A line protection scheme for hybrid cascaded DC transmission systems based on boundary transient power direction

Due to the complex structure of hybrid cascaded DC transmission systems， a DC line protection scheme based on boundary transient power direction is proposed to enhance protection adaptability. First， the smoothing reactor serves as the boundary， enabling the derivation of transient power direction from voltages and currents on both sides of the reactor during line faults. Then， internal and external faults are identified by analyzing the characteristics of the boundary transient power direction， and the faulted pole is determined by comparing the summation of positive and negative boundary transient power values. Finally， the Baihetan-Jiangsu UHV hybrid cascaded DC transmission system is modeled using simulation software to verify the protection scheme. Results show that the proposed protection scheme can accurately identify internal and external faults within 6 ms and withstand transition resistances up to 600 Ω.