Transformer Applications

Generally speaking, transformers fall into two categories, dry type transformers, and liquid or oil-filled transformers. Even though the primary components are similar between dry and oil-filled transformers (core, coil, leads, etc.) the insulation used can be quite different. Dry type transformers utilize safety certified CSA and UL recognized high-temperature insulation systems, while oil-filled transformers use oil and high dielectric strength wood based insulation materials.

The Gund Company manufactures a wide range of insulation components for the various types of transformers. Whether a large, power, oil-filled transformer, a dry type distribution transformer or low-voltage electronics transformer, our application engineering expertise can help our customers understand their options for insulation material selection and component design.

Select Your Transformer Type
Power Transformer Distribution Transformer Dry Type Transformer

Oil-Filled Transformers

Oil-filled transformers use dielectric oil to insulate and cool the windings of the transformers. With the benefit of the dielectric oil to cool the transformer winding, relatively low-temperature insulation materials can be used in these designs. Typical insulation materials are cellulose based products such as pressboard and laminated wood with relative temperature indices in the range of 80 °C to 105 °C. Oil-filled transformers are most commonly classified as power or distribution, depending on their size and application. The Gund Company supplies a range of insulating materials and components for each type of transformer.

Power Transformers

Power transformers are large, oil-filled transformers and are commonly divided by their rated power. The small power transformer classification typically starts around 5 MVA with a maximum service voltage of 145 kV. Medium power transformers range from about 30 MVA to about 250 MVA and commonly work as network and generator step-up transformers. The large power transformer classification includes transformers with ratings greater than 250 MVA and voltages greater than 345 kV. The Gund Company supplies a wide variety of insulation components for the full range of power transformer designs and ratings.

  • Core Insulation Kits
  • Step Blocks
  • Core Blocks
  • Oil Changing Plates
  • Core Filler Rods
  • Lead & Cleat Structure
  • Support & Yoke Beams Insulation
  • Fasteners (Rods & Nuts)
  • Winding Forms
  • Dovetail Strips
  • Key Spacers
  • Clamping Rings
  • Support Blocks
  • Washer Assembly
  • Tap Changer Boards
  • Tap Changer, Filament-Wound Housings
  • Composite Core Cooling Ducts & Spacers
  • Lead Clamps
  • Lead Conduit Tubes
  • Tap Changer Enclosure Gaskets
  • Bushing Gaskets & O-Rings
  • Radiator Gaskets
  • Hand-hole and Man-hole Gaskets
Common Materials Used in Power Transformers
Ranprex® Laminated Wood Filament Wound Tubes Nomex®
G-10, EPGC 201 G-11, EPGC 203 X, XX, XXX, PFCP 201, 202, 204
Low Power Factor – HP1 IEC Grade EPGM 203 U205, UPGM 205
Kraft Paper Pressboard Paper Phenolics
Viton®, Fluorocarbon Gaskets & O-Rings Neoprene Gaskets Fluorosilicone
Buna-N, Nitrile Gaskets & O-Rings Cork Rubber Gaskets
Click here to view additional material data sheets.

The Gund Company also offers a wide variety of elastomeric materials available for all your O-ring and gasket needs.


Ranprex® laminated wood from Rancan is a high density wooden laminate. Specially selected veneers of red beech wood are used as well as thermosetting resins, pressed under high temperature and pressure for its production. Ranprex® is available in several standard sheet sizes, as well as fully fabricated components, specific to your drawings/application.

Ranprex® features the following properties:

  • High Mechanical performance with respect to density
  • High Oil Absorption
  • Excellent Electrical Insulation
  • Negligible Shrinkage
  • * Visit our Italian Supply Partner Rancan to learn more about Ranprex® Laminated Wood or download the product brochure (left)

    Distribution Transformers

    Oil-filled distribution transformers are most commonly pole mounted, or pad mounted. Pole mounted distribution transformers are so named because they are mounted to utility poles in most older urban, or rural areas. In suburban areas, built within the last few decades, most distribution lines are buried underground, so transformers are mounted on concrete pads in residential, commercial, or light industrial applications. The Gund Company manufactures and fabricates a variety of insulation material solutions for distribution transformers.

