Insulated electrical transformers are one of the most critical and expensive investments in today’s electric power transmission and distribution systems. Their reliability typically depends on their insulation condition.

Any overlooked critical fault generated inside a power transformer may lead to a transformer catastrophic failure which could not only cause a disruption to the power system but also significant equipment damage. The cost of a failure can easily reach multi-million dollars.

Therefore, monitoring the health state of transformers with accuracy and prompt information is the critical prerequisite to predict fatal failures, optimize the maintenance, and make a vital decision on a transformer with suspicious conditions.

In high voltage transformers, a decisive significance is attributed to a secure and permanent insulation. The mineral oil and SF6 gas are good insulating materials. However, due to some inevitable factors such as electrical and thermal stresses as well as mechanical aging, the insulating medium inside a high-voltage transformer might be affected. For instance, some unwanted particles like water and gas can contaminate the oil. As a result, insulation strength gets reduced that may result in partial discharges and hot-spots in transformer oil.

Partial discharge is not only a precursor of insulation degradation but also a primary factor to accelerate the deterioration of the insulation system in a transformer. Monitoring of partial discharge activities through the measurement of changes in temperature and in current in the transformer insulating medium has been proven to be an effective procedure for transformer health state estimation.