A novel control–protection coordinated distance protection method for transmission lines connected to flexible direct current converter stations

The access of flexible direct-current (DC) transmission converters reshapes the fault characteristics of transmission lines, leading to degraded performance of distance protection methods that rely on fault characteristics typical of traditional synchronous power supplies. To address this issue, this paper proposes a control-protection coordinated distance protection method for transmission lines connected to flexible DC converter stations. During transmission line faults, an enhanced fault ride-through (FRT) control strategy is developed so that the converter station exhibits a positive-sequence controlled voltage-source behaviour, thereby improving the applicability of phase-comparison distance protection. Meanwhile, the reference voltage magnitude is reduced to strictly limit the fault current within the device overcurrent constraints, ensuring converter security. On the protection side, a steady-state validation criterion is employed to identify the completion of the converter transient control adjustment, and the distance element is released accordingly to achieve reliable and fast fault discrimination. PSCAD/EMTDC simulations on a representative flexible-DC-integrated system and dynamic experiment results verify that the proposed coordinated scheme can accurately identify fault direction and location while preserving selectivity under various fault types and locations.