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EMI Diagnostics: Steam Turbine Generators

Electro Magnetic Interference (EMI) is produced by electrical and mechanical defects in Steam Turbine Generator systems. The precise measurement of EMI will provide a condition assessment for the entire system, generator, exciter, bus and breaker. Data is collected during normal operation with a split core radio frequency transformer (RFCT) placed around the stator neutral conduit or a frame safety ground. No permanently installed coupler is necessary to collect EMI data. Data collection and preliminary analyses by a skilled test engineer requires around one hour. In this example EMI data was collected at full load for this hydrogen cooled steam turbine generator. Partial discharge (PD) activity was seen. There should be no PD in a 13.8 kV hydrogen cooled generator. A problem has developed.

The EMI Signature produced:

OH unit 4 max load 2011 steam

An inspection found endwinding movement:



The EMI pattern identified was unstable Partial Discharge activity. The frequency range of 2 MHz to 30 MHz indicated looseness in the stator endwindings. An inspection verified this was the condition detected. Stator repairs were applied before this deterioration became serious. Just in time maintenance prevented a winding failure. Advantages of using the EMI Test on steam turbine generators include:

  1. Test is made with no interruption to scheduled operation
  2. Problems are identified early so maintenance can be scheduled when needed
  3. The entire generator system is evaluated with the one test
  4. Risk of system failures is reduced to maximize production


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Doble Engineering Company

Doble Engineering Company