AE10.pdf

Emission can be applied to locate gassing sources

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emission can be applied to locate gassing sources that have been detected by periodic gas in oil analysis, to compare the condition of two sister units and to identify a trend in the acoustic data when a unit is tested regularly. Manufacturing, Repair or Refurbishment Early efforts applying acoustic emission testing to transformers took place in laboratory conditions, away from where power was generated and transmitted. The effort would save maintenance personnel hours or days in locating these problems. There are several advantages of testing a transformer under laboratory conditions: (1) control of the operating conditions, (2) elimination of external sources and (3) repeatability of test conditions. Acoustic emission testing is less sensitive than electrical monitors in this environment and can be used with various other tests such as induced voltage tests. Monitoring during Commissioning Static electrification is a phenomenon that can be detected using acoustic emission testing. It is more likely to occur in shell transformers and during commissioning of generator step-up units. Acoustic emission testing can be used to monitor the characteristics of the static discharge (amplitude, event rate, energy) 333 Electric Power Applications of Acoustic Emission Testing
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during the first hours after commissioning and provide an early warning of an increment at this stage or of a deviation from previous operation data. Monitoring during Special Operating Conditions Acoustic emission testing can be applied to monitor continuously the power transformer operating under special conditions such as overload or solar storms. The objective of these tests is to detect any change in acoustic activity caused by the development of an incipient fault. Case Histories Overheated Bus Bar A generator step-up transformer is shown in Fig. 1. The three-phase, oil cooled, shell transformer was rated at 25/500 kV and 784 MV·A and had two groups of pumps; it started gassing after 20 years of service. Dissolved gas analysis by all seven techniques applied indicated overheating. The gas generation rate increased periodically, so this unit underwent acoustic emission testing to locate sources of gassing inside the transformer. This transformer was tested continuously for 68 h while instrumented with 22 acoustic emission transducers. Many events were obtained over the course of the test with the event-to-time ratio changing dramatically. Analysis after testing indicated the existence of two clusters of events: cluster 1 close to a high voltage bushing and cluster 2 below a low voltage bushing (Fig. 2). Most of the events and the highest peak amplitude were obtained in cluster 2. Data analysis indicated that the number of events recorded was strongly related to pump operation. One pump group was running all the time and the second group was operating when it reached a preset temperature value. It is believed that these events were triggered by a high temperature value in the faulted area that reached its critical value just
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  • Fall '19
  • Test, Nondestructive testing, Acoustic Emission, Acoustic Emission Testing

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