A new regenerative cascaded multilevel converter and its control strategy based on V/f control has been presented. The converter employs insulated gate bipolar transistor-based rectifiers as active front end (AFE) only in part of cells and diode-based rectifiers in the rest cells. Those cells with AFE enable the converter to deliver the power generated by the motor to the grid. In the course of the motor's deceleration, the converter controls the output phase difference of the two types of cells to feed the motor generated electrical energy back to the grid through the cells with AFE and keep other cells' DC-link voltages stable. Simulation results of a 6-kV six-cell cascaded converter with one regenerative cell per phase and an experiment conducted on a laboratory cascaded converter verify that the control strategy is feasible and the drive system's speed regulation performance is improved. Compared with the converter with AFE in each cell, this converter has lower complexity and lower cost.
Lifetime is an important performance factor in the reliable operation of power converters. However, the state-of-the-art LC filter design of a buck DC-DC converter is limited to the specifications of voltage and current ripples and constrains in power density and cost without reliability considerations. This paper proposes a method to optimize the design of the LC filters from a reliability perspective, besides other considerations. An enhanced model is derived to quantify the lifetime of the capacitor in the filter considering the electro-thermal stress on it. Furthermore, the influence of different design aspects like the value of capacitance, the value of inductance, type of the capacitor have been discussed, focusing on their impacts on the key design objectives which are the cut-off frequency, lifetime and volume. Based on the analysis, an optimized design is proposed among different parameter sets. A 1 kW converter prototype is applied to verify the theoretical analysis and simulation.
Threaded joints are key components of core drilling tools. Currently, core drilling tools generally adopt the thread structure designed by the API Spec 7-1 standard. However, fractures easily occur in this thread structure due to high stress concentrations, resulting in downhole accidents. In this paper, according to the needs of large-diameter core drilling, a core barrel joint was designed with an outer diameter of Φ135 mm and a trapezoidal thread profile. Subsequently, a three-dimensional simulation model of the joint was established. The influence of the external load, connection state and thread structure on the stress distribution in the joint was analyzed through simulations, from which the optimal thread structure was determined. Finally, a connection test was carried out on the threaded joint. The stress distribution in the joint thread was indirectly studied by analyzing gas leaks (i.e., the sealing effect) under axial tension. According to the test data and the simulation results, the final joint thread structure was optimized, which lays a good foundation for the design of a core barrel.
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