Real time condition assessment monitoring of power transformers with Acoustic Emission

Nunez, Arturo; MISTRAS Group Inc.; United States

Núñez, A .; MISTRAS Group, Inc; USA

ID: ECNDT-0008-2018
Session: Structural Health Monitoring 4
Room: H1
Date: 2018-06-13
Time: 10:50 - 11:10

Power Transformers owners face several challenges
– Their assets are reaching or have already reached the end of their operating life
– Delivery times for a new transformer are very long (18 – 24 months)
– Ever increasing demand prohibits the transformers to be taken out of service for routine electrical testing.

There only a few techniques that can be applied with the transformer energized, these are Dissolved Gas Analysis, Oil quality, Infrared Inspection, Vibration and Acoustic Emission.

The most common practice is to perform periodic dissolved gas analysis and oil quality tests to determine if a fault is active and/or if the oil is been degraded. However these techniques do not provide information about the location of the fault. Infrared inspection is limited to indicate temperature differences only on the outside of the unit (connections, block radiators, hot spots on the walls, etc.); however it cannot detect defects on internal components. Vibration is a technique widely used for rotating machinery and although is now used in transformers, there are not comprehensive interpretation guidelines and it is only limited to mechanical sources

Acoustic Emission has been used in Power Transformers since 1970’s. Initially, only one sensor was used and it had to be moved around the test specimen to try to pinpoint the fault. For decades, it was believed to be only useful to detect electrical faults (partial discharge, arcing), however, a research project with the Electric Power Research Institute (EPRI) demonstrated that Thermal and Mechanical faults can also be detected.

Modern day technology allows the use of multiple sensors for the detection and location of the fault. The main advantages of this technique are:
– No transformer outage is required to perform the inspection
– Non invasive
– Global monitoring of the asset.
– Provides location of the fault.
– Offers real time information of when the fault is active
– Indicates under what operating conditions the fault is active
– Severity of the fault can be determined
This paper presents the latest developments on this technology and cases studies are presented