A new diagnostic tool for preventive maintenance on large generators - PD location in stator windings by the traveling wave method

The development of cost-effective maintenance strategies requires diagnostic tools which detect any defects as early as possible to enable the determination and selective planning of the requisite repair measures. The sensitive partial discharge (PD) measurement method enables the detection of individual weak points developing in generator high voltage stator insulation in their initial stages. However, a reliable evaluation also requires the most exact possible knowledge of the location of the insulation where partial discharge occurs. The following paper presents a method enabling the precise localization of internal partial discharges in the shielded slot section of stator windings based on the localization principle of transit time difference measurement between the two sides of the stator core. The PD pulses are decoupled from the winding bars using a patented new type of sensor [1]; digitized at a high sampling rate of 4 GS/s and subjected to mathematical analysis. Measurements on old winding bars from large generators and on complete stator windings (40-1640 MVA) yield high localization accuracies within the range of centimeters. In a rewound 880 MVA generator with an active core length of 6.6 m, a partial discharge source was localized which could not be detected by conventional equipment at the generator terminals. This demonstrates the excellent sensitivity of the new diagnostic tool.