Jun-Kyu Park

In this article, a new bidirectional dc circuit breaker (DCCB) topology based on Z-source principles is presented. The new DCCB topology is featured with the reprotection capability and safely protects the dc source and load even when a utility controller tries to reengage the DCCB with an uncleared fault. Our topology is efficient as it only includes one semiconductor device in its current path during normal operation and is symmetrically built to provide bidirectionality. With an additional current path for a fault, our new DCCB suppresses the reflection of the fault current onto the source side, enhancing equipment protection. Detailed mathematical models are built in this article to analyze the protection mechanism, operation performance, and short-circuit fault responses. This article also discusses how to expedite the fault-clearing and reclosing processes. In addition, this article explores the impact of the reverse recovery of semiconductor devices in DCCBs and provides a mitigation strategy. A laboratory-scale prototype is built to validate the theoretical analyses and verify the benefits of the proposed bidirectional dc circuit breaker topology.

This paper presents a structural optimization technique based on precise heat transfer analysis for transformers. Mechanical and thermal stresses during operation are particularly critical in confined environments, such as within wind turbine generators. To optimize the transformer’s structure, the temperature distribution is first calculated using computational fluid dynamics (CFD) based on the finite volume method. Eddy current losses, necessary for thermal analysis, are determined through electromagnetic analysis. These CFD results are then used as input conditions for structural analysis. Finally, structural optimization is performed on critical areas, primarily the radiation fins and panels. All models are analyzed in 3D, and the simulation results are validated through comparison with experimental data. Consequently, the optimized design results were validated against simulations, exhibiting a maximum deviation of 4%. These findings confirm the success of the proposed transformer design optimization.

The increasing concerns surrounding global warming and its adverse effects have contributed to the rising demand for SF ₆ -free eco-friendly gas circuit breakers (GCBs). In particular, CO ₂ /O ₂ mixtures possess better arc-quenching capabilities than pure CO ₂ , making them suitable insulation gases for eco-friendly GCBs. This study proposes a multiphysics model combining electric, fluid dynamics, and radiation models to evaluate the thermal interruption capability in a 72.5-kV GCB using a CO ₂ /O ₂ mixture gas. The pressure rise inside the thermal chamber and arc voltage of a prototype gas interrupter adopting a CO ₂ /O ₂ mixture gas were calculated using the proposed model. The feasibility of the model was verified through a comparison of the calculated pressure rise and arc voltage with the measurement results. Further, we calculated the temperature, pressure, and PTFE ablation distribution of the 72.5 kV GCB with the CO ₂ /O ₂ mixture gas. Based on these results, we estimated the thermal interruption capabilities, specifically, the post-arc current (PAC) and G ₂₀₀ indices. The numerical investigation of the thermal interruption capability can contribute to the design of GCB interrupters using CO ₂ /O ₂ mixture gas.

This study presents an efficient hybrid mechanical and power electronic (MPE) DC circuit breaker (DCCB) to solve the challenges of conventional hybrid MPE DCCBs. The main branch of the proposed topology reduces on-state losses to a negligible magnitude by utilizing only mechanical switches such as the residual current switch (RCS), fast mechanical switch (FMS), and monostable mechanical switch (MMS). Additionally, it significantly decreases the number of semiconductor devices by employing a series connection between the FMS and the main current breaker (MCB). The DC interruption capability of the proposed hybrid MPE DCCB topology was verified through a simulation based on equivalent electric circuits, and the DC interruption capability of the proposed medium-voltage (MV) class hybrid MPE DCCB was evaluated using a DC interruption test.

Common-Mode Voltage (CMV) induces shaft voltage and bearing current due to the electrical interaction with the parasitic capacitance of the motor. CMV, shaft voltage, and bearing current are considered the major causes of bearing fault. Motor fault in a traction system poses a risk of accidents. Therefore, it is necessary to reduce the CMV and the shaft voltage to ensure the reliability of the bearing. However, some existing CMV reduction methods are based on asynchronized space vector pulse width modulation (SVPWM), which will cause unacceptable harmonic distortion at a low switching frequency. Alternatively, some CMV reduction methods based on synchronized SVPWM burden the processor because they require a lot of calculation. In this paper, the method to reduce CMV and shaft voltage is proposed using carrier wave phase shift in SVPWM. CMV is explained in traditional SVPWM, and CMV is reduced by shifting the carrier wave phase of one phase. The simulation model is constructed through MATLAB/SIMULINK and Maxwell 2D/Twin Builder. Considering the proposed method, CMV, shaft voltage, and bearing current are analyzed by an equivalent circuit model. Moreover, the output torque behaviors with different input currents are analyzed through the simulation.

An 8-pole 48-slot dual three-phase synchronous reluctance motor has been tested under half control mode, that is, when only a single three-phase system is supplied while the other is not operating. Changing the terminal connections of the motor prototype available, two different winding configurations have been tested. This allows their performances to be experimentally compared. At first, the cross coupling effect between the axis in $dq$ reference is analyzed, by measuring the flux linkages for different current combinations. The torque characteristics are compared in terms of torque map, maximum torque capability and torque ripple. Then the finite element analysis has been used to explain the experimental results and to investigate the radial forces on the rotor. Finally, the impact of a lack of symmetry between the two three-phase system is investigated.

This article compares the characteristics of a dual three-phase synchronous motor under different operating conditions such as healthy, half-control, and postfault conditions. The inductances are analyzed to confirm the interaction between two three-phase systems, and induced voltages of each phase are measured to verify the interaction behaviors. The postfault characteristics are analyzed under open-circuit faults (single-open-switch and single-open-phase faults) and short-circuit faults (single-short-switch and one-three-phase-short faults). The predicted results are validated by experimental tests. Moreover, the half-control mode is investigated as a strategy to operate with high efficiency and tolerant mode.

This study investigates experimental test on 8-pole 36-slot dual three-phase synchronous reluctance motor according to different two winding configurations. The characteristics of motor for the two winding configurations were analyzed by changing the winding distribution of two three-phase system. The experiment was carried out using one three-phase inverter instead of two inverters for the convenience of experiment since this paper deals with the symmetric models The flux and torque characteristics are analyzed according to different dq-axis currents under healthy and half control mode conditions. In addition, torque and power versus speed characteristics are analyzed under half control and short-circuit modes.