High-voltage motors and generators are the most conservatively designed pieces of rotating equipment found in industry because all other processes depend on them. However, the voltage level at which they operate — from 7.7 to 13.8 kV — results in continuous stress to insulation systems.
Moments of power surges in motors and mal-synchronization in generators add additional stress, according to Walt Dryburg, Plant Manager with Rockwell Automation in Washington, Pa. Load surges cause mechanical bending and rocking of coils that further increase the tendency for the insulation to crack and abrade conductor slots.
Where there are weak points in conductor insulation—due to small gas voids, cracks, or pinholes in the varnish—a phenomenon known as partial discharge occurs between coils. Partial discharge is progressive and, over time, weakens the insulation walls and reduces the efficiency of the rotating unit. Eventually, a motor or generator that experiences partial discharge will short out and have to be rebuilt.
At a paper mill in North Carolina, a 10 MW generator had to be shut down for a rewind after several months of declining performance. The generator had entered service several years earlier, but dielectric tests indicated that the condition of the insulation on the conductors could not be improved by temporary measures (dip-and-bake treatment), so a complete rewind was needed. Because of the need for long service life and the generator’s exposure to a mildly corrosive environment, the mill decided to completely re-manufacture the generator.
How a high-voltage machine is rebuilt has a lot to do with how long its “second life” will be. State-of-the-art technology involves both new varnish materials and new application technology, known as coil sealing systems. The latest materials are epoxy-based resins that not only provide excellent thermal properties (Class F temperature rise) and extremely low dissipation factors (0.75%), but also are highly resistant to corrosives such as seawater or pulp plant environments—far more than the conventional polyester varnishes. These resins encapsulate the coils in such a way as to form a solid homogenous mass. With compression strength on the order of 40,000 psi and flexural yield strength of 60,000 psi, the resin greatly increases the mechanical rigidity of the coil.
“As important as the new resins are to the performance of a rebuilt generator or motor, how they are applied is equally important,” says Dryburg. “The vacuum-pressure impregnation (VPI) process is a technique of alternating the atmospheric pressure around a conductor to be rebuilt. On the vacuum phase, gases that are trapped between coils are drawn out, then the tank is flooded with the resin and pressurized and the fluid resin is driven in to replace all evacuated voids.”
The restoration process at the paper mill began by burning out (carburizing) the existing insulation and removing the old conductor from the rotor and stator. This enabled technicians to reverse engineer the dimensions for the new conductor. The slots were then cleaned and the new conductor installed. Supporting insulation strips and tape were fastened. Finally, leads were encapsulated with a vulcanized sleeve to prevent the service environment (high humidity and mild corrosives) from entering the unit.
When all the mechanical work was completed, the generator was lowered into Rockwell’s 14-ft diameter VPI tank and subjected to two vacuum-pressure cycles to ensure complete encapsulation of the conductor. After baking for eight hours for each cycle, the resin was crosslinked into a hard, solid mass.
After the reinsulating phase, the generator was given an IEEE water-immersion test to verify the thoroughness and quality of the insulation. Finally, the rotor was spun balanced at its operating speed of 3600 rpm.
The generator has performed flawlessly since the retrofit, with no decline in output, according to Dryburg. During regular inspections every six months, the generator has been monitored for electrical and mechanical problems, but none have been observed. The repaired unit is guaranteed for five years, but its life expectancy is as much as 30 years.