柔性直流配电网中直流线路故障电流具有上升速率快、峰值高、发展迅速等特征,增大了直流断路器设计的难度.本文利用送端换流器可控性强的特点,提出了一种故障限流的控制策略,无需检测是否发生直流侧故障,等效增大限流电抗器的电阻,并自适应调整虚拟电阻值:并利用Matlab/Simulink仿真平台进行仿真验证.仿真结果表明,当直流线路发生故障时,通过此控制策略能快速将部分电容能量馈入交流电网,有效地降低故障电流峰值和电流变化率,减小直流断路器的开断应力;且在有效限制故障电流的同时,对系统正常运行的稳态特性和暂态特性的影响很小.

由于单相并网逆变器系统缺少一个自由度,无法直接将静止同步坐标系中的交流量转换为基于dq坐标系下的直流量.采用直接将实际物理量延时1/4个工频周期和复杂算法的引入所构建的坐标变换得到的虚拟正交变量,严重影响了系统动态响应特性和增加了系统复杂性.针对该问题提出了一种基于间接延时构建正交虚拟量的方法,首先将真实物物理量延时一个很小的时间度,再将所得到的延时量与真实物理量作为输入量推导出所需要的正交虚拟量.该方法采用一个很小的延时替代1/4个工频周期延时,且无需引入复杂数学算法,极大提高了系统的动态响应特性.最后,通过Matlab仿真验证了该方法的有效性和正确性.

Grid unbalanced faults can cause core saturation of power transformer and produce lower-order harmonics. These issues increase the electrical stress of power electronic devices and can cause a tripping of an entire HVDC system. In this paper, based on the positive-sequence and negative-sequence impedance model of a VSC-HVDC system as seen from the point of common connection (PCC), the resonance problem is analyzed and the factors determining the resonant frequency are obtained. Furthermore, to suppress over-voltage and over-current during resonance, a novel method using a virtual harmonic resistor is proposed. The virtual harmonic resistor emulates the role of a resistor connected in series with the commutating inductor without influencing the active and reactive power control. Simulation results in PSCAD/EMTDC show that the proposed control strategy can suppress resonant over-voltage and over-current. In addition, it can be seen that the proposed strategy improves the safety of the VSC-HVDC system under unbalanced faults.