Instrument transformers are critical components on the substations, they are used to measure voltage and current on the system. External contamination (ozone, water,  etcetera) is the main factor causing degradation on the surface of these equipment. Partial discharge can develop either internally or superficially.

Acoustic Emission can be used to detect partial discharges on time before they reach critical levels without taking the equipment out of service or manipulating its operating conditions.