbore based on the linear elastic well bore stress model. The research results prove that, the present form of Mohr-Coulomb criterion suffers from two major limitations. Firstly, it represents the strength of rock as a linear function of confining pressure. Secondly, the effect of intermediate principal stress is not considered by this criterion. As a result, the collapse pressure predicted by Mohr-Coulomb criterion is conservative. The effect of intermediate principal stress on rock strength is restricted by σ3. Besides, in normal faulting stress regime, horizontal wellbore is most prone to collapse. In reverse faulting regime, the wellbore collapse pressure is smaller when drilling along the direction of horizontal maximum In-situ stress. In strike-slip stress regime, the risk of wellbore instability is highest in vertical wells, while the horizontal borehole is more stable. The quantitative study of drilling fluid density to ensure that the stability ofwellbore provides basis and suggestions for well structure, optimization of hole trajectory and reasonable determination of drilling fluid density. [ABSTRACT FROM AUTHOR]