柔性低频输电系统的主变稳态负序差动保护

为了解决常规纵差保护难以适用于低频系统的问题，通过分析低频系统的序分量抑制策略，并根据低频系统故障进入稳态后单侧有负序注入的特征，提出一种基于稳态负序分量的主变差动保护方法。为防止该差动保护在区外故障时因电流互感器饱和出现误动，通过对比分析三类制动电流计算方式的制动性能，得到了差动保护的制动方程和制动系数。为解决励磁涌流、电流互感器断线可能导致误动的问题，提出了相电压突变量的差动保护开放条件。仿真分析表明，该方法能够灵敏反映区内金属性故障及轻微故障，在区外故障伴随电流互感器饱和时有可靠的制动裕度。

Steady state negative sequence differential protection of a transformer in a flexible low-frequency transmission system

It is difficult to apply traditional differential protection to low-frequency systems. To improve this, a transformer differential protection method based on steady-state negative sequence components is proposed. It achieves improvement by analyzing the sequence component suppression strategy of low-frequency systems and considering the characteristic of negative sequence injection on one side after low-frequency system faults enter steady state. By comparing and analyzing the braking performance of three braking current calculation methods, the braking equation and braking coefficient of differential protection are obtained to prevent differential protection misoperation caused by current transformer saturation during external faults. The method adopts the differential protection opening condition of phase voltage sudden change to cope with special working conditions such as excitation inrush current and current transformer disconnection. Simulation analysis shows that this method can sensitively reflect internal metallic faults and minor faults, and maintain reliable braking margin when external faults are accompanied by current transformer saturation.