On-Load Tap Changer Internal Contact Health Assessment Based on Ion Detection in Oil

Internal contact deterioration causes most electrical faults in on-load tap changers (OLTCs), which are difficult to diagnose promptly and accurately by traditional methods. This article proposes a novel approach for internal contact condition evaluation based on metal ion detection. The variation of metal ions under the action of dc breakdown and natural corrosion for metal contacts with different materials are experimentally studied first. The formation mechanism of metal ions indicates that electric erosion is the primary cause of the short-term increase of characteristic ions. In addition, both chemical corrosion and electrochemical corrosion play a significant role in the long-term growth of metal ions. Based on the assumption of maximum damage to OLTC contacts, the characteristic metal ion concentration when the contacts require mandatory replacement is calculated. An early warning method for the damage condition of the internal contacts is proposed by combining the accumulated metal ion concentration and the relative metal ion growth rate. An on-site fault transformer case demonstrates that the multilevel warning value can well meet the requirements for the health status assessment of OLTCs internal contacts.