特高压直流分层接入方式下预防换相失败的协调控制策略

分层接入的特高压直流输电系统中，非故障层换相失败预测控制的启动时刻存在延时，无法及时反应故障严重程度以输出合适的触发角提前量，可能导致高低端换流器同时发生换相失败的风险增加。对此，提出一种基于故障层触发角提前量前馈的高低端换流器换相失败预测控制协调策略。利用故障层换流器可更灵敏反应故障严重程度的特点，所提策略将故障层换相失败预测控制触发角提前量引入非故障层换相失败预测控制，实现非故障层换相失败预测控制启动时刻提前。此外，还引入直流电流协调系数以得到更合适的触发角提前量。在PSCAD/ EMTDC中搭建了仿真模型，对不同工况下所提控制策略进行了验证。结果表明，该策略能快速应对逆变侧交流系统故障，减小高低端换流器同时发生换相失败的风险，对预防分层接入系统高低端换流器同时发生换相失败有一定借鉴意义。

Coordinated control strategy for preventing commutation failure in a UHVDC system hierarchically connected to an AC grid

In a hierarchically connected UHVDC system, there is a delay in the start time of the non-fault layer commutation failure prevention (CFPREV). This makes it unable to respond to the severity of the fault in time to output an appropriate trigger angle advance. This may lead to an increased risk of commutation failure of the high and low converters at the same time. Thus a CFPREV coordination strategy for high and low converters based on the feedforward of the trigger angle advance of the fault layer is proposed. This strategy uses the characteristics of fault converters that can more sensitively respond to the severity of faults, and it introduces the trigger angle advance of the CFPREV of the faulty layer into the CFPREV of the non-faulty layer, so that the start time of the CFPREV of the non-faulty layer is advanced. In addition, the strategy also introduces a DC current coordination coefficient to obtain a more appropriate trigger angle advance. A simulation model is built in PSCAD/EMTDC to verify the proposed control strategy under different working conditions. The results show that this strategy can quickly respond to inverter-side AC system faults, and reduce the risk of simultaneous commutation failures of high and low converters. The proposed control strategy plays a role in solving the problem of simultaneous commutation failure of high and low converters in the hierarchically connected system. This work is supported by the Science and Technology Program of Sichuan Province (No. 2021YFG0026).