Transformer Differential Protection Revisited

Short Abstract This paper takes a fresh look at transformer differential and restricted earth fault protection. The paper provides a new way to rule out magnetizing inrush as a source of the differential current, allowing tripping for most transformer faults in as fast as a quarter of a power cycle. The paper also provides a new way to restrain the differential element for ratio errors and by doing so allows increasing protection sensitivity. Some of the presented ideas require the transformer relay to have access to the voltage signal, but today, such access is not a limiting factor. The paper also addresses protection dependability for faults that occur during or prior to transformer energization.

Full Abstract Power transformers are expensive assets with long lead times and complicated on-site delivery and installation steps. Transformer faults may result in limited damage, allowing the faulted transformer to be repaired, or in a catastrophic failure, resulting not only in scrapping the faulted transformer but also in a substation fire with collateral damage and environmental costs due to an oil spill. The speed and sensitivity of the transformer protective relay can make a difference between these two outcomes. The transformer differential (87T) protection element in microprocessor-based relays is one of the most conventional implementations of the original principle devised many decades ago within the limitations of the electromechanical relay technology. This backwards compatibility and limited innovation contributed to the fast adoption of microprocessor-based 87T relays but resulted in no or only limited performance improvements over the decades. This paper takes a fresh look at the speed and sensitivity of the 87T element. Magnetizing inrush current is the single most critical obstacle to the 87T speed (having to rule out inrush slows down the 87T element to about 1.5 cycles). Current transformer (CT) ratio errors and operation of the on-load tap changer are key obstacles to the 87T sensitivity, forcing reliance on the oil-and-gas relays. This paper describes simple out-of-the box solutions to these speed and sensitivity challenges. The paper proposes a new way to rule out initial magnetizing inrush as the cause of the differential current, and by doing so, it offers an opportunity to reduce the 87T operating time to about a quarter of a cycle. The paper discusses and proves that the new method works well during recovery inrush (clearance of a nearby external fault) and sympathetic inrush (an energized transformer drawing a gradually increasing inrush current because of dc offset in the voltage that is caused by an initial energizing of a nearby transformer). The paper proposes a new way to restrain the 87T element for CT ratio errors and on-load tap changer operation. The new method is not concerned with fast and deep CT saturation during external faults because this challenge is solved by the tried-and-true external fault detection logic. Instead, the paper considers small CT ratio errors and the challenge of attributing the small differential current either to an internal fault (trip) or to CT and relay measurement errors or transformer ratio mismatch (restrain). Finally, the paper discusses the challenges of and solutions for detecting internal faults during inrush conditions. The paper uses field records to illustrate and explain the challenges and the proposed solutions. Users can implement several of the proposed ideas in today’s relays by using programmable logic.