Adaptability Analysis of Traveling Wave Protection for Cascaded Converter Hybrid HVDC Transmission Line

The inverter side of the cascaded converter hybrid HVDC transmission system (CCHHTS) adopts the structure of LCC and MMC in series. Therefore, it is necessary to investigate the adaptability of the conventional traveling wave protection (TWP) to the CCHHTS due to the structure being significantly different from that of the traditional HVDC system. Firstly, the impedance frequency characteristic curves are given based on the primary system parameters of the CCHHTS. Then, the refraction and reflection coefficients curves of voltage and current traveling wave on the rectifier and inverter side at different frequencies are obtained. The refraction and reflection coefficients curves show that after the forward external fault of the cascaded converter hybrid HVDC transmission line (CCHHTL) occurs, the voltage change and voltage change rate in the voltage TWP criterion are larger than those of the traditional HVDC system. It will increase the probability of the protection false operation. Finally, the correctness of theoretical analysis is verified by simulation cases based on the PSCAD/EMTDC.