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September 2011

Case Study: Power Factor Testing on High-Voltage Rotating Machines

Power factor, dissipation factor or tan delta, testing of rotating machines is a way to asses the overall insulation losses of the stator insulation. The tests are useful in detecting deteriorated or insufficiently cured insulation and moisture or other contaminants. If you are not currently using this test in your maintenance program, you may want to add it in to the list of assessment tools.

The power factor test is normally performed at several test voltages when permitted by the stator winding voltage rating, For example, for a 13.8 kV winding, the power factor test is routinely performed at 2, 4, 6 and 8 kV, the normal operating to ground voltage. It can perhaps also be performed at a slightly higher test voltage after reviewing the history and condition of this winding. The power-factor test was never meant to be a high-potential test.

The lowest voltage measurement is performed at a test voltage below the discharge inception voltage of the stator winding. This measurement tells us something about the general dryness and cleanliness of the insulation.

The higher voltage measurements tell us something about the amount of cavities presence in the dry type insulation, the compactness of the groundwall insulation, and the tightness of the slot wedges.

Included is a case that shows how the power factor tip-up and capacitance were affected by the tightness (or looseness) of the slot wedges.

To be most useful, results should be compared with the other two phases tested, with
similar units, with factory data and most importantly with previous test results.