Improvements in protection and commissioning of digital transformer relays at medium voltage industrial facilities

The application of multifunction digital relays to protect medium voltage power transformers has become a common industrial practice. Industrial transformers, unlike utility transformers, frequently use neutral grounding resistors to limit ground current during faults to 200-400A range on medium voltage systems. This paper will discuss why these types of transformers require sensitive ground differential protection. The paper will also discuss the basics of transformer protection including: phasing standards, through-fault withstand capability, differential/fusing/overcurrent protection, slope, CT requirements, harmonic restraint, and communicating these requirements properly when programming and commissioning new digital relays. The rationale for providing transformer overexcitation protection on all major transformers within industrial facilities is also addressed. Advancements in digital technology have allowed relay manufacturers to include more and more relay functions within a single hardware platform as well as address more and more transformer winding configurations. This has resulted in digital transformer relays requiring an experienced protection engineer to set and an experienced relay testing technician to commission. Since there are fewer experienced professionals among us now, 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.