Impact factors on the 2nd zone of distance protection under integration of modular multilevel converters

The introduction of an increasing number of converters in the existing AC systems has led to a series of challenges. One of them is securing the stability of the existing AC grids in case of faults. Several publications indicate a negative impact of modular multilevel converters (MMC) on the AC distance protection, especially on its second zone. This work reviews the existing contributions and analyses the considered topologies and impact factors. Based on this analysis, study cases are derived to compare the impact of an MMC on the distance protection to other impact factors. Thereafter a differentiation between the factors resulting from the chosen system configuration and the impact of the MMC is given. The results show that there is an impact of the MMC on the second zone due to the given system topology. However, due to its limited fault current contribution capability, the impact does not exceed the variation due to other impact factors. Depending on the previously existing system conditions, the impact of an MMC inserted into an existing line is negligible in a meshed AC grid due to its limited short circuit current contribution.