    • Layer Insulation
    • Layer Insulation Tape
    • Core Step Blocks
    • Coil Supports
    • Hardboard Coil Packing & Shims
    • Phase Barriers
    • Ground Barriers
    • Sight Window Barriers
    • Lead Supports, Spacers, & Clamps
    • Fuse Boards
    • Fuse Tubes
    • Tap Changer Boards
    • Enclosure Gaskets
    • Bushing Gaskets
    • O-Rings
    Common Materials Used in Distribution Transformers
    GPO-1, UPGM 201 HP1 Low Power Factor Filament Wound Tubes
    Angles and Channels GPO-3, UPGM 203 Threaded Rods & Nuts
    G-10, EPGC 201 Standoff Insulators G-11, EPGC 203
    Ranprex® Laminated Wood Laminated Pressboard S2S Hardboard
    Kraft Paper / Diamond Coated Kraft Paper Duct Paper – Stick Paper Crepe Paper
    Click here to view additional material data sheets.

    The Gund Company also offers a wide variety of elastomeric materials available for all your O-ring and gasket needs.

    Dry Type Transformers

    While oil-filled transformers use dielectric oil to insulate and cool the windings of the transformer, dry type transformers are most commonly used in applications where the use of dielectric oil in the transformer design is not permitted. Without the cooling presence of dielectric oil, dry type transformer designs must use higher temperature insulation materials. Most dry type transformers made in North America are certified to UL and CSA recognized insulation systems.

    The Gund Company is a vertically integrated manufacturer of these high-temperature insulation materials, including materials rated at 155 °C, 180 °C, and 220 °C that has been specifically tested per UL 1446 for chemical compatibility with a long list of UL recognized insulation systems. The Gund Company offers a variety of insulation components for the following types of dry type transformers.

    Layer Wound, Dry-Type Transformers

    Also known as barrel wound transformers, layer-wound transformers are most commonly used in low-voltage (<600v Class) applications. Layer-wound units are so called because the coil is made by wrapping a layer of conductor around an insulated winding form and then wrapping a layer of insulation over the conductor. The coil is built by wrapping a layer of conductor and then a layer of insulation in successive layers per the design. This design typically relies on high-temperature spacer sticks called "dogbones" that are used to create the air cooling passages. Between the primary and secondary coils, it is common to have a layer of flexible insulation such as Nomex® Aramid Paper or even a flexible glass polyester laminate such as Grade N200F or Grade FHT.

    • Standoff Insulators
    • Bus Support Angles & Channels
    • Winding Forms & Filament Wound Core Tubes
    • Layer “Turn” Insulation
    • Layer Insulation Tape
    • Axial Spacer Sticks “Dogbones”
    • Yoke Insulation
    • Coil Support Blocks
    • Wedges, Shims, & Filler
    • Lead Supports, Spacers, & Clamps
    • Fuse Boards
    • Terminal Boards
    • Lead Insulation – Sleeving
    • Phase Barriers
    • Fasteners – Rods & Nuts
    • Enclosure Gaskets

    Disc-Wound, Dry Type Transformers

    Disc-wound transformer designs are typically rated for higher voltage applications than barrel or layer-wound transformers. Disc-wound transformers are so-called because the conductor is wrapped around the coil in such a way as to make the coil appear to be a series of stacked discs, separated by airflow gaps. These units use either radial spacers or winding combs to form the conductor support and winding path around the coil while providing the air gaps necessary for air flow through the coil. These radial spacers and winding combs are typically made with high-temperature (130 °C, 155 °C, 180 °C, or 220 °C) glass polyester insulation material such as NEMA GPO-3, NEMA GPO-1 (Grades: N155, N180, N220, SG-200, or HST-II).

