Tutorial on Protection of Three Single-Phase Transformers

At the design stage of a new power substation, one of the decisions to be made is whether to select a single three-phase power transformer or a bank of three single-phase transformers. At Churchill Falls Terminal Station 2, of the Lower Churchill Project, a bank of three single-phase 3×280 MVA 735 kV power transformers was selected because of the required capacity. During the transformer protection relay configuration stage, the protection engineers typically encounter relays designed towards protecting the more standard three-phase transformers. In order to adequately protect the bank of three single-phase power transformers, and ensure both security and dependability, some measures have to be taken to adapt the existing relay parameters to the reality of the primary equipment. This paper will briefly cover the pros and cons of using a bank of single-phase transformers. It will then address the main protection functions used for large utility electric power transformers, and how the protection setting calculations and actual relay settings are affected when protecting three single-phase transformers.