Herve Perrin, Tapco, UAE
Under heavy rain in early 2006, a GE 9A3 generator operating unroofed and outdoors at a water and power plant in the United Arab Emirates tripped during operation.
The differential protection in the generator, which was part of the 308.3 MW/22.5 MIGD Initial B Extension facility at the Taweelah Complex near Abu Dhabi city, started the unit’s emergency shutdown cycle, with turning gear engaging and the associated GE Frame 9E DLN1 gas turbine and combined cycle plant tripping too.
After electrical isolation of the generator, an inspection through its manholes showed heavy soot in the compartment, indication of a flashover. A first insulation test revealed that one phase appeared healthy but that two phases gave low values.
Dry cleaning and another insulation test after 6 hours showed no improvement, so the decision was taken to disassemble the end shields for visual inspection. This inspection revealed that an electrical flashover had occurred at the excitation side of the generator end winding.
The damage extended to terminals of stator bars at the two phases where the flashover had occured and had melted copper. External generator specialists were invited immediately to visit the site to advise on the extent of repair required and what procedures this would involve.
A minor inspection of this generator in 2003 had shown some corona tracks on bar surfaces, supports and one winding cap at the turbine end of the end winding. Corona tracks are caused by the effect of humidity on the winding surface when it is subjected to voltage stress. Humidity can reach the generator internals through the connection joints of the external covers.
While these seemed to have been properly sealed off with silicone, they appeared to have been possible entry points for the water. Other possible points included the cooler ends located below ground level. However, this location is under pressure when the generator is in operation and water ingress appears less likely in these conditions. At that time, the inspection was followed by cleaning and repainting with insulating varnish.
In 2006, it was a flashover that started between the upper bar of slot 48 (the line end bar of the U phase) and the lower bar of slot 19 (the line end bar of the V phase) that caused the plant to shut down.
An inspection revealed that damage extended to stator bar terminals
The fault also affected the lower bar of slot 20 and the upper bar of slot 49. The melting out area of all of these bars were found at the bottom and top sides of the bars coming out from the insulating cap and partly extending to the outer support ring.
Inspection of the lower part of the generator showed that all of the winding bends had damaged insulation. Heavy soot deposits were found over the winding and on the rotor. Data from the digital generator protection showed the fault is from phase to phase. Although there were no obvious water traces, old traces were present.
The extent of the damage to the copper coils was such that a localized repair was not possible. Specialists from VA Tech, General Electric and Cegelec confirmed that at least a partial rewind would be required. Work began to remove the generator rotor but some special tools were not immediately available, requiring special orders and local manufacture.
Elin/VA Tech in Austria made the generator. Engineers from VA Tech and GE visited the Initial B Extension plant soon after the incident to assess the damage. All assessments require the removal of the rotor, and this one showed that a partial repair seemed possible and that a quick repair would be possible if the damage were limited to the visible portion of the rotor, any other damage requiring a longer overhaul. GE’s workshop proposed that the inspection of the rotor could take place at the company’s Saudi workshop. This would include removal of the end bells to allow inspection of the winding. The root cause of the damage was found to be humidity or liquid water entering the generator, compounded by defective silicone insulation in the short end caps. TAPCO focused sharply on the stator bars as their availability would determine the duration of outage of the plant. The plant prepared itself for being unavailable for several months.
Plant personnel began the removal of the rotor while discussions were under way with specialist companies bidding to carry out the repair. TAPCO eventually chose to have the rotor partially rewinded. This work included the removal of the rotor, inspection and cleaning of the stator, removal of defective bars, the electrical testing of the remaining bars, repair of the damaged bar and replacement of those bars that could not be repaired.
In the absence of an overhead crane, a 250 tonne mobile crane removed the rotor. The next step was a thorough cryogenic cleaning of the rotor using dry ice. In the mean time, the plant had ordered six new bars. Four were being air freighted from the manufacturer, who had two upper and two lower bars in stock. The first high voltage test, at 32 kV DC, showed that the repair solution was to replace only the most damaged bars following the existing design of the generator. The end caps were removed to de-braze the copper connection.
