Simplified calculation method of threshold value for the non-unit transient-voltage based protection in multi-terminal VSC-HVDC grid

Abstract The non-unit protection based on transient voltage shows outstanding application prospect in the multi-terminal VSC-HVDC grid, due to the advantages including high operation speed, ability to withstand large transition resistance, et al. However, for this kind of protection, the threshold value is generally determined according to the simulation, because the theoretical calculation method is still lack of study. In this paper, the fault traveling wave propagation process in the multi-terminal VSC-HVDC grid is analyzed. And the calculation methods of the key parameters are deduced, such as the reflection coefficient, line-mode transition resistance and line-mode fault point voltage. Then the impact of the frequency-dependent line-parameter characteristics is discussed in detail. On this basis, the simplified calculation method of threshold value for the non-unit transient-voltage based protection is proposed. Compared with the simulation-based method, the computing load of the proposed method is reduced greatly. In addition, in contrast to the method developed for ROCOV-based dc protection, the rational fitting and inverse transform are not required in the proposed method, presenting easier implementation for engineering application. Finally, simulation cases prove that the proposed method calculates the fault transient voltage accurately. And the protection with the calculated threshold value distinguishes internal and external faults reliably.