LCC-FHMMC混合直流输电系统受端交流系统故障穿越控制策略

送端采用电网换相换流器（LCC）、受端采用全半桥子模块混合型模块化多电平变流器（FHMMC）的LCC-FHMMC混合直流输电系统，当受端交流系统发生故障时，受端交流电压跌落，受端功率传输受阻，盈余的功率导致子模块电容过电压，甚至可能造成设备的严重损坏。为此，提出了一种基于FHMMC直流电压降压运行的受端交流系统故障穿越控制策略，使其直流电压始终低于逆变侧交流母线的电压有效值。同时，整流侧LCC保持常规的定直流电流控制，保证逆变侧的直流电流在额定值附近运行，从而实现了进入直流系统的有功功率与逆变器向受端交流系统输出的有功功率之间的平衡。最后在PSCAD/EMTDC仿真平台上对LCC-FHMMC混合直流输电系统受端交流系统发生的对称故障和不对称故障分别进行了仿真分析，仿真结果验证了所提控制策略能够快速有效地穿越受端交流系统故障，并抑制子模块电容过电压。

Receiving-end AC system fault ride-through control strategy of LCC-FHMMC hybrid HVDC transmission system

When the LCC-FHMMC hybrid HVDC transmission system, in which the sending end adopts line commutated converter(LCC) and the receiving end adopts full and half bridge submodule hybrid modular multi-level converter(FHMMC),has a fault at the receiving-end AC system, the AC voltage at the receiving end drops, and the power transmission at the receiving end is impeded. The surplus power leads to the overvoltage of the sub-module capacitor, and may even cause serious damage to the equipment. A receiving-end AC system fault ride-through control strategy based on the DC voltage step-down operation of FHMMC is proposed, which always sets the DC voltage of FHMMC to be lower than the root mean square value(RMS) of AC bus voltage at the inverter side. Meanwhile, the rectifier-side LCC adopts the constant DC current control to ensure that the inverter-side DC current is near the rated value, thereby achieving the balance between the active power injecting into the DC system and the active power output by the inverter to the receiving-end AC system. The symmetric faults and asymmetric faults occurred on the receiving-end AC system of LCC-FHMMC hybrid HVDC transmission system are simulated and analyzed on PSCAD/EMTDC platform finally. The simulative results verify that the proposed control strategy can rapidly and effectively ride through receiving-end AC system faults, as well as suppress the overvoltage of sub-module capacitors.