Compact gas-insulated systems for high voltage direct current transmission: Basic design

The design of insulating elements for HVAC GIS is optimized for a capacitive field distribution. An intrinsic difference between AC and DC is however that the DC conductivity of insulation materials is strongly temperature dependent, while their permittivity varies only weakly with temperature. As a consequence, the resistive field is enhanced where the DC conductivity is at its minimum, i.e. in cold regions of the insulation. Besides, the accumulation of space and surface charges have to be observed as well as the specific load at superposition of impulse voltages. Using of multi-physics simulation tools the analysis of temperature and electrical field distribution is now possible with high accuracy, taking the following parameters into consideration: temperature and electrical field dependent characteristics of the insulating materials, accumulation of space and surface charges and the superposition of DC and impulse voltages.