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城轨柔直T型三电平双向变流器环流抑制与中点电位平衡
Circulating Current Suppression and Midpoint Potential Balance of Flexible DC T-Type Three-Level Bidirectional Converter for Urban Rail Transit
【目的】双向变流器集牵引、回馈、无功补偿为一体，是城轨直流牵引供电系统主设备升级的主要选择方案。在低压直流牵引供电领域，变流器具备低电压大电流特性，T型三电平多单元并联结合载波移相技术更易满足城轨直流牵引供电系统对主设备低成本、低谐波、高功率密度、高效率、宽直流电压运行范围的特殊要求，但存在零序环流过大和中点电位不平衡问题。【方法】首先针对双向变流器的环流问题，建立零序环流模型，对环流分类分析，提出变压器低压绕组分裂、共用中线、L型滤波器和软件控制算法抑制零序环流；其次针对中点电位不平衡问题，在实现等效空间矢量脉宽调制的基础上，通过分析T型三电平单相拓扑结构，发现中点电位与相电压、相电流极性及空间矢量种类有关，据此提出一种基于反馈补偿的中点电位平衡控制方法。【结果】基于PSCAD/ EMTDC开展仿真测试，发现各类零序环流可以得到有效抑制，在变流器四象限运行时实现三电平变流器中点电位平衡。【结论】所提环流抑制方法不仅可以抑制各类零序环流，还有助于减少三电平中点电位量测成本；所提中点电位控制方法无需进行复杂的空间矢量调制过程，计算量小，实现过程简便，可有效控制中点电位。
[Objective] A bidirectional converter integrates traction， feedback， and reactive power compensation and is the primary choice for upgrading the main equipment of an urban rail DC traction power supply system. In the field of low-voltage DC traction power supply， converters have the characteristics of low voltage and high current. The T-type three-level multi-unit parallel connection combined with carrier phase-shifting technology can meet the special requirements of low cost， low harmonic， high power density， high efficiency， and wide DC voltage operation range of urban rail DC traction power supply systems for the main equipment. However， this approach has problems such as excessive zero-sequence circulating currents and unbalanced midpoint potentials. [Methods] First， a zero-sequence circulating current model is established to address the circulating current problem in bidirectional converters. The circulating current is classified and analyzed， and a transformer low-voltage winding splitting， shared neutral line， L-shaped filter， and software control algorithm are proposed to suppress the zero-sequence circulating current. Second， in response to the problem of midpoint potential imbalance and based on the implementation of the equivalent space vector pulse width modulation， the relation of the midpoint potential to the phase voltage， phase current polarity， and space vector type is established by analyzing the T-shaped three-level single-phase topology structure. Accordingly， a feedback-compensation-based midpoint potential balance control method is proposed. [Results] Based on PSCAD/EMTDC simulation testing， it was found that various zero-sequence circulating currents could be effectively suppressed， achieving a midpoint potential balance in three-level converters during four-quadrant operation. [Conclusions] The proposed circulating current suppression method can not only suppress various zero sequence circulating currents， but also help reduce the cost of measuring the midpoint potential of three-level systems. The proposed midpoint potential control method does not require complex space vector modulation processes， has low computational complexity， is easy to implement， and can effectively control the midpoint potential.
柔性直流 / T型三电平 / 双向变流器 / 环流抑制 / 中点电位平衡
flexible DC / T-type three-level / bidirectional converter / circulation suppression / midpoint potential balance
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In order to reduce the sensitivity of AC line differential protection in flexible DC transmission system of inverter new energy station, an optimization scheme of differential protection based on improved criterion is proposed. Firstly, according to the AC line fault characteristics of the flexible DC transmission system of the inverter new energy station, the adaptability of the AC line differential protection is analyzed. In case of fault in the normal area, the braking component is set to 0 to improve the sensitivity of the protection. When the phase angle difference on both sides exceeds 90°, the current phase angle difference on both sides is added to the criterion to reduce the reduction of sensitivity. Then, compared with the method of directly reducing the braking coefficient, the optimization scheme can take into account the reliability of differential protection by changing parameters, and the optimization scheme is not affected by the strength of the external system of the inverter new energy station. Finally, the 4-terminal flexible DC system model of inverter new energy station connected to Zhangbei is built in PSCAD simulation software, and the simulation verifies the effectiveness of the optimization scheme. |
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Commutation failure on the receiving side can be caused by a fault in the line commutated converter-based HVDC (LCC-HVDC) on the sending side, and an over-compensated reactive power increases the risk of commutation failure. In this paper, for a transmission system in which the sending-side LCC-HVDC is in parallel with a synchronous condenser and voltage source converter-based HVDC (VSC-HVDC), the reactive power control mechanisms and response characteristics of synchronous condenser and VSC-HVDC are discussed. We observe that the over-compensated reactive power of VSC-HVDC increases the risk of commutation failure of the LCC-HVDC, and the reactive power regulation capacity of the synchronous condenser is not fully utilized. Based on this, a reactive power-coordinated control scheme between the synchronous condenser and VSC-HVDC is proposed to accelerate the reactive power response of the VSC-HVDC, reduce the excess reactive power compensation to reduce the risk of commutation failure of the LCC-HVDC, and improve utilization of the reactive power regulation capacity of the synchronous condenser. Finally, simulation results of typical examples reveal that the proposed scheme can fully utilize the dynamic reactive power regulation capacity of the synchronous condenser, inhibit transient low voltage at the moment of fault, and reduce the risk of commutation failure after the fault is cleared.
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