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Application Guide (High Voltage)

Application Guide (High Voltage)


Application Guide (High Voltage)


1. Single Conductor Insulation

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Rectangular Copper Wire with layers of Conductofol® 2009 or 2159

Recommended number of layers:

  • Up to 2 kV: 1 x 1 (butt lapped) PETP 23 μ + 1 x 1 Conductofol® 0.09mm
  • Up to 4 kV:
    • 2 x 1 Conductofol® 0.09mm or
    • 2 x 30% Conductofol® 0.09mm or
    • 1 x 50% (over lapped) Conductofol® 0.09mm
  •  Above 6 kV:
    • 1 x 50% PETP 23 μ + 1 x 50% Conductofol® 0.09mm or
    • 2 x 33% Conductofol® 0.09mm or
    • 2 x 50% Conductofol® 0.09mm

Recommended Tape widths:

For cross section (Cu) <8mm² 8-16mm² >16mm²
Tape width 8mm 12mm 15mm

2. Main Wall Insulation

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 Porofol® 2076 Polyester-backed mica tape for operating voltages from 6.6 kV up to and including 11 kV (wall thickness max. 2.5mm) 

  • Alternatively Poroband® 4037 Glass-backed mica tape for operating voltages up to 15kV

The number of layers depends upon the operating voltage and the mechanical stress.

Recommendations are:

3.3 kV: 3 layers 50% lapped
6.6 kV: 5 layers 50% lapped
11 kV: 8 layers 50% lapped

It is also crucial that the correct number of layers of tape be added to achieve a tight fit in the slot, generally with a 0.3mm clearance.

The number of layers of tape in the overhang is normally reduced to 60% of that in the slot area.


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The taping process itself is crucial to achieving good results on the finished machine. Care should be taken that taping is done at the correct tension to achieve a tightly taped coil, but not at too high a tension, which could cause wrinkling. This could result in resin drainage and hence voids in the system (see below).


Corona protection in the slot area

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Wilec recommends conductive corona-protection tapes from operating voltages of 6 kV upwards:

Contafel® H0865 0.12mm 1 x 50% lapped

The tape should extend 10mm past the last protruding metal object ie, the compression finger.

Additional end-winding insulation

Wilec recommends voltage-grading tapes from operating voltages >6 kV:

As a voltage grading tape we recommend B-staged EGSB® 2709 0.24 x 25mm. The de-stressing tape should overlap the corona protection tape by approximately 5–10mm.

Recommended lengths of the EGSB® 2709 wrapping depending on the test voltage:


Stress grading tape EGSB® 2709 0.24 x 25mm
Operating Voltage Length of wrapping (beyond the corona protection tape)
6.6 kV 50mm 1 x 50% lapped
11 kV 80mm 1 x 50% lapped

To prevent drainage of VPI resin from the overhang we recommend Isoseal® MF 0611 0.10mm x 20mm or 25mm sealing tape, 1 x 50% lapped. This tape shrinks during the VPI curing cycle to provide a smooth, waterproof anti-tracking finish.

3. Strips and interlayers (if necessary)

As a filling material under the lower coil, between the lower and upper coil, and under the wedges, we recommend Poromat® 2248 (which swells during the preheating cycle) in sheets of 0.4mm, 0.75mm or 1.2mm.

4. Slot Wedges

Isoval® TM is a non-conductive rigid laminate and consists of glass cloth impregnated with the temperature resistant version of the ISOVAL® epoxy system. Laminates exhibit excellent thermal and chemical resistance as well as high mechanical strength at elevated temperatures. It is available in various thicknesses, and machined slot wedges are available on request.

Alternative: Magnoval® 2067 has good magnetic conductivity combined with high resistivity and high mechanical strength. It is available in various thicknesses, and machined slot wedges are available on request.

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5. Supporters for the overhang

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ISOVAL® TM sheets of different thicknesses, alternatively Poromat swellable laminate.

6. VPI Procedure using Epoxylite® 006-0841 Epoxy Resin

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The predrying-cycle is a crucial part of the VPI-process. This period is necessary to swell the Poromat® filling material, to bond the layers of EGSB® 2709 together and to remove any volatiles from the insulation.

The machine should be pre-heated up in an oven for approximately 8 – 16 hours with a circulating air-temperature of 100ºC. This temperature should not be exceeded, in order
to prevent shrinkage of the Isoseal® MF 0611 sealing tape. The machine should be removed when a temperature of 60ºC is reached.

Vacuum drying

After placing the machine into the impregnating vessel a vacuum of

Preparing the resin

The resin should be well evacuated before use. The temperature for impregnation depends on the insulation thickness: and the viscosity of the resin. An impregnation viscosity of 3–5 poise is recommended for 11kV windings.

Impregnation Process

After the resin is flooded into the vessel, a pressure of 5–6 bars should be applied.


The heat-up period during curing should be as short as possible to prevent excess resin drainage. The fact that the machine temperature will lag significantly behind the oven temperature must be taken into account when determining cure cycles (see below). For most impregnating resins we recommend a curing time of approximately 16 hours at 160ºC.

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