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Motor current signature analysis (MCSA) has become an essential part of the preventive maintenance program for monitoring the condition of the rotor cage in medium voltage induction motors in the pulp and paper industry. However, many cases of false indications due to interference from the motor or load have been reported. False indications can result in unnecessary inspection and outage costs (false positives) or major repair/replacement costs and loss of production (false negatives). The objective of this paper is to present the potential root causes of false indications, and provide guidelines on how commercially available off-line and on-line tests can be applied for identifying false indications from a field engineers' perspective. Case studies of false MCSA indications and results of alternative commercial tests forimproving the reliability of the diagnosis are provided through measurements on 6.6 kV and laboratory motor samples. Finally, new test methods under research and development for reliable rotor fault detection are summarized and unresolved problems are listed. This paper is expected to help field maintenance engineers prevent unnecessary motor inspection and forced outages, and guide researchers target future research towards industrial needs.

-To evaluate the condition of stator winding insulation in generators that have been operated for a long period of time, diagnostic tests were performed on the stator bars of a 500 MW, 22 kV generator under accelerated thermal and electrical aging procedures. The tests included measurements of AC current (∆I), dissipation factor (tanδ), partial discharge (PD) magnitude, and capacitance (C). In addition, the AC current test was performed on the stator winding of a 350 MW, 24 kV generator under operation to confirm insulation deterioration. The values of ∆I, ∆tanδ, and PD magnitude in one stator bar indicated serious insulation deterioration. In another stator bar, the ∆I measurements showed that the insulation was in good condition, whereas the values of ∆tanδ and PD magnitude indicated an incipient stage of insulation deterioration. Measurements of ∆I and PD magnitude in all three phases (A, B, C) of the remaining generator stator windings showed that they were in good condition, although the ∆tanδ measurements suggested that the condition of the insulation should be monitored carefully. Overall analysis of the results suggested that the generator stator windings were in good condition. The patterns of PD magnitude in all three phases (A, B, C) were attributed to internal discharge.

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Forced outage of generators due to stator winding insulation failure can result in significant financial loss because of the high cost repair and loss of production. In recent years, the demand for insulation diagnosis is increasing to prevent unexpected failures, as the capacity of generators has increased. Insulation diagnosis is composed of the insulation resistance measurement, polarization index measurement, dissipation factor (DF) tip-up test, AC current increasing ratio measurement, and the partial discharge (PD) measurement. In this paper, the results of the PD measurement and PD pulse pattern analysis performed on a healthy generator and two generators that experienced dielectric breakdown failure during operation is presented.