When thinking about transformer testing, TTR ranks number one in the routine maintenance testing for most engineers. It provides a good starting point for transformer diagnostics, in that it tests that the transformer is doing what it is supposed to do.
When it comes to uncovering problems that may not yet be impacting operations, DC winding resistance testing is regarded as another essential routine screening tool. It indicates problems such as loose, defective or incorrect connections, which cause enough transformer failures each year to be regarded as a failure category of its own. Winding resistance tests may also reveal problems with a transformer’s tap changer (s), on-load (OLTC) and/ or de-energized (DETC) – a major and, by some statistics – leading, component category that fails in transformers. However, the test tends to be neglected due to a number of concerns and challenges surrounding winding resistance.