The reliable power transmission using HVDC systems depends on the secure operation of its high voltage equipment. So far the general understanding of the electrical field strength distribution within insulation systems under high DC voltage stress used to base on the assumption of a dominant ratio of specific conductivities of the insulation materials. This assumption then also served as a base for design and diagnosis. Successful adapted RC-network models for the dielectric behaviour of the common insulation materials under high AC voltage stress have been used for high DC voltage stress, too. Fundamentally, these state of the art models do not cover the influence of space charges on the electrical field strength distribution and therefore mispredict the electric stress in insulation systems. The contribution provides a summary on the physical assumptions of electric conductivity mechanisms in insulation materials and the alternative calculation method of charge density based modelling. By adapting these models, the field strength distribution in insulation systems can be calculated for high DC voltage stress. Fundamental effects on the charge carrier density and therefore field strength distribution are discussed in order to provide fundamentals for the diagnosis of possible root causes of failures in HVDC equipment such as partial discharges and breakdowns.