Analysis of common-mode voltages in flexible DC system with single-pole grounded fault

In some flexible DC transmission systems with certain grounding modes, single-pole grounded faults won't cause large fault current. If the system keeps operating and clears the fault in time, the power supply reliability of the system will be greatly improved. SVPWM is widely used in the converter. However, the SVPWM will generate third harmonic voltage in the system. Firstly, the topology of multi-terminal flexible DC transmission system including structures and grounding modes is discussed. Subsequently, a common-mode equivalent circuit mode for analyzing the influence of third harmonic voltage is established. Thirdly, the formula for calculation of the third harmonic voltage and current is derived. The fault characteristics are also analyzed and a diagnostic method of fault location is proposed. Finally, the simulation experiments are made in PSCAD/EMTDC. The simulation results show that the proposed protection strategies can accurately locate the fault line after pole-to-ground fault occurs.