Between Sept. 25 and 27, 2024, Hurricane Helene passed through the Appalachian Mountains and dumped an unprecedented amount of rain on already saturated mountains and full rivers. The most impactful part of the storm was the destruction of municipal water systems in the region, which led to months without safe drinking water or working sewer systems for many residents. Three months later, residents are still without safe water, and estimates for full reconstruction of the water systems span into years.
In addition to the drinking water and sewer systems in the mountains, hydropower generation plants across the Southeast were impacted by this storm. Tennessee and North Carolina, the two states hardest hit by Helene, are both in the top ten for state hydroelectric power generation, with over 70 utility-scale hydroelectric plants between them—most of which are in the Appalachian Mountains (National Hydro Association).
According to the National Weather Service, the highest recorded rainfall total from Helene was 30.78 inches in the North Carolina mountains. The multiple feet of rain that fell and ran down the mountainsides compounded into rivers that ended up nearly three stories over their banks. The French Broad River in Asheville, for instance, peaked at 24.67 feet over its banks, an all-time record (National Weather Service). The water in the rivers combined with mud, trees, furniture, vehicles, etc. to create a devastating battering ram headed toward the reservoirs and the hydroelectric dams that create them.
While all the dams themselves miraculously held up, the power plants were full of silt and debris and, in many cases, several feet of standing water. While this created an arduous cleanup job for the mechanical components (turbines, gates and piping), the generators and associated electrical switching and transmission equipment presented a much more significant challenge.
Below is an outline of the most critical elements to repairing and evaluating the time needed to get these units up and running for local communities.
1. Testing the rotor poles before and after cleaning is an essential step to return any generator to working condition. This testing provides an indication as to any variations in resistance after cleaning.
2. Following testing, it is recommended to set up containment to trap and contain any residue and grime coming off the generator. This will also prevent adding any additional pollutants to the already disrupted flood waters.
3. Once all the necessary prep work has been completed, a flooded generator needs to be cleaned and dried as quickly as possible. Moisture and electricity are never a good combination, and gritty sediment can act as an abrasive that wears down the insulation between the turns of the winding as the generator spins.
When working with a repair partner with experience in hydro generator repair, cleaning and testing, power producers can expect a team to be deployed to the affected hydroelectric plant to commence with dry-ice blasting and hand-cleaning the generators. Technicians should clean a portion of the stator windings to make sure insulation is not being damaged by the cleaning.
Older insulation tends to flake, and if that issue arises, it is recommended to resort to solvent cleaning instead of ice blasting (an electron dielectric solvent that is Environmental Protection Agency [EPA] approved is recommended).
4. After close inspection of the windings for any visible damage, heating tents can be set up to fully dry out the units. Once the units are dry, a thorough electrical testing process should be performed prior to reinsulating and starting the units. Industry standard is to reinsulate the generator components with an ER41 red insulator to extend the insulating properties and life of the generator components after cleaning.
5. The transformers associated with the generators should also be evaluated, cleaned and tested with a trusted partner. While this process is labor intensive, quick reaction and attention can return damaged generators to operation, avoiding extended downtime and the prospect of full replacement and associated disaster insurance claims.
