Power transformers represent the largest portion of capital investment in transmission and distribution substations. The financial consequences of losing a single power transformer unit can have a multi-million-pound impact with a build or replacement timeframe of 18-24 months. Yet, MISTRAS says, more than half of all transformer failures can be prevented.
A failing transformer removed from service in time can normally be economically reconditioned. However, using AE techniques, it is often possible to avoid having to remove a faulty transformer from site altogether, as the precise location of the fault can often be determined and rectified on site.
Fault conditions in a power transformer are detected in several ways. One 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. However, this energy, released through fault processes such as overheating, partial discharge (PD) and arcing, is often sufficient to generate the fault gases initially in the form of bubbles. Also, high moisture conditions and sudden overloads can cause the inception of moisture vapour bubbles released from conductor insulation.
When it has been established that a transformer is gassing from abnormal dissolved gas analysis (DGA) results, most transformer owners implement a programme of more frequent testing to try to understand what is happening inside the transformer and to prevent a catastrophic failure.
For many years, AE techniques have been used to detect and locate PD/arcing. However, there are some cases where acoustic emissions have been detected in the absence of PD.
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, such as rain, snow, ice, or dust impacting the transformer, areas of turbulent oil flow within the unit, load tap changer operations, and magnetostrictive noise.
“MISTRAS was the first company in the world to document, through laboratory and on-site tests, that Acoustic Emissions can be produced by other fault mechanisms such as overheating or mechanical defects,” said Tim Bradshaw, general manager, MISTRAS Group UK Operations. “Over the years, MISTRAS has developed data filtering techniques that enable our engineers to differentiate between relevant and irrelevant data. Adequate data filtering is absolutely critical in order to obtain a true indication of the condition of the transformer.
“AE techniques can be used to proactively screen a transformer while it is operating. Or, if a fault has already occurred, AE can help to pinpoint that fault and identify the type of problem you need to rectify. You can also use AE to monitor precisely when and under what conditions the fault is occurring,” he concluded.
