Page 10 - Basic PD Theory

Partial Discharge for Stator Windings

In older windings (before about 1970), the organic varnishes used were either asphalt or shellac. Both of these varnishes deform
or flow when under thermal stress, i.e., thermoplastic. These thermoplastic resins retain part of their distorted shapes when
cooled. In newer windings, thermoset resins of epoxy or polyester are used. Thermoset windings are less affected by thermal
stresses at normal winding operating temperatures, and usually do not deform until the glass transition temperature is exceeded.

1.3 Coil Manufacturing Process
Different manufacturers use different manufacturing processes, but the basic steps are about the same. Which process the
manufacturer chooses should be based on your specifications and their experience and capability.

The basic coil manufacturing steps are as follows:
Step 1: bundle the insulated strands together
Step 2: apply turn insulation (if used)
Step 3: form bundle into coils
Step 4: apply ground insulation tapes
Step 5: impregnate or press cure
Step 6: seal the winding

1.3.1 Impregnation Processes

1.3.1.1 Individual VPI
This individual Vacuum Pressure Impregnation (VPI) process is most commonly used on short core or small diameter machines
that are too large to be globally impregnated. Dry mica paper or mica-splitting tapes are applied to the coils. These “green coils”
are inserted in a mold (one mold per coil). The molds are then placed in batches into an impregnation tank. A vacuum is drawn
on the tank to remove the air trapped in between the layers of tape. Next, a low viscosity epoxy or polyester resin (based on the
manufacturer) is injected into the tank and the tank pressurized. The resin is maintained at proper temperature and pressure
levels to enable the resin to flow between the mica tape layers. Finally, heat and pressure are applied to the coils to cure the resin
and reduce the void size. This last step also fixes the dimensions of the slot portion of the stator coil. Coils can then be tested
for quality of consolidation or curing.
Next the coils are inserted and wedged into the slot, and for motors the complete winding is often dipped and baked to seal and
bond bracing components. If possible, the complete core/winding system is then heated to cure bonding resin in endwindings
and connections.

1.3.1.2 Global VPI
A commonly used impregnation process for small diameter cores is the global VPI process. As with the batch VPI process
described above, the coils are first insulated with dry mica paper or mica splitting tapes. Then the “green coils” are placed in the
slots, side-packed, and wedged. The endwindings are blocked and tied. Following this, the entire stator is placed in a VPI tank
and then impregnated and baked for curing. A second VPI process is often conducted to “seal” the winding and provide
additional mechanical stability.
Global VPI steps:
1. Stator heated to about 60 °C (140 ° F) and placed in VPI Tank
2. VPI Tank is Sealed and Vacuum Drawn (<1.0 mm Hg)
3. Impregnating Resin Introduced and Allowed to Soak in at Atmospheric Pressure – monitor while looking for the
capacitance readings to stabilize
4. VPI Tank then Pressurized to about 90 psig to further force Resin into Insulation System
The advantages of a global VPI stator are that the manufacturing costs can be reduced, the process can be accelerated, and the
final product is mechanically very sound. The mechanical soundness is the result of wedging and blocking the coils prior to the
impregnation process; thus, the impregnation will help to secure the coils in place and increase the mechanical stability of the
stator. The quality of this type of winding is mainly dependant on the impregnating resin penetrating to the conductor stack in
each coil to ensure an essentially void free, well bonded insulation system. There is no practical way to monitor or test the quality

www.irispower.com

7 | Pa g e