TL;DR:
- Oil duct spacers are precompressed pressboard blocks and sticks that hold open the cooling channels between transformer winding discs and layers.
- Spacer thickness sets the oil duct height — typically 3–8 mm radial and 6–12 mm axial — which directly controls oil velocity and winding hot-spot temperature.
- High-density precompressed pressboard (IEC 60641-3) is the standard material for oil-filled transformers; FRP/GPO-3 and epoxy glass laminates serve dry-type and high-mechanical-load designs.
- Poorly chosen spacer pitch can raise the hot-spot factor and cost years of insulation life per IEC 60076-2 thermal rules.
- SIDA supplies pressboard, laminated wood and epoxy-glass spacer materials as sheets, machined parts or complete CNC-cut spacer kits.
Transformer oil duct spacers are insulating blocks, sticks and strips — usually made from high-density precompressed pressboard — placed between winding discs and around winding cylinders to hold open the ducts through which transformer oil circulates and removes heat. This article explores what oil duct spacers are and why every liquid-filled transformer needs them, which materials are used and how they compare, and how spacer design affects oil flow, cooling performance and hot-spot temperature, closing with practical guidance on ordering spacer kits for your transformer design.
What Are Oil Duct Spacers and Why Do Transformers Need Them?
In a disc or helical winding, the copper conductors would stack into a solid, uncoolable mass without something to keep them apart. Oil duct spacers solve this: radial spacers (small dovetailed blocks) sit between adjacent winding discs to create horizontal cooling ducts, while axial sticks or runners glued to pressboard cylinders create vertical ducts between the winding and its neighboring insulation barrier. Together they form the labyrinth of channels through which mineral or ester oil flows, absorbing heat from the conductor surfaces.
Spacers do double duty as mechanical structure. During a through-fault, axial short-circuit forces try to compress the winding like a spring; the spacer columns transmit and resist these forces, so they must withstand permanent compressive stresses of 20–40 MPa without creeping. This is why spacers are punched or machined from precompressed, oil-impregnable board rather than ordinary paper — the material must match the cellulose insulation system thermally (Class A, 105 °C) while behaving structurally like an engineered laminate.
Related product: SIDA Pressboard PSP-3050 (precompressed transformerboard)
What Materials Are Used for Oil Duct Spacers?
High-density precompressed pressboard to IEC 60641-3 (types B.3.1A / PSP-3050) is the default spacer material for oil-immersed transformers. Made from 100% sulphate wood pulp densified to 1.15–1.30 g/cm³, it impregnates fully with oil, ages at the same rate as the winding’s kraft paper, and machines cleanly into dovetail profiles. For thicker load-bearing parts — clamping ring segments, thick radial spacers, jacking blocks — laminated pressboard (IEC 60763) and laminated densified wood provide higher compressive strength while remaining oil-compatible.
Outside the oil-filled world, resin-based laminates take over. GPO-3 glass polyester and FR4/G10 epoxy glass spacers serve dry-type transformers, air-core reactors and applications where moisture immunity or Class F/H temperature capability matters more than oil impregnability. High-density transformerboard such as G4 grade bridges the two, offering maximum stability for spacers in large power transformers. Material choice is ultimately a system decision: everything inside the tank should age together.
Related product: SIDA G4 Transformerboard High Density
How Does Spacer Design Affect Oil Flow and Cooling?
Spacer geometry is a thermal design variable, not just a mechanical one. The spacer thickness fixes the radial duct height — commonly 3–8 mm between discs — and the number of spacer columns around the circumference fixes how much disc surface is blocked from oil contact, typically 25–35%. Narrower ducts raise oil velocity and heat-transfer coefficient but increase pressure drop; wider ducts do the opposite. CFD studies of disc windings show hot-spot temperature can swing by 10 °C or more purely through duct-height and spacer-pitch choices (Torriano et al., 2010).
Flow guidance matters as much as duct size. In directed-oil (OD) cooling, pressboard washers force the oil into a zigzag path so it sweeps every horizontal duct instead of short-circuiting up the innermost vertical channel; the spacers must seal against these washers to make the scheme work. Every extra degree at the hot spot roughly halves-per-6-8 °C the insulation life defined in IEC 60076-2 and IEEE C57.91, so specifying spacer dimensions carelessly quietly costs transformer life — a theme we also cover in our guide on cutting transformer insulation costs without sacrificing performance.
Related product: SIDA Laminated Densified Wood Machined Components
Pressboard vs FRP vs Epoxy Spacers: Material Comparison
Each spacer material family has a clear home turf. Precompressed pressboard is unbeatable inside oil: it wets out completely (no trapped voids to trigger partial discharge), matches cellulose aging, and costs the least. FRP pultrusions and GPO-3 offer roughly triple the compressive strength and total moisture immunity, but they do not impregnate with oil, so designers must verify PD behavior at oil–laminate interfaces. Epoxy glass grades (G10/G11) sit at the top for mechanical and thermal capability — and price.
| Property | Precompressed Pressboard | FRP / GPO-3 | Epoxy Glass (G10/G11) |
|---|---|---|---|
| Standard | IEC 60641-3 | ASTM D709 / NEMA GPO-3 | IEC 60893 / NEMA G10-G11 |
| Thermal class in oil/air | 105 °C (Class A, in oil) | 130–155 °C | 130–180 °C |
| Compressive strength | ~150–250 MPa | ~300–450 MPa | ~350–500 MPa |
| Oil impregnation | Full — PD-safe in oil | None (surface only) | None (surface only) |
| Moisture sensitivity | Must be dried (<0.5% for assembly) | Immune | Immune |
| Relative cost | 1× | 2–3× | 3–5× |
| Best for | Oil-filled power & distribution transformers | Dry-type transformers, reactors, bus supports | High-load structural parts, Class F/H systems |
The selection logic mirrors other insulation choices: match the material system, don’t maximize single properties. An oil-filled transformer gains nothing from moisture-immune FRP spacers — the surrounding paper still needs drying — while a cast-resin dry-type cannot use pressboard at all. For mixed structures, many OEMs combine pressboard radial spacers with laminated-wood or epoxy-glass clamping parts, similar to the grade-matching logic in our presspaper thickness and grade guide.
