Yuchao Hou

Modular multilevel DC converter (MMDC) is a main solution of medium-/high-voltage DC transformer with the advantages like centralized transformer, simplified control and drawbacks include bulky volume and significant cost. To improve the power density and footprint of DC transformers in the DC power grid, an arm multiplexing MMDC (AM-MMDC) is proposed in this paper. Compared with the traditional MMDC, the AM-MMDC can reduce the number of submodule and capacitor by 25 %, which can improve the volume of DC transformer effectively. The AM-MMDC adopts quasi-two-level modulation strategy to realize the functions like voltage matching and power regulation. Furthermore, a specific soft switching strategy of arm-transfer switch is proposed to avoid the challenge of dynamic voltage-balance. Additionally, the simulation and experimental results prove the correctness and effectiveness of the proposed AM-MMDC and its control strategy. Comparison between the AM-MMDC and other MMDC topologies reveals that the AM-MMDC can achieve a better balance in cost, efficiency, volume and reliability.

The hybrid modular multilevel converter (HMMC) based on multimaterial devices has been proved to achieve a better balance among cost, loss, and feasibility compared with others. However, due to the use of hybrid-frequency modulation method, the implementation of traditional circulating current control strategy would increase output voltage harmonics of the HMMC. In this article, the mechanism of the above problems is analyzed in detail, and the specific circulating current suppression (SCCS) method is proposed. Based on the voltage balancing operation of the high-frequency submodules, the corresponding redundant voltage generated by the low-frequency submodules in the arms is analyzed. By utilizing the redundant voltage levels during the operation of HMMC, the number of inserted submodules is adjusted to reduce the circulating current of the system without compromising the quality of the HMMC output waveform. Finally, the simulation and experimental results verify the effectiveness and feasibility of the proposed SCCS method.

The advanced traction power supply equipment (ATPSE) based on power electronics technology presents an opportunity to address issues of negative-sequence power quality and neutral section simultaneously. However, existing power-frequency/high-frequency isolation schemes still face challenges such as low power density and high investment costs. To address these issues, a high-frequency isolated ATPSE (HI-ATPSE) based on triple-port high-frequency isolated dc converter and existing traction transformers is proposed. Compared to existing high-frequency isolation schemes, the device volume and initial cost are effectively reduced by sharing output ports in the proposed HI-ATPSE. In addition, considering the power quality issues within the proposed HI-ATPSE, a control strategy is proposed to suppress the dc side double-frequency ripple and negative-sequence while reducing the dc side's capacitance value. Finally, the feasibility and reliability of the proposed HI-ATPSE and control strategy are validated through simulation and experimental verification.

Since the voltage amplitude of the arc suppression device is different during the normal operation and single line-to-ground fault, the problems of high cost and low module utilization rate are serious. An integrated grid-connected converter (IGCC) with reactive power compensation and fault regulation ability is proposed. First, the topology and operation mechanism of IGCC are introduced in this article. A common unit combining neutral point clamped (NPC) and cascaded H-bridge is formed by improving the traditional arc suppression device. By adding a fourth leg in the NPC module and connecting with the arc suppression inductance, the integration of the two structures is realized. The access of NPC unit not only reduces the number of modules of traditional arc suppression device, but also provides an integrated port for arc suppression device and reactive power compensation device. Second, the parameters of active and passive part of IGCC are optimally designed to ensure the stable operation of IGCC. In addition, compared with existing schemes, the superiority of IGCC in cost and volume is proved. Finally, the correctness, feasibility and effectiveness of the proposed topology and functions are verified by the simulation and experiment results.

To resolve the issue of the difficultly in effectively balancing the output performance improvement, cost reduction, and efficiency improvement of a medium-voltage modular multilevel converter (MMC), a novel MMC (NMMC) topology based on high- and low-frequency hybrid modulation is proposed in this study. Each arm of the NMMC contains a high-frequency sub-module composed of a heterogeneous cross-connect module (HCCM) and N − 1 low-frequency sub-modules composed of half-bridge converters. The high-frequency bridge arm of the HCCM in this study adopts SiC MOSFET devices, while the commutation bridge arm and low-frequency sub-module of the HCCM adopt Si IGBT devices. For the NMMC topology, this study adopts a high/low-frequency hybrid modulation strategy, which gives full play to the advantages of low switching loss in SiC MOSFET devices and low on-state loss in Si IGBT devices. In addition, a specific capacitor voltage balance strategy is proposed for the HCCM, and the working state of the HCCM is analyzed in detail. Furthermore, the feasibility and effectiveness of the proposed topology, modulation strategy, and voltage balancing strategy are verified by experiments. Finally, the proposed topology is compared with the existing MMC topology in terms of device cost and operating loss, which proves that the proposed topology can better balance the cost and efficiency indicators of the device.

