Three-Winding Autotransformer Fault Study and Impact on Protection Application

Autotransformers (ATs) have been widely used for many years. For large size ATs, a tertiary winding is usually included. Sometimes the tertiary winding is loaded for station service, local loads, VAR compensation, etc. Other times, the tertiary winding is buried and acts as stabilizing winding which provides a path for zero-sequence current and 3rd harmonic current. Three-winding ATs present some interesting and different problems for fault study and relay application. Some of these problems are not well understood among protection engineers. The delta-connected tertiary winding may not have large impact on positive and negative sequence network. However, it has a significant impact on the zero-sequence network. Very often we encounter one minus impedance in transformer equivalent T model. The minus impedance will further complicate the zero-sequence current magnitude and direction. If protection engineers do not have proper understanding of transformer model and the fault study, misapplication and misoperation of protection relays are likely to happen. This paper will discuss the equivalent model of 3-winding for fault study, zero-sequence current distribution, and impact on zero-sequence current based polarization and overcurrent application.