Related product: SIDA FR4/G10 Epoxy Glass Sheet/Tube/Rod
How to Order Spacer Kits for Your Transformer Design
A complete spacer inquiry needs five data points: material grade (e.g., IEC 60641-3 B.3.1A pressboard, GPO-3, laminated wood), part geometry (dovetail radial spacer, axial stick, washer segment — drawings or DXF preferred), dimensions with tolerances (thickness tolerance is critical; ±0.1 mm is standard for calibrated spacers), quantity per transformer and annual volume, and delivery state — flat sheets for self-punching, or ready-machined parts. Reputable suppliers provide material certificates showing density, compressibility, oil absorption and moisture content per IEC 60641-2 test methods.
Buying spacers as CNC-cut kits rather than raw board shifts the punching, milling and edge-chamfering work to the supplier and guarantees column-height consistency, which matters because a single over-thick spacer concentrates clamping force on one disc. SIDA machines spacers, sticks, washers and clamping components from pressboard, laminated densified wood and epoxy laminates, bundles them per winding as kitted sets, and exports worldwide — see our note on sourcing insulation materials from China for logistics and QC practices.
Related product: SIDA Machined Spacer Components & Kits
FAQ
What thickness are transformer oil duct spacers?
Radial spacers between winding discs are typically 3–8 mm thick, chosen to balance oil velocity against pressure drop, while axial sticks forming vertical ducts usually run 6–12 mm. Distribution transformers sit at the thin end and large power transformers at the thick end of these ranges. The critical requirement is thickness consistency: spacers in the same column are calibrated to ±0.1 mm so clamping pressure distributes evenly across every disc. Spacer height is a thermal design output — it should come from the winding’s cooling calculation, not from a catalog default.
Can FRP or GPO-3 spacers be used inside oil-filled transformers?
They can be, and sometimes are for high-mechanical-stress locations, but with caution. FRP and GPO-3 do not impregnate with oil, so any surface voids or delaminations can become partial-discharge sites in high-field regions; designers must verify PD performance and usually keep resin laminates away from the highest-stress zones. Standard practice remains precompressed pressboard for everything oil-wetted near the windings, with laminated wood or epoxy glass reserved for clamping rings, support beams and other lower-field structural parts.
Summary
Transformer oil duct spacers are small parts with outsized influence: they define the cooling duct geometry that sets hot-spot temperature, and they carry the short-circuit forces that would otherwise crush a winding. Use precompressed pressboard spacers for oil-filled transformers, FRP/GPO-3 for dry-type designs, and epoxy glass or laminated densified wood where clamping loads dominate. Specify material grade, geometry, calibrated thickness tolerance and delivery format, and treat spacer pitch and duct height as thermal design variables per IEC 60076-2.
SIDA supplies the complete oil duct spacer material chain — precompressed pressboard PSP-3050, G4 high-density transformerboard, laminated densified wood, GPO-3 and FR4/G10 laminates — as sheets, rods or finished CNC-machined spacer kits with material certificates. Send us your drawings for a same-day quotation.
Contact SIDA for oil duct spacers and transformer insulation kits:
📞 +86-15958243831
📧 jessie.feng@sidanm.com
💬 WhatsApp: https://wa.me/8615958243831
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References
- IEC 60641-3-1:2008. Pressboard and presspaper for electrical purposes – Part 3: Specifications for individual materials – Sheet 1: Requirements for pressboard, types B.0.1, B.0.3, B.2.1, B.2.3, B.3.1, B.3.3, B.4.1, B.4.3, B.5.1, B.5.3 and B.6.1. International Electrotechnical Commission, Geneva. https://webstore.iec.ch/publication/2707
- IEC 60763-3-1. Laminated pressboard for electrical purposes – Specifications for individual materials. International Electrotechnical Commission, Geneva. https://webstore.iec.ch
- IEC 60076-2:2011. Power transformers – Part 2: Temperature rise for liquid-immersed transformers. International Electrotechnical Commission, Geneva. https://webstore.iec.ch/publication/591
- IEEE C57.91-2011. IEEE Guide for Loading Mineral-Oil-Immersed Transformers and Step-Voltage Regulators. IEEE, New York. https://standards.ieee.org/ieee/C57.91/4699/
- Torriano, F., Chaaban, M., & Picher, P. (2010). Numerical study of parameters affecting the temperature distribution in a disc-type transformer winding. Applied Thermal Engineering, 30(14–15), 2034–2044. https://www.sciencedirect.com/science/article/abs/pii/S1359431110002152
- Del Vecchio, R. M., Poulin, B., Feghali, P. T., Shah, D. M., & Ahuja, R. (2017). Transformer Design Principles (3rd ed.). CRC Press, Boca Raton. https://www.routledge.com/Transformer-Design-Principles-Third-Edition/DelVecchio-Poulin-Feghali-Shah-Ahuja/p/book/9781498787536
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