<p>It has been admitted globally that climate change is real, driven by fossil fuels, leading to alternate sustainable energy systems. As most of the renewable energy sources (RES) and modern loads operate on direct current (DC), the interconnection of these can be more efficiently achieved via a DC bus, leading to direct current distributed energy systems (DC DES). Development of system level models is crucial for analysis of DC DES before deployment. The integration of RES and various loads in DC DES has been accomplished via deploying off the shelf power electronic converters (PECs). Behavioural modelling technique has been successful to capture the dynamics of these off the shelf PEC in stand-alone mode. However, in modern DC DES, the dynamic interactions at the input–output of PEC lead to terminated transfer functions which result in behavioural model that is unable to model the true behaviour of the system. The current work proposes a methodology to address this issue and acquire unterminated transfer functions for system level analysis. The proposed method is based on the modular concept, i.e. it can be applied to a single PEC and also extended to a system involving multiple PEC. For validation, it is first applied to a cascaded converter system and then extended to a DC DES. The results demonstrate that using the proposed technique, the unterminated transfer functions can be obtained for PEC in any complex system, thus leading to construction of its behavioural model that accurately represents the true behaviour of the system.</p>