兼顾直流电压安全与无功支撑的柔性直流输电故障穿越控制

Fault Ride-Through Control Method for VSC-HVDC Balancing between DC Voltage Security and Reactive Power Support

受端电网三相故障下柔性直流输电(VSC-HVDC)的直流电压骤升,可能引发过电压闭锁并威胁电网稳定。现有基于卸荷的VSC-HVDC故障穿越方法大多以直流电压为投切条件,未计及受端换流站交流母线电压和无功功率对直流电压安全的影响,可能因投切条件不合理或受端换流站无功功率过大引发卸荷非必要投入,对交直流系统造成额外冲击并增加能量损失。为此,该文建立了受端换流站功率可行域,以刻画受端电网故障下受端换流站的功率控制能力;分析发现了受端换流站功率可行域的缩减特征,进而提出避免直流电压越限的受端电网故障极限切除时间的概念和计算方法;通过解析引发直流电压越限的门槛电压,提出基于最大限度避免直流电压越限的协调控制点的VSC-HVDC故障穿越控制方法。算例表明,所提方法在受端强电网或VSCHVDC卸荷风险较高的场景中,能够兼顾VSC-HVDC的直流电压安全和对受端电网的无功功率支撑,在最大限度避免卸荷投入的同时,尽可能地为受端电网提供无功功率。

Voltage source converter-based high voltage direct current(VSC-HVDC)is essential for building modern power systems.The rapid rise of DC voltage under the three-phase fault in the receiving-end grid may trigger overvoltage blocking and threaten grid stability.The existing fault ride-through method based on the dynamic braking resistor(DBR)for VSC-HVDC uses DC voltage as the activation condition.It does not take into account the impact of the AC bus voltage and reactive power of the receiving-end converter(REC)on DC voltage security,which may lead to non-essential activation of DBR due to unreasonable activation conditions or excessive reactive power of REC,causing extra shocks to the AC-DC system and increasing energy dissipation.According to Davina's theorem,the faulty receiving-end grid is equated to a two-port network.Among them,the ideal voltage source and series impedance are the equivalent potential and the equivalent impedance of the receiving-end grid during normal operation,respectively.The parallel equivalent fault transition impedance is used to equate the effect of a short-circuited branch circuit on the equivalent impedance of the receiving-end grid.The analytical equations of the DC voltage and AC bus voltage of REC concerning the active and reactive power of REC are deduced.The power feasible domain of REC under the limitation of the maximum permissible AC current with the guarantee of DC voltage security is established to depict the power control capability of REC under the receiving-end grid fault.The minimum active power of REC that precisely guarantees DC voltage security increases with the increase in fault duration,and the range of active and reactive power of REC under the limitation of the maximum permissible AC current is constant,so that the power feasible domain of REC exhibits a shrinking characteristic.If the power feasible domain of REC under a certain fault duration has only one point,the fault duration is the critical clearing time of the receiving-end grid fault.Since REC can provide the maximum active power under the limitation of the maximum permissible AC current,the DC voltage will exceed its maximum permissible value at the critical clearing time.Therefore,the critical clearing time characterizes the security margin of the DC voltage under the receiving-end grid fault.The active current of REC required to maintain DC voltage stabilization cannot be increased without limit as the degree of AC bus voltage drop increases.The control reference value of REC shall be adapted to the different fault severities in the receiving-end grid.Suppose REC can provide a certain reactive current while restoring its active power to its initial value before the fault.In that case,the DC voltage can be kept stable,and REC can continue to use constant DC voltage control.When the receiving-end grid fault is more severe and the active power decreases due to the reactive power provided by REC,the DC voltage will continue to rise with the increase in fault duration.REC should be switched to the fixed active and reactive power control and cooperate with DBR.The coordination control point is the operating point of REC that can withstand the power imbalance for the longest time.The time required for DC overvoltage can be maximized when REC is operated at the coordination control point.If the critical clearing time exceeds the fault duration,the DC voltage during the receiving-end grid fault is less than the maximum permissible DC voltage.If the fault duration exceeds the critical clearing time,the unbalanced power is minimized,and the energy dissipated to ensure DC voltage security is minimized.Therefore,no matter how long the fault duration is,when the AC bus voltage of REC is lower than the switching threshold voltage,REC should be controlled to operate at the coordination control point and activate DBR according to the critical clearing time.