The Load tap changer (LTC) is a mechanical switching device and is the most expensive and vulnerable accessory on a power transformer. They cause more failures and outages than any other component of a power transformer.

The function of a LTC is to change the turns ratio without interrupting the load current. LTC failures are categorized as electrical, mechanical, and thermal but most are mechanical at the beginning and develop into electrical faults. This occurs due to problems on the contacts, transition resistors and insulation breakdowns.

The LTC can be evaluated on-line without affecting its normal operation by using a combination of acoustic emission and vibration techniques (AE/VA).

Acoustic Emission assessment is based on the fact that no acoustic activity is expected from inside the LTC compartment if the tap changer is not being operated and if it is in good condition.

Vibration techniques consist of obtaining the signature of one operation of the tap changer and performing the comparison of its characteristics (time, amplitude, energy, etc.) with another signature obtained some time in the future or with a sister unit in the same operation.

When using a combination of both techniques, the evaluation of the out-of-service condition of the tap changer, is performed using acoustic emission; whereas the evaluation during an operation is by vibration techniques.