The reliable power transmission using HVDC systems depend on the secure operation of its high voltage equipment. The general understanding of the electrical field strength distribution within insulation systems under HVDC stress is based on the assumption of a dominant ratio of specific conductivities of the insulation materials. These assumptions then also serve as a base for the design. For decades successful adapted RC-network models for the dielectric behaviour of the common materials under HVAC stress is one reason for the usage under HVDC stress. 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, one can yield physical sound, calculated field strength distribution in insulation systems when stressed with high DC voltage. At the example of oil-paper insulation systems for HVDC power transformers and SF6-resin systems for GIS respectively. Fundamental effects on the discharge behaviour such as partial discharges and breakdown are discussed with the charge carrier based electric conductivity model in order to provide fundamentals for the diagnosis of possible root causes of failures in HVDC equipment.