Partial Discharges in Large Rotating Machines - Continuous On-line Monitoring and Localisation, Based on a New Developed Technique (InTech)

Abstract: Partial discharges (PD) that originate from dielectrically defective parts of the insulation system result in its degradation, which may ultimately lead to a failure of high voltage apparatus. This has resulted in development of numerous PD measuring techniques.

One of the most widespread applications of PD measurement is in rotating machines. PD measurements in rotating machines unfortunately suffer from common difficulties, such as noise rejection, attenuation of the PD signals in stator winding and calibration accuracy.

This thesis presents work on development of a new technique as well as a new measuring system for on-line measurement and localisation of PD in large rotating machines. The technique, which is denoted InTech, has considerably improved the aforementioned difficulties. In this technique, inductive sensors are positioned on the upper stator bars, so that every stator coil is situated between two sensors. The measurement is then focused on individual stator coils. Treatment of the sensor signals results in a measurement of the PD level in the stator coil. Thus, the stator coil from which the PD originates is also identified.

The measuring system is controlled by a computer and the measurement values are documented for future usage during insulation condition control. InTech was primarily developed and tested in the laboratory using a pilot generator, and afterwards it was installed with good results on large power generators operating at power stations. The detection sensitivity of the measuring system for conventional rotating machines in service conditions is about 1nC.

In addition, a complete measuring system has also been installed since August 1998 on a PowerformerTM, a new type of high voltage generator based on a cable winding. Here the system has been used for monitoring eventual PD activity in the cable joints. One of the features utilised is remote access to the system using a telephone link or a data network.

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