交流故障下永富直流换流器差动保护误动风险分析

Mal-operation Risk Analysis of Yongfu DC Converter Differential Protection Under AC Fault

永富直流的逆变站（广西侧）交流系统稍弱于云南侧,因此当其逆变侧交流系统故障时,极易引起直流系统换相失败,甚至连续换相失败。永富直流继续沿用了延时3 s的100 Hz保护,因此急需对交流系统故障下直流系统保护是否存在误动风险进行分析预判。以永富直流系统仿真模型和现场采用的阀组保护逻辑为基础,仿真分析了逆变侧交流系统不同故障类型、不同故障相角及直流不同运行方式下,现行阀组保护的动作特性并统计了其动作概率,并基于此给出了换流器差动保护误动的风险概率。分析表明：逆变侧交流系统发生单相接地、两相接地以及三相故障时,阀组差动存在误动的风险,而桥差保护不会误动;逆变侧交流系统发生单相接地故障时,直流系统不同的运行方式下阀差保护均有误动的风险,而桥差保护不会误动。因此提出了采用交流故障特征量来闭锁阀差保护以及采用延时来保证保护的选择性的改进策略,其有效性得到了仿真实验验证。

The AC system of inverter side（Guangxi side） is weaker than that of Yunnan side for Yongfu HVDC. Therefore, when the inverter has faults of the AC system, it is very easy to cause the failure of the commutation of the DC system and even the failure of the continuous commutation. The 100 Hz protection with a delay of 3 s is continuously adopted in Yongfu HVDC, so it is urgent to analyze and predict the risk of mal-operation of DC system protection under AC system failure. Based on the simulation model and the protection logic of valve group used in the field practice, the operation characteristics of current valve group protection under different fault types, different fault phases and different operation modes of the inverter side system were simulated and analyzed, and the probability of their action was also counted. Based on this, the risk probability of the mal operation of the converter differential protection was given. The analysis results show that, while the single-phase grounding, two-phase grounding and three-phase faults occur on the inverter side AC system, there is a risk of mal-operation of the valve group differential, while the bridge differential protection will not be in maloperation. When a single-phase grounding fault occurs on the inverter side AC system, the protection of the valve differential is in mal-operation under different operation modes of the DC system, while the bridge differential protection will not be in maloperation. Therefore, an improved strategy is proposed, which uses AC fault characteristic quantity to lock valve difference protection and delay to ensure selectivity of protection, and its validity is verified by simulation experiments.