Understanding the Differences

In certain machine applications, a winding of individually VPI'd coils or bars has no parallel in performance, reliability and long-term value.

Not all stator coil systems can meet the stringent performance specifications needed for the long-term reliability of generators operating in today's demanding high-voltage duty cycles. Comparing the different methods for injecting resin into a coil system is akin to comparing apples to oranges and the long-term life of the winding is the defining factor.

GVPI or Globally Vacuum Pressure Impregnated stator windings are distinct from NEC’s VPI system in that the entire generator stator – windings and all – are VPI’d. This process has a size limitation since the entire stator must be hung from a crane and inserted into a large VPI chamber full of resin. Therefore, across the industrial and utility sectors, many GVPI machines are not the largest and highest rated. However, more recently, the GVPI process has become quite commonly used for large air-cooled generators, with ratings as high as 400 MVA and up to 21 kV.

The GVPI process has the major drawback that if a coil fails the final acceptance Hipot or final commissioning test, removal and replacement of a single coil or bar can be extremely difficult.

Resin rich insulation systems are also distinct from NEC’s individual coil/bar VPI system.  The resin rich systems fall into two distinct categories.  The first is the resin rich system manufactured with expensive autoclave equipment, similar to NEC’s VPI chambers.  The second is the resin rich system manufactured with heated presses.. 

The first system, notably used by GE for many years, can provide coils and bars that meet standard industry tests.  This system incorporates an autoclave chamber, filled with asphalt, used to conform the coil to its final shape.  This type of manufacturing also requires a significant investment in expensive equipment.  The resin rich system, when functioning correctly, has all the resin that is needed already loaded into the tape.  Instead, the autoclave is used to conform the asphalt to the coil or bar outer surface. No resin is inserted under pressure as is with the NEC VPI system.

Additionally, adding to the buyer's confusion, there are also resin rich systems that do not use the expensive autoclave equipment. These type of resin rich coils have limited application for high-voltage machines and the variation of quality among suppliers can be huge. Companies like NEC will tend to invest in the types of sophisticated equipment, engineering design and quality management systems needed to consistently produce high-quality results. Conversely, many industrial shops producing resin rich type windings will have very little invested in the skills, processes and equipment needed to produce a highly reliable winding suitable the higher voltage applications in industrial sectors or for the lower voltage (under 11kV and 30 MVA) applications in the utility sector.

 

Global VPI Stators

As a class, large GVPI air-cooled machines have not had a good reliability record.  Many suffer from high levels of partial discharge, requiring resin injection to dampen the effects of PD deterioration within the first 4 or 5 years of operation.

One major OEM has exited this business, likely
due to the poor reliability record it had with this process.

failed GVPI stator coilIf a coil in a GVPI machine's winding fails, owners face options such as junking the machine or replacing the entire winding. The photo at the right shows a non-NE C-manufactured GVPI coil cross section. Voids in between the strands and between the strands and the groundwall insulation are visible, as is the irregular positioning of the strands themselves.

 

Resin Rich windings vary widely in quality and price

NEC has been active in the high-voltage coil and bar manufacturing market for for the utility industry many decades. Many industrial market coil manufacturers come into high-voltage market with
little invested, a short track record, and no significant losses of reputation or capital when their coils fail after only a short time.

The usual method of producing an inexpensive resin rich coil is to utilize a stock B-staged resin rich tape, apply it by hand to the coil, and hot press only the cell portion of the coil.  When the cells are heated it is presumed that the resin will flow sufficiently to properly bond and cure the resin rich tapes. Unfortunately, much of the equipment used in this type of manufacturing operation is initially purchased for low-voltage (less than 6,900 volts) coil manufacturing and cannot be retrofitted sufficiently for the demands of high-voltage coil/bar manufacturing. Low-voltage manufacturing processes may also lack the extra steps and procedures necessary to achieve a consistent level of quality expected by the high-voltage machine owner. Typical problems include:
  • Voids and inclusions don’t show up in testing because with this manufacturing process, only the cell portion is energized.
  • Coils with voids are put into your machine and failures can occur out of warranty
  • Moisture and air pockets trapped in tape - tapes not dried and degassed
  • Lack of fill and consolidation around internal Roebel and twist transpositions
  • No industry-wide evidence of long-term reliability – no known track record.
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