When access to an external power source is constrained for an electrified railway, the traction substation supplying electric locomotives is commonly co-located with the power substation supplying communication and signaling facilities. This configuration is referred to as an integrated traction and power substation. The single-phase asymmetry and strong impact characteristics of high-power traction loads cause three-phase voltage unbalance on the high-voltage grid side and voltage sags on the low-voltage power-supply side. Based on the principle of co-phase power supply, a co-phase traction power supply system topology structure suitable for integrated traction and power substations is first proposed. The topology consists of a single-phase traction transformer, a modular multilevel converter (MMC)-based power flow controller (PFC), and a quasi-Z-source dynamic voltage restorer (DVR), where the DC bus of the PFC supplies DC power to the quasi-Z-source DVR. An integrated negative-sequence and voltage-sag compensation strategy is then proposed by combining the negative-sequence optimal compensation strategy of the PFC with the voltage-sag compensation strategy of the quasi-Z-source DVR. Calculation methods are derived for the PFC compensation current constrained by the negative-sequence power limit and the quasi-Z-source DVR compensation voltage satisfying the voltage-sag limit. Considering the influence of PFC DC-bus voltage fluctuations on DVR performance, predictive current control and voltage-balancing control are proposed for the MMC-based PFC. These control methods are coordinated with quasi-Z-source DC-link voltage stabilization control and DVR AC inverter control to achieve integrated compensation of negative-sequence components and voltage sags. Finally, MATLAB/Simulink simulations and StarSim HIL real-time simulation experiments verify the correctness and effectiveness of the proposed topology and integrated negative-sequence and voltage-sag compensation strategy. The results show that the co-phase traction power supply system integrated with the quasi-Z-source DVR compensates for negative-sequence components on the high-voltage grid side and voltage sags on the low-voltage power-supply side under different traction load conditions. The PFC maintains the three-phase voltage unbalance factor on the high-voltage grid side at approximately 2%, while the quasi-Z-source DVR generates the compensation voltage within two cycles and maintains a stable voltage on the low-voltage power-supply side. The proposed system provides a highly integrated structure, a fast compensation response, and effective coordination among the control functions.
在电压暂降治理方面,现有治理装置主要分为以下3类:电压补偿型、电流补偿型、混合补偿型。文献[19]在风电并网系统中,采用动态电压恢复器(Dynamic Voltage Restorer,DVR)在风电机组低压穿越期间生成补偿电压,实现电压暂降的治理,提升了风电机组的稳定性与系统运行的可靠性。文献[20]通过静止同步补偿器动态注入无功电流维持并网点电压稳定,快速支撑节点电压。文献[21]采用电能质量调节器,通过同时注入补偿电压和补偿电流,可治理由无功、雷击、短路等引起的电压波动,保证电压的稳定性。基于DVR的电压暂降补偿方法由于使用场景丰富、拓展性强、能够快速响应电压的变化等特点得到了广泛的关注[22-24]。文献[25]建立了基于级联H桥多电平变流器的高压直挂式DVR模型,提出dq解耦控制策略,保证了DVR的补偿精度与响应速度。文献[26]提出了一种基于直接AC-AC变换器的DVR拓扑结构,基于该拓扑的DVR在幅值和相位矫正方面的补偿能力得到提升。然而,以上围绕DVR的建模与控制都是建立在其直流侧为理想直流电压源的基础上,而当DVR应用到电气化铁路场景时其直流侧接入同相牵引供电系统PFC的直流环节时,由于电力机车在运行时具有强冲击和单相不对称性,造成PFC的直流侧网压波动,容易影响DVR的稳定运行。针对DVR直流侧电源波动问题,集成准Z源DVR的方案被提出,该方案将准Z源作为并网逆变装置,可同时实现单级升压和降压的作用[27],对直流链电压和交流相电压进行调制,适用于输入电压变化范围较大的应用场合[28-29]。然而,由于牵引供电系统与铁路电力系统共用外部电源,如何在牵引负荷强冲击下兼顾电压不平衡与电压暂降的补偿问题需要研究。
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