Development of simple coupling-capacitor voltage transformer for GIS

A magnetic voltage transformer (VT), with a secondary load of more than 50 VA, is usually used for a GIS. However, the capacity of the load is too large and not economical since the load of A/D converter of a digitized control switchboard (DCS) is recently limited to only several VA. Therefore, the authors have developed a simple voltage transformer (SVT) of coupling-capacitor type to meet the performances of DCS with low load, by inserting a cylindrical capacitor electrode into the branch bus of a GIS. To maintain the required accuracy of ±3%, the secondary output voltages from the SVT were corrected by the numerical control process unit of the DCS in this system, since the voltages were influenced by variation in external load. Coaxial cables were also applied as a measure against temperature variation. As a result, they found that the developed SVT provided accuracy of within ±3% in ambient temperature between -20°C to +60°C