Application and Commissioning of Multifunction Digital Transformer Relays

The application of multifunction digital relays to protect power transformers has become a common utility practice. This paper discusses the basics of transformer protection including: phasing standards, through-fault withstand capability, differential/fusing/overcurrent protection, slope, CT requirements, harmonic restraint, and communicating these properly to new digital relays. The rationale for providing transformer overexcitation protection on all major transformers is also addressed. Hopefully, this information will be helpful to less experienced engineers. Advancements in digital technology have allowed relay manufacturers to include more and more relay functions within a single hardware platform as well as address increasingly more transformer winding configurations. This has resulted in digital transformer relays requiring an Einstein to set and an Edison to commission. Since there are few Einsteins or Edisons among us, the next generation of transformer relays needs to concentrate on this complexity issue in addition to technical improvements. This paper addresses these issues that the author believes are the major shortcomings of existing digital transformer protective relays