Coupling characteristics between MMC-HVDC and AC transmission lines on the same tower and their impact on protective relaying.

• Improvement schemes for zero-sequence overcurrent and directional relayings of AC line are proposed to avoid the coupling influence of adjacent MMC-HVDC line. • Coupling characteristics of AC line to MMC-HVDC line on the same tower during normal operation and their influence factors are investigated. • Coupling influence between MMC-HVDC line and AC line on the same tower after DC or AC faults and its impact on protective relaying are analyzed. Significant coupling exists between DC and AC lines erected on the same tower or sharing transmission corridors. Electromagnetic and electrostatic coupling between DC and AC lines are investigated based on ± 500 kV MMC-HVDC and 500 kV double-circuit AC lines on the same tower. The interactive coupling characteristics and their impacts on protective relaying are analyzed during normal operation and after DC or AC faults. Voltage coupling ratio and current coupling ratio are defined. The 50 Hz components in MMC-HVDC line may be higher than ripple criterion under the coupling of AC line and can be even more significant when a fault occurs on adjacent AC line. The DC reactor with larger inductance can reduce its impact on MMC-HVDC system. Zero-sequence overcurrent relaying and zero-sequence directional relaying for AC lines may mal-operate when a fault occurs on adjacent MMC-HVDC line. And there is also an interruption risk for AC systems. Improvement schemes are proposed to avoid the mal-operation of protective relaying. Simulation case studies verify the correctness of coupling characteristic analysis and the effectiveness of proposed improvement scheme. [ABSTRACT FROM AUTHOR]