HOOD AS 1919109 - Caterpillar

If the insulation deteriorates how is the generator affected?
Usually insulation breakdown occurs in specific locations rather than generally deteriorating throughout the windings. This means current leakage is concentrated in just those locations where the insulation has deteriorated. Localized current leakage through the insulation generates heat and causes further breakdown in the insulation. Eventually, the insulation capacity is reduced so much that short circuiting and winding damage can occur.What causes the reduced insulation capacity?
Of all the factors that can affect insulation, none causes more problems than moisture. Moisture, in the form of water vapor or humidity, is present nearly everywhere. The cooling air drawn through the windings by the generator fans brings the humidity into contact with the insulation. If local conditions are very humid or if the generator is improperly stored, a small amount of the water vapor will be retained in minute voids or cracks in the insulating material. Water is generally a poor conductor of electricty, but will combine with other materials to provide a conductive pathway for electrons to leak through the insulation. The ability of moisture to conduct electricity past the insulation is increased if there happens to be a small amount of salt or similar conductive chemical disolved in the water vapor. Generators operated near seacoasts or unprotected onboard a ship are more likely to have current leakage as the insulation deteriorates. Also, generators operating under these conditions can experience a salt buildup on the windings.How does salt buildup occur?
As the cooling air flows through the generator, some of the water vapor containing dissolved salt evaporates on the windings and leaves a salt residue behind. The salt buildup pulls moisture from the air and retains it when the generator is not operating and the windings are cold. This puts more moisture in contact with the windings.Other types of airborne material can also become caked on the windings and cause moisture retention. These include grain dust, saw dust, limestone dust from rock crushers, dirt, and various chemicals. No matter what type of material is allowed to build up on the windings, the result is nearly always increased moisture in contact with the windings.Does moisture cause problems other than reduced resistance through the insulation to ground?
Yes. If a conductive pathway exists between two bundles of wire because of moisture, current will leak from the higher voltage bundle to the lower voltage bundle. This produces heat and causes further breakdown and charring of the insulation. If the problem goes undetected the windings can be severely damaged. Illustration 2 shows a "hole" that was blown in the windings because of a breakdown of the phase insulation.
Illustration 2. Hole blown in stator winding.What other factors can contribute to reduce insulation capacity besides moisture?
Potentially, any environment that contains abrasive or chemically active airborne material can have a bad effect on the insulation over a period of time. Epoxy is strong and chemically inert, but it can be gradually worn down by mechanical abrasion or nicking and chemical attack.The operating