Corona on HV rotating Machines
There are many factors caused by insulation that may lead to corona formation such as ageing, overvoltage, contamination of windings and more. According to CIGRE Study Committee SC11, EG11.02, "Hydro Generator Failures – Results of the Survey", (2003) a statistics of 1,199 hydro generators showed that 56% of hydro generators' breakdowns are due to insulation damages.
Generating unit outages due to failures can create appreciable loss of revenue and no manufacturer is safe from it. Corrective action must be taken to remedy the deterioration before it becomes critical or breakdown. This entails shutting down generating units, which is clearly undesirable in an increasingly competitive market for electricity sales. An early detection of the corona phenomenon will provide a simple and easy repair with high ROI.
When E field along the coating is higher than the dielectric breakdown of air, the local current will increase causing insulation material to burn. This will cause corona in slots and may lead to breakdown of the insulation inside the slot & near the slot exit.
There are many factors that can cause corona such as scratches, chipped paint, inadequate spacing, miss-positioned RTD and more. But there are 2 even more common factors than these mentioned; the first is slot discharge; when coils become loose winding current is dissipated to the ground so that the ground wall insulation system is compromised leading to winding failures and corona dusting. The second is winding contamination (such as moisture, oil mixed with dust/dirt) where a conductive path between two adjacent coils of different potential can form tracking (often from different phases) forming corona discharge & permanent deterioration (phase-to-phase or phase-to-ground) which may lead to failure of the ground-wall. Gladly, once contamination is removed corona activity will cease
A case study of stress-grading system in hydro generators presented in Iris Rotating Machine Conference on June 2007 in San Antonio showed signs of surface discharge on a winding after 4 years of service. As a result, inspection revealed white powder at the junctions and grading coatings in the stator winding. It is important to mention that such coatings are tricky to design and implement and that the white powder along the coating decompose the insulating material.
In order to locate the source of discharge and it's cause, various inspection modes were used i.g. capacitive coupler, ultrasonic, corona probe electromagnetic, naked-eye and UV camera.
Eventually, the method of UV camera was chosen for inspection for many reasons. First and foremost the use of a corona camera is very safe, there was no need to be near the bars and it was used under regular lighting conditions. Moreover, the UV camera was able to measure several bars simultaneously, testing 80 bars by batches. None of the other methods were able to capture corona as easy as with the UV camera; the capacitive coupler could not be used for simultaneous detection on several bars and it didn't detect external discharge at the junction. With ultrasonic it was difficult to get close enough to distinguish between internal and external discharges, with corona probe the detection could cover an area of only 1x5[cm] and the dial gauge supplied with the probe could not detect corona discharges and finally, with the black-out test (naked eye) it was required a 15-minute wait in complete darkness for the eye to become sensitive enough in each inspection.
The findings with the corona camera were explicit. Corona was recorded at the stress-grading junction at a minimum inception voltage of 5.2 kV. Such value is clearly unacceptable since bars on the phase side of the stator-winding are permanently subjected to higher voltages. Due to the UV corona inspection maintenance staff could then assign a priority to each site.
Since Corona is a progressive problem that needs to be eliminated imperatively, several corrective actions were taken; silicon carbide stress-grading tape was used instead of the paint used earlier since coatings controls the E-field in stators' end-windings and spots with visible UV were repaired (by sanding, cleaning or applying stress-grading paint).
To conclude, with a corona camera locating the fault was easy and fast, giving a reliable maintenance inspection and a trend analysis method, even during assembly, inspection with corona camera ensured that defects do not slip. Corona inspection for motors & generators is a safe inspection due to a non intrusive & non destructive instrument even while the rotating machine is operated. Furthermore, it is possible to perform corona inspection in parallel to any other standard inspection and it is the only equipment with which it is possible to pinpoint and locate the exact source of corona fault. Routine corona inspection allows extending the life of the motor or generator while preventing unscheduled cease operations and related operating expenses.