In addition, the study also indicates that the decisive factors for the selection of the grounding impedance of a symmetrical monopolar station are the asymmetrical operation of the whole grid and its contribution to pole-to-ground fault currents.

The analysis concludes that an inappropriate earthing resistance magnitude may lead to a poor performance of heterogenous HVDC grids during asymmetrical operation. The impact on the protection system is also investigated. For that reason, this paper focuses on the influence of the earthing system resistance of symmetrical monopolar stations on the performance of a heterogeneous HVDC grid during asymmetrical operation. Furthermore, when symmetrical monopolar stations exist in the DC grid, asymmetrical contingencies will also affect its healthy pole. The effects of these contingencies on an HVDC grid are very dependent on the configuration of the HVDC stations and their earthing system.

Such stations may suffer contingencies affecting just one pole (asymmetrical contingencies). As a response to the future challenges that power systems will have to face, a gradual growth of present High Voltage Direct Current (HVDC) links is expected, leading to an HVDC supergrid integrating different configurations of HVDC stations.

Asymmetrical modular grids