Root causes of condensation in ring main unit cable boxes

Ring main units (RMUs) are widely used in Malaysia medium voltage distribution network, for both indoor and outdoor substations. In a few particular designs of RMU, liquid droplets were observed to be formed inside the cable box. The cases of the liquid droplets formation can be classified into two different situations. In the first situation, there was no further anomaly observed on the components inside the cable box apart from the liquid droplet formation. In the second situation, the liquid droplets formation was found together with severe corrosion of the cable box´s metallic parts as well as severe degradation of the insulation material. The root cause for the first situation was that the liquid droplets formation were condensation of the highly humid air inside the cable box. The high humidity level was caused by the direct exposure of the cable box to the humid ground and the lack of ventilation inside the cable box. The second situation root cause was that the liquid droplets were formed as by-products of existing corona discharge activities in the cable box, which was further aggravated by the high humidity level. Water was initially formed, which later became acidic due to reaction with corona discharge gaseous by-products. These acidic products caused the severe metal corrosion and insulation degradation. The paper also discusses the mitigation methods associated with the two categories.