Frequency domain analysis of HVDC grid non-unit protection

Owing to the fact that travelling wave propagation in an HVDC grid is a heavily frequency dependent phenomenon, frequency domain analysis has been identified as a useful means of assessing the capabilities and limitations of non-unit protection. This exploits the fact that the main difference between an internal and external fault is the presence of the series inductor in the fault path. The inductor acts as a high impedance element for high frequency components and consequently the transient voltage frequency response in each fault case is recognisably different. On this basis, the investigation in the frequency domain can reveal significant information for protection design purposes. Towards this aim, the transient voltage at the relay location is meticulously represented in the frequency domain by taking into account the transmission medium length and geometry, the number of other attached feeders to the same bus, the converter parameters, the inductive termination, the fault resistance, and the travelling wave behaviour of DC faults. By performing a sensitivity analysis on these parameters, a deeper understanding of their impact on HVDC non-unit protection is obtained. In addition, the factors that can be adjusted to extend the reach of the protection systems are revealed and a generic approach for analytically calculating the protection threshold is developed.