    • Standoff Insulators
    • Bus Support Angles & Channels
    • Winding Forms & Filament Wound Core Tubes
    • Conductor Wrap
    • Winding Combs
    • Axial Radial Spacers
    • Spacer Sticks “Dogbones”
    • Yoke Insulation
    • Coil Support Blocks
    • Wedges, Shims, Filler
    • Lead Supports, Spacers, & Clamps
    • Fuse Boards
    • Terminal Boards
    • Lead Insulation – Sleeving
    • Phase Barriers
    • Fasteners – Rods & Nuts

    Cast Coil Transformers

    Cast coil transformers have coils that have been manufactured using a vacuum molding process that fully impregnates and insulates the conductors with a polyester or epoxy resin system in most designs. The cast coil design allows for higher short circuit strength and overload capacity. Due to significantly different coil design, cast coil units require different insulating components than other types of dry type transformers. Common cast coil insulation components supplied by The Gund Company include saturated DMD, NMN, and glass epoxy B-stage layer insulations, as well as “dogbone” spacer sticks.

    • Standoff Insulators
    • Bus Support Angles & Channels
    • Layer “Turn” Insulation
    • Layer Insulation Tape
    • Axial Spacer Sticks “Dogbones”
    • Coil Support Blocks
    • Lead Supports, Spacers, & Clamps
    • Lead Insulation – Sleeving
    • Fasteners – Rods & Nuts

    Electronic Transformers

    By using the term “electronic transformers”, we are intending to capture a wide range of low-voltage transformers, commonly used in electronic equipment applications. These units are typically rated at less than 600 volts. They are used in a wide range of application from current transformers to instrument transformers, to lighting ballasts. Other specialty magnetics fall into this category as well, such as shunts and chokes. The range of electronic transformer types and applications is nearly unlimited. Common insulation material applications in electronic transformers include:

    • Fabricated Winding Bobbins
    • Core Insulation
    • Layer Insulation
    • Layer Insulation Tape
    • Lead Insulation – Sleeving
    • Terminal Boards
    • Fuse Boards

    Reactors

    Reactors have either iron core or air core construction. They are used to help control quality by limiting load faults and surges, or high power lines. Reactors are typically used in substation construction, close to a load that may cause significant swings in power quality. Reactors are often used in conjunction with capacitor banks to help control power quality. All air core reactors have conducting windings that are wound much like a disc-wound, dry type transformer coil.

    Common Materials Used in Dry-Type Transformers
    GPO-3, UPGM 203 GPO-1, UPGM 201, APO1 G-11, EPGC 203
    Class F GPO-1, UPGM 201, N155 Class R GPO-1, UPGM 201, N220 UPGM 205, U205
    Class F, Class H, Class R Spacer Sticks “Dogbones” Nomex® Meta-Aramid Paper Threaded Rods & Nuts
    GPO-3 Angles & Channels Standoff Insulators Polyester PET Film
    NMN DMD Rag Mylar RM, RMR
    NEMA G-10, EPGC 201 Elastomeric Materials FHT, HTF, N200F, Flexible High-Temperature GPO
    Copaco® FR4, G-10 XX Convolute Wound Tubing Glass Epoxy Filament Wound Tubing
    3M® Paper: Thermavolt®, Tufquin®, Cequin® SG-200 & HST-II Polyimide Film – Kapton®
    Click here to view additional material data sheets.

    The Gund Company also offers a wide variety of elastomeric materials available for all your O-ring and gasket needs.

    The Gund Company uses a “Dry Type Transformer Checklist” to assist our customers with reviewing material and component specifications. The checklist is also helpful for setting up kitting programs as well as vendor managed inventory programs by transformer design. Contact one of our transformer application material specialists today to learn more.

    The Gund Company is a vertically integrated manufacturer and fabricator of engineered material solutions. Since 1951, we have listened to our customers and learned about the demanding operating environments of their industries. We are AS9100C Certified and ITAR Compliant. Our custom fabricated parts are manufactured according to ISO 9001:2008 certified quality systems.

    We understand the challenges of material selection and the demanding operating environment of your application. Our Application Engineering Team takes a consultative approach to understanding your requirements. By relying on our material specialists, our customers gain valuable insight into how to improve component designs to increase efficiency and functionality while reducing cost. In addition to helping with material selection, we challenge ourselves to optimize production for material yield or fabrication efficiency. As a lean enterprise, we are focused on continuous improvement and finding the most cost-effective, efficient solutions for our customers.

    Please contact us today if we can help answer material property questions or provide an application specific quote. Thank you for the opportunity to earn your business.