During repairs to a generator at its 672-MW Mactaquac project on the Saint John River, New Brunswick Power (NB Power) in New Brunswick, Canada, discovered excessive stator core and end-winding contamination. This contamination was a combination of thrust bearing oil and dust from both the brake and the slip ring brush. To provide a work surface that was free of oil and other contamination, NB Power chose a citrus-based solvent that acted as a degreaser and helped restore the dielectric properties of the electrical equipment. Used in combination with dry ice blasting to reach the end-windings and air vents, this solvent removed all traces of contaminant, with only minimal stripping of the epoxy paint on the end-windings and stator iron.
Why the generator windings needed cleaning
During planned maintenance outages in 1999 and 2000, personnel measured excessive voids between the stator core iron and coil sides on the 110-MW Unit 2 generator at Mactaquac, as well as found four loose and/or missing wedges. This generator had been placed in service in 1967, but had been completely rewound in 1997. These voids were an indication of poor or inadequate side packing pressure as a result of the rewind. In July 2000, NB Power contacted the original equipment manufacturer (OEM) with these findings and repairs were begun under warranty.
Initially, NB Power thought removing two adjacent rotor poles would allow sufficient access to repair and/or replaced the wedges in the four locations. This would allow repairs to be completed within a week, and a longer outage of five to seven weeks, plus the time to do repairs, could be planned for 2001, when the stator could be jacked. However, after removing the two poles, the OEM and plant personnel determined that clearances for repair work were inadequate. At that time, NB Power decided to proceed with jacking of the stator to complete all necessary inspection, testing, and repair work.
Personnel dismantled the generator, jacked up the stator frame, and installed a work platform. Before further work could proceed on the repair, personnel needed to remove excessive stator core and end-winding contamination. Plant personnel knew the contamination existed, but the full extent was not clear until the stator was completely exposed. The contamination was a combination of thrust bearing oil and dust from both the brake and the slip ring brush. The oil contamination likely was caused by a generator over-speed during start up in 1999. Because of the presence of this oil, dust from normal brush and brake operation remained on the stator.
The OEM indicated that the only feasible method to repair the void between the coil and stator iron involved injecting almost 80 percent of the slots with CRTV (carbon silicon). The injection equipment for this repair required access through the stator air vents, and the CRTV could only be applied to a work surface that was free of oil and other contamination. To minimize the length of this unplanned outage, NB Power needed a fast and effective method of cleaning the stator.
Finding the best ways to clean the stator
Traditionally, NB Power personnel used a solvent called SS80 to clean electrical equipment and generator end-windings, cores and core faces, and air vents at its hydro plants. In addition to containing d-limonene, a citrus extract, this solvent contained trichloroethane 111. In 1996, the Montreal Protocol banned use of trichloroethane 111 after determining it was a compound responsible for ozone depletion. Since then, manufacture and use of this compound has been phased out throughout most of the world.
During the late 1990s, NB Power performed multiple trials of alternative solvents containing d-limonene at its hydro facilities. As a result of these trials, NB Power selected Biogenic Regent from Rochester Midland Corporation in Rochester, N.Y., as a suitable replacement for the SS80. Biogenic Regent was adequate as a degreaser and helped restore the dielectric properties of the electrical equipment. The main deficiency of this product when compared with SS80 was a longer evaporation time.
Using cloths saturated with Biogenic Regent proved effective in cleaning the stator core face of Unit 2’s generator at Mactaquac. However, this method could not be used to clean the stator end-windings and air vents because of the difficult access. This method would have required more manpower and time, as much as several weeks in the air vents and area of the core clamping fingers. As a result, NB Power began looking for other options to clean the end-windings and air vents.
In the past, several companies had proposed dry ice (CO=) blasting for stator cleaning, but generator experts opposed this method because of concerns that it could cause damage to the winding insulation. This damage included stripping of the grading, semi-conducting, and armor tape, as well as the interface between these tape layers.
Another option considered was corn blasting. Experience from previous cleaning trials at NB Power facilities using corn blasting had given mixed results and required extensive preparation and cleanup.