Aiming at the problems of low equipment utilization and the high-capacity requirements of existing arc-suppression devices, a multi-functional reactive power compensation device with the capability of grounding fault regulation (MF-RPCD) is proposed. Firstly, the topology and operation mechanism of MF-RPCD are introduced in this paper. MF-RPCD works in a reactive power compensation mode when a power grid is under normal operation to ensure the unit power factor operation of the power-grid side. When a single line-to-ground fault occurs in the power grid, MF-RPCD works in a fault-regulation mode to effectively suppress the ground-fault current. Secondly, the parameters of the active and passive parts of MF-RPCD are optimally designed to ensure the stable operation of MF-RPCD. Finally, the correctness, feasibility and effectiveness of the proposed topology and functions are verified using simulations and experiments.

Conventionally, parallel-connected or series-connected power quality control devices cannot compensate for both the voltage fluctuation and reactive power simultaneously. To improve the equipment utilization and reliability of the existing power quality control devices, a multi-functional integrated converter (MFIC) is proposed, which can effectively integrate the functions of both dynamic voltage restoration and reactive power compensation. When the grid voltage sags or swells, the MFIC works in the series access mode to achieve voltage regulation function. When the grid voltage is normal, the MFIC works in the parallel access mode to achieve unity power factor operation at the grid side. Compared with the traditional series-connected power quality control device, the MFIC only has one more coupling capacitor, markedly improving the utilization factor of equipment. In addition, the port voltage of the converter module in MFIC is clamped to zero when a short circuit or grounding fault occurs. Meanwhile, a large equivalent impedance is formed to effectively limit the line fault current, which improves the reliability of the MFIC itself and its connected grid as well. Furthermore, the effectiveness and feasibility of the proposed topology and strategy are verified by simulation and experimental results.

The novel hybrid advanced traction power supply device (NH-ATPSD), which consists of a traction transformer and a “two-phase-parallel-input to single-phase-cascaded-output” (2AC-AC) converter, has been proposed recently. It provides opportunities for solving the neutral section and negative-sequence power quality problems. However, different from the single-level power electronic device with cascade structure, the negative-sequence current and healthy module overload may occur when the input- or output-module fault occurs in NH-ATPSD. Thus, a dynamic fault-tolerant control for NH-ATPSD is proposed in this paper. For input module faults, the healthy module overload is avoided by the proposed dynamic modulation coefficient, which can dynamically adjust the power allocation between output port modules. In addition, the negative-sequence current is suppressed by controlling the partial power imbalance between the 2AC input modules. For output module faults, the active bypass of faulty module and increased voltage methods are adopted to keep the rated voltage and power output. Finally, the simulation and experiment results verify the effectiveness and efficiency of the proposed control strategy under different fault types.

The advanced cophase traction power supply system (ACTPSS) integrating with photovoltaics (PV) and energy storage (ES) can realize the efficient consumption of regenerative braking energy (RBE) and renewable energy sources. However, the existing single-level energy management strategies for ACTPSS that only focus on the main-station or the sub-station have poor performance in the economy and timeliness. Therefore, a multi-timescale energy management strategy including the minutes-level optimal dispatch for main-station and the seconds-level real-time control for sub-stations is proposed, which can improve the economy and reliability of the ACTPSS with PV and ES simultaneously. In terms of the main-station layer, an min-level optimal dispatch model is established with an objective of minimum operating cost to obtain reference working points of output port converter, PV, and ES in each sub-station. In terms of the sub-station layer, an s-level adaptive working point control strategy based on improved Arctan droop is proposed to ensure the voltage and frequency stability of ACTPSS, while suppressing the PV/RBE power feedback to three-phase power network. Finally, the feasibility and superiority of the proposed multi-timescale energy management strategy are verified by simulations and experiment results.