Effects of Water Absorption on Epoxy-Mica Based Stator Winding Insulation Systems
How does varying types of water exposure affect stator winding insulation properties and ultimately does it reduce the expected service life of the winding?
Need and Benefit
•In general the power generation industry believes epoxy-mica based insulation systems are impervious to water
absorption. Data obtained in previous research, IEEE and ASTM papers and standards, along with extensive
experience proves epoxy-mica insulation systems used in all large power generators are, in fact, not water-proof.
While this little known fact exists there is a specific lack of knowledge about the effects of exposure to water based on
the water entry method and/or the exposure type.
•There are many different ways the insulation system may be exposed to water during a life-cycle of a stator winding.
This research aims to examine 1.) The effects of water absorption on critical epoxy characteristics and resulting loss
of stator winding life, and 2.) The amount of water absorption based on the type of water exposure or entry method.
For example during winding construction water is often used to cool brazed joints when connecting the coils or bars
together. Often, due to inexperience or negligence, water is exposed to the ends of the windings where the copper
conductors exit the insulation layers. The amount of water exposure varies but is often not avoidable if water is
chosen as the conductor cooling method. In this exposure case water is readily absorbed into the insulation system in
this type of exposure which is vastly different than exposing the surface of the winding to water (e.g. air cooler leak
during machine operation).
•The type of water ingress/absorption method and surfaces involved matter as these two types of exposure will have
different impedance to moisture absorption. Furthermore, once water is absorbed it causes irreversible damage to the
epoxy. It is proposed to quantify the changes in critical epoxy characteristics when water is absorbed and attempt to
quantify the reduction of the stator winding life, if any.
•All of Reclamations power plants will benefit from this research in regard to the following:
o This examines the general effects of water exposure on the stator winding insulation for potential use of water-
mist based fire suppression systems.
o Knowledge gained from experiments can directly affect rewind specifications. This applies to any rewind where
the contractor may use water as a method to cool the copper conductors during brazing.
--> There have been many cases of water exposure during winding construction which may have already reduced the
expected life of stator windings. Each rewind performed without guidance may be reducing winding life if water is
used during construction.
--> Fire suppression systems typically use CO2 systems which don't have any effect on winding life. Due to concerns
of personnel asphyxiation by CO2 the industry is looking to water-mist systems as one possible solution. If this
solution is chosen then rotating machine asset owners need to be informed about the effects, if any, on the life of the
stator winding and how to restore the unit to a serviceable condition if the system is operated. Additionally there are
usually several fire suppression system ‘mis-fires' per year which would unnecessarily expose the windings to water.
Therefore it is important to have a working plan to recover from a mis-fire in order to return the machine to service
quickly without further damage.
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