Large electric motors and generators operating at voltages in the range 6.6 kV - 11 kV are vital elements in almost all offshore oil and gas production installations. A high proportion of failures which occur in these machines are due to breakdown of high-voltage stator winding insulation and the cost of the consequent outage is often much more serious in terms of loss of production than the actual repair cost. There is thus a clear potential advantage in the application of condition monitoring to give early warning of degradation or failure so that remedial work may be carried out at a time of the operator's choosing rather than as an emergency repair.

Techniques which were developed for the purpose of condition monitoring of high-voltage machines have been available for many years and are widely applied but they are essentially off-line techniques, i.e. the machine to be tested must be stopped and isolated from its normal high-voltage power supply. There are a number of disadvantages associated with this procedure, the most obvious being that the machine is out of service and not available for production. Also the test equipment is inherently large and heavy, since it must be capable of exciting the stator winding to rated voltage at least, so that access to site may present problems. The tests are therefore carried out infrequently with the risk that a rapidly developing fault may not be detected before failure occurs.

From a diagnostic viewpoint, an off-line measurement of insulation condition has the drawback that the test conditions are artificial, in that the distribution of voltage in the winding is quite different from normal service, and that the characteristics displayed by the machine are therefore not those which it generates in normal service.

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