After weighing the two options, NB Power contacted two local motor repair shops to provide equipment to perform dry ice blasting. Both shops had only recently received this equipment and had not used this method to clean a large generator stator core. Because of the work involved in cleaning the windings, NB Power contracted the work to the Siemens Westinghouse motor repair shop. To more closely address concerns about possible damage to the generator winding, the blasting at Mactaquac was performed under the author’s supervision.
Initial testing of dry ice blasting at various flow rates did not provide adequate results. This method simply “pushed” the oil and brake and brush dust from one location to another. In addition, because of the amount of dry ice consumed, the author determined that the supply would run out with only about a quarter of the work completed. The only source of dry ice was a six-hour drive away.
Another concern with this method was the fact that the nozzle would be held in one area. This dramatically increased the risk of damage to the paint and insulation system of the winding. The paint could be stripped within two to three seconds.
Combining dry ice blasting and the citrus-based solvent
To avoid the expense and time involved with cleaning the stator by hand, the author decided to try using Biogenic Regent in combination with dry ice blasting. One major concern of this experiment was the possibility that the solvent could transport the conductive oil and dust mixture into the core slots. To minimize this problem, NB Power applied Biogenic Regent using a hand pump spray bottle just ahead of the dry ice nozzle. This application method helped ensure the solvent did not have time to run down the coil before it was removed.
Applied first to the core and end-windings, this method yielded immediate results. All traces of contamination were removed. After further trial and error, personnel determined that only the top end-winding needed to be sprayed with the solvent immediately ahead of the dry ice nozzle. All other areas — including the core face, circuit rings, supports, and bottom end-winding — could be lightly presoaked in advance of the blasting.
The combination of dry ice blasting and use of the citrus-based solvent proved extremely effective in cleaning the air vents of the stators. In fact, tarps draped behind the back core of the stator were saturated with a mixture of oil and dust after the dry ice evaporated. The dry ice blasting equipment used rice-sized pellets, which could easily be directed into the air vents.
To ensure the dry ice particles did not damage the winding insulation system, the equipment operator constantly moved the nozzle. However, even with this additional attention, about 0.5 percent of the epoxy paint on the end-windings and stator iron was stripped. This required minor repair after the re-wedging was completed.
The core cleaning was accomplished in two continuous 12-hour shifts, with one man operating the dry ice blasting equipment and one man pre-soaking the winding and core and refilling the dry ice. This cleaning included two complete passes, as well as additional time on top of the stator where the circuit rings were located and underneath the stator from the back iron side.
The amount of dry ice consumed decreased to the point that dry ice was left over to clean the rotor slip rings, hub, and some of the rotor poles.
Once cleaning was completed, the generator repairs were successfully accomplished and the unit was placed back in service in September 2000.
Similar results were obtained during rotor pole repair work on the two 10-MW generators at the 20-MW Tobique project on the Tobique River in 2003 and 2004. This work required removal of the rotor on both generators. Inspection revealed the accumulation of more than a decade of bearing oil, brake dust, exciter brush dust and dirt, and winged insects on the stator and rotor. The accumulation had blocked about 20 percent of the stator air vents and most of the rotor air vents.
Based on the success of the cleaning at Mactaquac, a similar practice was proposed for the Tobique generators. First, personnel removed all the air vent blockages using dry ice blasting. As at Mactaquac, this method pushed the oil and other contaminants around but did not remove them from the windings. Personnel used Biogenic Regent as a presoak and then cleaned the generators using dry ice blasting.
Citrus-based (d-limonene) solvents have been used to clean electrical equipment, rotating excitation equipment, motors, and windings since the mid-1990s at all of NB Power’s hydro stations. These products are easier to handle, use, and dispose of than previously used solvents. When combined with dry ice blasting, results have proven to be more effective and efficient than all other methods tried.
— By K. Fon Hiew, P.Eng., Electrical Engineer, NB Power, Mactaquac Generating Station, 451 Route 105, Keswick Ridge, New Brunswick E6L 1B2 Canada; (1) 506-462-3827; E-mail: fhiew@ nbpower.com.