The answer to this month’s trouble shooter is provided by MISTRAS Group Limited.

Published:  12 October, 2017

Q:

How can I detect and diagnose fault conditions within a power transformer while it is still operating i.e. without having to remove it from site?

A:

One common method is based on detection of the degradation products of the insulating oil, typically dissolved gases that are produced as a result of an abnormal dissipation of energy within the transformer. Through oil sampling these gases can be detected providing the operator with an indication of an issue.

The Acoustic Emission (AE) technique is another, less widely known method that can be used to not only detect but also locate faults within the transformer. Faults such as partial discharge (PD)/arcing are most commonly detected while there are some cases where acoustic emissions have been detected in the absence of PD and produced by fault mechanisms such as high temperature faults or mechanical defects.

AE techniques can be used to proactively screen a transformer while it is operating. If a fault is active, AE can help to pinpoint its location and identify the type of problem that needs rectifying. AE can also be used to monitor precisely when and under what conditions the fault is occurring.

The mechanisms that produce signals are directly related to a fault in the transformer. However there are other mechanisms that will generate AE activity that are not directly related to a problem in the unit. Some of these include environmental sources (e.g. rain, snow or dust), areas of turbulent oil flow within the unit (pump operation), load tap changer operations, and magnetostrictive noise. This means that adequate data filtering techniques need to be applied in order to differentiate between relevant and irrelevant data.

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