Load tap changer (LTC) is a mechanical
switching device; they are the most expensive and vulnerable accessories on a
power transformer and they cause more failures and outages than any other
component of a power transformer.
LTC function is to change turns ratio
without interrupting the load current. LTC failures are categorized as
electrical, mechanical, and thermal. Most of the failures are mechanical at
the beginning and developed to electrical faults mainly occurring due to
problems on the contacts, transition resistors, and insulation breakdowns.
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 technique consists
in obtaining the signature of one operation of the tap changer and performs
the comparison of its characteristics (time, amplitude, energy, etc.) with
another signature obtained some time in the future or with a sister unit
having the same operation.
When using a combination of
both techniques, the evaluation of the condition of the tap changer when it is
not being operated is performed by using acoustic emission whereas the
evaluation during an operation is made by the vibration technique.
