AEP experience with protection of three delta/hex phase angle regulating transformers

American Electric Power installed three 138 kV, 150 MVA delta/hex phase angle regulating (PAR) transformers on the south Texas transmission grid during 2006. These transformers help optimize power flow on the transmission grid until planned 345 kV line construction projects can be completed to improve the strength of the grid in this portion of the power system. The delta/hex PAR transformer is a new single-tank design that promises to be much more economical to build and install than previous designs - improving the economic viability of deployment of these devices on the transmission grid. This paper addresses the challenges of providing fully redundant, sensitive, and secure protection for all types of faults, including turn-to-turn faults in this type of transformer and its surrounding bus work. The protection system includes a mix of conventional protection concepts and a completely new differential protection system that can compensate for the variable phase angle shift introduced by operation of the PAR transformer. The complete protection system was tested using digital model power system transient testing prior to installation in the field.