Transformer Differential Protection Revisited

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.