Application and commissioning of multifunction digital transformer relays

The application of multifunction digital relays to protect power transformers has become a common utility practice. These relays provide more than just differential protection, but offer a full range of transformer protective functions including: overexcitation, restricted earth fault, overcurrent backup, thermal, and breaker failure protection in a single hardware platform. The paper presents an overview of reasons for transformer failure as well as IEEE through fault-withstand standards. The paper also presents element design changes made possible by digital technology such as: use of multiple harmonics and cross-phase inrush restraint to enhance security on transformer energizing, and overexcitation harmonic restraint to desensitize tripping rather than block all tripping. Advancements in digital technology have provided many technical improvements and have allowed relay manufacturers to include more relay functions within a single hardware platform - including addressing more transformer winding configurations. This has increased complexity resulting in digital transformer relays that require an Einstein to set or an Edison to commission. Since there are few Einsteins and Edisons among us, the next generation of transformer relays needs to concentrate on this complexity issue in addition to technical improvements. The author believes that these issues are the major shortcomings of many existing digital transformer protective relays which have resulted in improper setting of many transformer protection relays by less-experienced relay engineers