Due to the development of High Voltage Direct Current (HVDC) technology which is particularly suitable for the transmission of large amount of energy over long distances and for off-shore energy integration, new technology for HVDC Gas Insulated Switchgears (GIS) and Gas Insulated Lines (GIL) is developing thanks to its compactness and its high reliability for application in HVDC switchyard.
Development and dimensioning of HVDC GIL and GIS requires deep knowledge on electric field distribution in the gas/solid insulating systems. The electrical field distribution changes progressively from a capacitive state which depends only on permittivity to a resistive state which depends on permittivities and conductivities of gaseous and solid insulations. Moreover, various factors like the temperature gradient due to the nominal current, the charge carrier generation in gas due to the gas moistures or the roughness/imperfections of high voltage electrodes, the interface charge accumulation at the interfaces between gas/solid, the space charge accumulation inside solid spacer can influence strongly the electric field distribution and thus the withstand voltage and the reliability of the HVDC system.
For a reliable design of HVDC GIL/GIS, the phenomena understanding, the material’s characteristics, the simulation methodology/model must be mastered in order to take into account the specific nonlinear properties of insulating materials and the specific phenomena at the operating conditions: high DC voltage and with temperature gradient. Moreover, simulation for DC apparatus requires, unlike under AC voltage, consideration of many physical phenomena as described in table below.
Finally, as stresses and test requirements for DC apparatus are not the same as the one Under AC voltage, specific design criteria must be defined.