Unconventional diagnostic methods for testing generator stator windings

This paper presented a comparison of different methods of measuring insulation resistance and tan delta (dissipation factor), and of documenting partial discharge activity. The measurements were made on several service-aged generators. The results for one in-service generator, obtained between service inspections, are discussed in detail. Comparing classical insulation resistance measurements and the dielectric response behavior in the frequency domain, good agreement is found. With the insulation time-constant, the dryness can be determined relatively. In the case of resin-impregnated insulation systems, additional investigations are required to determine the humidity absolutely. The determination of tan delta using various test systems and measuring methods was carried out on-site. Because tan delta is frequency and temperature-dependent, the results of series resonance circuit measurements must be modified by correcting for the difference between the measurement frequency and the power frequency. The results obtained using various PD-test systems depend on bandwidth and the circuit configuration. The multi-synchronous system has the significant advantages that the PD activity at several points can be monitored in one measurement, and the three-phase amplitude relation diagram makes it easy to identify the PD source. However, the calibration process is lengthy, so little time is saved overall, and a full set of quadripoles and coupling capacitors is needed for each channel.