Protection and Commissioning of Digital Transformer Relays: Improvements in Medium-Voltage Industrial Transformer Protection

The application of multifunctional 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 the 200-400 A range on medium voltage systems. This article discusses why these types of transformers require sensitive ground differential protection. The basics of transformer protection are also reviewed, including phasing standards, through-fault withstand capability, differential/fusing/overcurrent protection, slope, current transformer (CT) requirements, and harmonic restraint. How to best communicate these requirements when programming and commissioning new digital relays is discussed. The rationale for providing transformer overexcitation protection on all major transformers within industrial facilities is also addressed.