Software plays an essential role in the design and maintenance of the electric power system. To understand the studied phenomena and the underlying modelling tool, a certain degree of openness is needed. For that reason, open-source modelling has been gaining momentum regarding open (i.e., non-black-boxed) models and actual open-source modelling tools. However, the actual electric power system is also becoming more software-defined, in particular, due to the emergence of more and more power electronic converters. The control and protection software of power electronic converters can cause unwanted interactions in the power system. This paper provides an overview of such unwanted interactions and implemented solutions. Because of many unknowns in the changing power system, this paper argues that open control and protection software for power electronic converters can be a valuable complement to standardization.