Resin Rich v VPI

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A question Preformed Windings are often asked is which specification of coils is more suitable for high voltage applications, VPI or Resin Rich. We hope to outline some of the advantages of each below, although it should be noted that we manufacture both VPI and Resin Rich coils for our customers:

Resin Rich
  • Highly uniform impregnation of the resin in the slot portion reducing the risk of partial discharge occurring
  • Rigid resins are used in slot cell section of the coil and different resins with resilience to coil movement are used in the endwinding i.e. resin containing flexibilisers
  • Capability to control partial discharge in the slot, and remedial action in service is practical during insulation system life
  • Partial repairs are practical i.e. coils which have failed can be replaced
  • Coils can be installed on site as there is no requirement for further processing (i.e. the VPI process)
  • Endwinding bracing systems can be improved & made more robust to meet operational requirements
  • The coils are more expensive (typically offset by the additional cost of the VPI process)
  • Endwinding can contain voids, but since the endwinding surface becomes charged to coil voltage, breakdown of internal voids is not a problem other than at the first bend under the stress grading tape
  • Endwinding insulation and blocking is not a homogeneous system therefore more dependent on competence of winders
  • Resin rich coils in the endwinding can be more prone to chemical attack than VPI windings (Our Resin Rich coils incorporate a sealant tape mitigating this risk)
  • Reduced manufacturing cost (often offset by the cost of the VPI process)
  • Fast to install due to high level of flexibility
  • No scarf joint at end of coil slot straight meaning the straight length can be reduced
  • Reduced risk of voids in the end winding
  • Homogeneous insulation & bracing
  • Gives good insulation resistance
  • Inherently Resistant to moisture & chemicals
  • Cost of buying and maintaining the VPI equipment and materials such as tank and resin
  • Only one resin type can be used in the VPI process since the complete stator and winding is impregnated in one process. To maintain stability of coil in slot the resin needs to be a rigid thermo setting resin
  • Multiple processing required for good surface finish
  • After 15 -20 years service the rigid resin can cause cracking in the endwinding due to the frequency related movement
  • Maintaining the coil parallel in the slot portion can be difficult (increasing the risk of partial discharge)
  • Due to differential expansion the coil to coil bracing in the endwinding can result in loosening of the bracing between coils which will require regular rectification during the life of the machine
  • There are a range of resins used in the VPI process The lower viscosity resins used by manufactures are difficult to maintain and need a high resin turnover
  • Retaining resin in the stator winding during curing generally requires stator rotation during the curing process, which is easier for manufacturers who wind a stator core pack before inserting the cured stator core pack into the stator case
  • Since the complete stator needs to be processes the VPI vacuum/pressure vessel needs to be large enough to accommodate the stator case
  • Stators which have been VPI impregnated are very difficult to repair.
  • Complete rewinds are more expensive due to difficulty of removing the old winding This becomes a major issue for machines which can only be rewound on site

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