Filling the end caps was a flexible silicone, within the mass of which many porous areas were visible. The most damaged bars (lower 19 and 20, upper 48) were removed for replacement. This required the removal of 38 bars from the stator. The other removed bars were refurbished locally. An additional 40 kV DC, high-voltage test that followed IEC guidelines failed two additional bars. The removal of the bottom bars required the removal of all the remaining upper bars in the stator.
On inspection, an additional suspect bottom bar was also removed. This left the stator with all upper bars and five lower bars removed.
The rotor remained on site and received extensive cleaning and inspection to ensure that no carbon or copper deposits infiltrated the winding area. No defect was found on the rotor.
The local workshop did not have the capability to repair the most damaged bars. These were sent to the manufacturer for assessment and possible repairs. Out of the seven sent, only three were repairable, leaving a shortfall of one lower bar.
The manufacturer repaired the bars and insulated them (Vacuum Pressure Impregnation) at its premises.
The local workshop cleaned all bars individually and retaped them, painted them with resin and tested them. The manufacturer accelerated its fabrication programme for the spare bars ordered, which were installed as soon as they arrived on site.
As all the upper bars were removed, the decision was taken to include the latest modifications to the winding supports. There are now additional supports inserted, an arrangement that will provide stronger resistance to resonance.
The modification also included changes to the end winding caps, which now have a longer coverage along the bar and are filled with a hard resin. All areas were cleaned and varnished. The rotor was reinserted in the machine and covers reinstalled. At the cover joints, the recommended TIL was implemented and additional sealing was installed around the generator.
The focus of improvements has been on prevention of further incidents, especially as this generator and the other at the Initial B Extension plant operate outside. But other generators at the station were also improved. To prevent water ingress into the generator, sealing was improved as per GE’s Technical Information Letter and concrete at the cooler base was modified. A roof was also installed over the generators.
A generator operating unroofed and outdoors tripped after a heavy bout of rain
The rebuilt stator was high-voltage tested during the rebuild and after completion of the work according to the requirements of new factory built units. Its end caps received modified filling material to give them improved flash resistance compared with the original installation.
The improved end winding supports reduce problems that resonance can cause. All the at the station now benefit from have partial discharge monitoring. The frame sealing is better than the original installation. The generator is now expected to continue running for its initial expected lifetime.
This article is based on a paper entitled “Generator Failure at Al Taweelah Power Plant”, which was presented at POWER-GEN Europe 2007.
Taweelah Complex is 80 km east north east of Abu Dhabi city in the United Arab Emirates. The Project is the fifth and the largest independent water and power project developed in Abu Dhabi under the government of Abu Dhabi’s programme for the privatization of the water and power sector.
Three generation entities make up the plant: A1, A2 and B. Taweelah Asia Power Company (TAPCO) owns the B plant, which consists of the Initial B plant, the Initial B Extension plant and the New B Extension plant.
The generator failure occurred on gas turbine generator unit #81in the Initial B Extension plant, which was completed in 2000 and has an installed net power generation capacity of 308.3 MW and a net desalinated water production capacity of 22.5 MIGD.
It comprises two gas turbines that operate outdoors without external covers on the generators, two heat recovery steam generators, a single back pressure steam turbine and three MSF desalination units.
Initial B was commissioned between 1995 and 1997 and has an installed net power capacity of 619.9 MW and a net desalinated water production capacity of 69.1 MIGD, provided by six steam generators, six steam turbines and six multistage flash desalination units.
New B Extension is being built. It will provide a total power capacity of 2000 MW and 160 MIGD when combined with existing B Plant.
It will comprise three gas turbines, three heat recovery steam generators, a single back pressure turbine and four MSF desalination units.