Diagnosis of HVDC insulation systems by use of oil-conductivity measuring methods

HVDC (high voltage direct current) transmission is a very effective way of power transmission. For DC applications the oil conductivity is an important quantity for the design, the diagnosis and the condition assessment of HVDC insulation systems. Insulating oil has a strongly nonlinear conductivity which depends on various parameters (field strength, temperature, water content and ageing) and is time-dependent because of charge carrier drift processes. Special measuring methods are required to provide the information necessary for the condition assessment. In this paper measurements of the relations between initial and steady-state conductivities at different temperatures are presented and the consequences of different conductivity measuring methods for the condition assessment of insulation systems, e.g. in oil-filled converter transformers, are discussed. Moreover, measurements of new and aged oils are compared and taken into account for a diagnosis of oil-filled HVDC insulation systems.