DC Partial Discharge Studies Applied to O-Ring Applications

Abstract:
In recent years, DC transmission is becoming more and more relevant in order to increase the efficiency of power trans-mission lines and eliminate losses related to reactive power. For DC transmission, converters are necessary to convert from AC to DC and vice versa. These converter units consist of many semiconductor switching units which are connected in series. A cooling system is utilized to cope with switching and conduction losses in the semiconductor switches. This paper investigates the behaviour of O-rings employed within a cooling system subjected to DC voltage stress in a single switching unit. The cooling system applied to the switching module creates polarization of the sealing material due to voltage drops resulting from the surrounding DC field. This results in an increased potential on the outer edge of the O-ring, which is therefore defined as the high voltage (HV) boundary condition for the analysis. The lower potential is given by the coolant, which is assumed as ground. For this investigation, a single cooling pipe unit is selected, with a single slot for an O-ring. A metallic pipe is utilized to represent ground potential at the inner side of the O-ring, while high voltage is applied to the cooling pipe containing the O-ring within the slot, creating a global electric field. Partial discharge (PD) measurements are conducted using the circuit of the measurement setup developed by Freudenberg et al. The O-rings utilized in the study are categorized into three groups based on their condition: good, medium used, and fully damaged O-rings, reflecting varying levels of damage developed over time. PD occurrence is primarily attributed to the degree of damage present in the O-rings, with higher occurrences observed in medium to highly damaged O-rings. Additionally, the presence of glycol in the coolant during testing was found to enhance PD occurrence. To explore the short-term aging effects on O-rings, a high voltage stress of 2.5 kV was applied for one hour, shedding light on the impact of short-term aging. Moreover, the effectiveness of employing silicon grease as a remedy to reduce PD formation was examined and validated.