MISTRAS Group Limited has introduced a suite of diagnostic and monitoring techniques for power transformers based on Acoustic Emission (AE) technology. This is supported by the new Transformer Clinic from MISTRAS, an online diagnostic and monitoring service that helps to determine the fitness and health status of power transformers, including online real time remote health monitoring of power transformers.
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 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 – perhaps on a weekly or daily basis – 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 (eg, rain, snow, ice, or dust impacting the transformer), areas of turbulent oil flow within the unit (pump operation), 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. Over the years, MISTRAS has developed data filtering techniques that enable our engineers to differentiate between relevant and irrelevant data. Several different factors determine which filtering scheme we will use for a particular application. But adequate data filtering is absolutely critical in order to obtain a true indication of the condition of the transformer,” commented Tim Bradshaw, General Manager, MISTRAS Group UK Operations.
“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,” adds Bradshaw.
In the UK, Mistras has carried out power transformer AE inspections for both the Power Distribution Industry and for industrial users. In a recent example, says Bradshaw, MISTRAS was asked to inspect a power transformer located on a housing estate.
He explains: “The power distribution company received a DGA alarm on the power transformer and were considering taking the unit offline to diagnose the exact fault, which would have been costly and time consuming. By asking MISTRAS to inspect the transformer on site using AE techniques, these costs were avoided and power supply to the housing estate was maintained.”
“Using AE, we monitored the transformer while power was applied to it. At 100 per cent load, minor AE activity was detected towards the top of the transformer. By pinpointing the precise location of the fault, we were then able to drain off the oil from the top of the unit without removing it from site. This revealed a loose mechanical joint that was vibrating, which in turn was generating a hot spot in the unit. Pinpointing and rectifying the problem on site allowed the transformer to be put back into service at around 80-85 per cent of full load, until the power company’s next planned maintenance window became available,” explains Bradshaw.
What can AE techniques detect?
AE diagnostic and monitoring techniques can be used on power transformers to detect a variety of faults such as:
* Partial discharge
* Hot spots
* Loose connections
* Static electrification in Generator Step Up (GSU) transformers
* Core clamping problems
*Mechanical defects such as loose fasteners/joints
Other parameters (load current, vibration, gas monitoring data, pump current, temperature, and load tap changer motor current) are acquired along with Acoustic Emission data in order to correlate this information with the operating conditions on the transformer during the test/monitoring. A grading system provides information on the intensity of the fault. All results are compared to a database of thousands of field tests carried out by MISTRAS more than 20 years.