Products developed and commercialized through collaboration of CEATI International and Iris Power Engineering

Over 20 years of extensive research, funded by CEATI International, Iris Power Engineering (Ontario, Canada), and Ontario Power Generation, refined partial discharge technologies that can be used to detect most motor and generator stator winding problems during normal service have been developed.

This CEATI partial discharge test technology has won worldwide acceptance with all major utilities.

Partial discharges (PD) are small electrical sparks that occur within the high voltage electrical insulation in stator windings. The PD occurs whenever there are small air gaps or voids in or on the surface of the insulation. Normally, well-made stator windings in good condition have very little PD activity. As a stator winding deteriorates due to winding vibration, operation at high temperatures, or contamination from oil, moisture and other chemicals, the PD activity will increase.

Thus on-line PD monitoring detects the main root causes of stator winding failure. Based on the CEATI-developed technology, PD monitoring can now be performed during normal motor or generator operation, and generally gives 2 or more years of warning that there is a risk of failure providing a very powerful tool for utility predictive maintenance programs. Although PD testing has been done for decades, the key reason for the success of the CEATI technology is that it can explicitly separate stator winding PD from other similar types of electrical signals, called noise. Noise, which is usually relatively harmless, comes from power system corona, brush sparking on slip rings and shaft grounding arrangements, power tool operation, etc.

Unlike other PD measuring technologies that require considerable human expertise to separate PD from noise or that need expertise to continuously adjust filters and gates to separate PD and noise, the CEATI/Iris development allows objective measurements to be made by plant staff. On-line PD monitoring involves the permanent installation of sensors, which are periodically monitored via a specialized portable instrument or a permanently connected monitoring system.

A key aspect of the CEATI-funded work was the development of specific sensors and instrument technologies for different machine types. With this technology, the type of sensor, its mounting location, and the instrument type used for the measurement, are all dependent on the machine to be monitored and have been researched to provide maximum sensitivity and high reliability.

• For large turbine generators, specific sensors called stator slot couplers (SSC™) were developed and are installed inside the machine underneath the stator wedges. Utilizing a TGA-S™ instrument, PD signals originating from the machines winding can be detected by the SSC's and separated from noise signals. Stator slot couplers are now routinely installed by many utilities with several hundred large turbine generators outfitted with the technology.

• For smaller generators and motors, a second technology involving the installation of capacitive couplers on the output bus of the machine was researched and developed. Utilizing a TGA-B™ instrument, these couplers are able to discriminate between PD from within the machine, and noise pulses coming from outside the machine.

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