Methods of shielding primary windings on high voltage instrument transformers

One of the main problems encountered in the design and construction of high voltage potential transformers is achieving the ability to withstand high voltage transients. These transients can be the result of lightening, switching surges, equipment failure and flash-overs. Issues that arise from these transients tend to occur in the outermost layers of the primary coil, resulting in damage, such as layer to layer insulation puncture and creepage failure. A common technique used in averting this, involves the utilization of a line shield, over the last layer of the primary winding. This paper will discuss the methodology involved in proper line shielding, using the design for a 115 kV class voltage transformer, as an example.