A hybrid UHVDC transmission system applying LCC as the rectifier and MMC as the inverter combines the advantages of both converter types

which makes this protection scheme more complicated. A new pilot protection scheme for a three-terminal hybrid DC transmission system applying energy functions is proposed. The energy function for LCC is applied to MMC to derive the energy level of the hybrid system. Furthermore

an improved Hausdorff distance (IHD) algorithm is proposed to detect the difference in energy levels between the normal and fault states. An abrupt change in energy level is characterized by IHD change rate. Time points at which the IHD change rate exceeds the threshold at converter stations are applied to determine the fault line and to estimate the fault section. The proposed protection scheme is then verified by a simulation model of the Wudongde ±800 kV three-terminal hybrid UHVDC transmission project. The appropriate sampling frequency is selected for a real-time calculation

and the threshold is selected considering the effect of noise. Results show the proposed scheme can identify and trip fault lines quickly and effectively

even for a 600 Ω grounding fault. Other waveshape similarity algorithms are compared and analyzed. Compared with existing protection schemes

the proposed scheme transmits less data to improve communication speed and reliability.