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Introduction

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 more serious in term of loss of production than the actual repair cost. There 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 highvoltage 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.

Frost 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 frost normal service, and that the characteristics displayed by the machine are therefore not those which it generates in normal service.

Some of the above comments are particular examples of general problems which are associated with off-line diagnostic measurements on both electrical and mechanical plant and there is an increasing interest in the possibility of making on-line measurements with a view to obtaining more relevant data more frequently, and with minimum disturbance to production processes.

This paper describes a new technique for on-line monitoring of stator winding insulation condition in high voltage machines. The technique is based upon the non-invasive measurement of highfrequency pulses of electric current in the stator winding generated by partial discharge activity in the insulation structure. It is easier to apply than traditional off-line methods and has the important advantage of being able to display the discharge characteristic which the machine exhibits in normal service rather than the somewhat artificial condition of an off-line test.

2. INSULATION DEGRADATION

Any monitoring technique must obviously be applied in the light of known mechanisms of deterioration and failure. Degradation of high-voltage machine insulation may be a very complex process but some mechanisms have been found to be process but some mechanisms have been found to be common to a number of failures in service and all of these may be expected to modify the partial discharge characteristic of the winding.

Partial discharges are low level spark discharges Partial discharges are low level spark discharges occurring on the surface of, or in cavities in, the insulation structure of the stator winding insulation. Owing to the high electric stresses necessarily present in a high-voltage winding, all high-voltage machines generate partial discharges, to some extent, but the discharge activity may become more intense in the event of physical damage to, or contamination of, the physical damage to, or contamination of, the winding. There is then a possibility that the partial discharge activity may contribute to partial discharge activity may contribute to failure by causing erosion of the insulation material at the point of damage. This is the main reason why most condition monitoring methods which are applied to high-voltage machines are based on making sane measure of partial discharge activity.

Physical damage to the winding insulation may Physical damage to the winding insulation may typically be the result of stator core or tooth vibration, or loose wedging, which leads to movement and abrasion of the slot section of a coil. This process can initiate slot discharge between the slot wall and the coil side, leading to gradual removal of insulation by a combination of physical abrasion and discharge erosion (Ref 1).

Excessive end winding movement, typically the result of very high electromagnetic forces acting on the coils during starting operations, may also lead to deterioration of insulation.

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