Design guidelines for dielectric mineral-oil-filled power transformers using synthetic esters include:

• Material compatibility – ensure seals, gaskets, and insulation are resistant to ester fluids.
• Thermal design – synthetic esters have higher viscosity; adjust cooling system and winding design accordingly.
• Moisture management – esters absorb more water without losing dielectric strength, but drying procedures are still important.
• Fire safety – take advantage of esters’ higher fire point to improve safety ratings.
• Standards compliance – follow IEC/IEEE guidelines for ester-filled transformers (e.g., IEC 61099, IEC 60076-14).

Thanks, Mr Farid

These are general points, known to all working in this area.

I was looking for some design tips on radial axial clearances inside the transformer, and also the care to be taken in the non-homogeneous field

1) Start from the right standards (what must not change)

• Insulation levels & tests: keep your LI/SI/AC withstand selections per IEC 60071-1/-2 (insulation coordination) and verify by dielectric tests per IEC 60076-3. These determine your target withstands; the liquid change does not relax them. (Iteh Standards)
• Liquid specification: select a qualified synthetic ester per IEC 61099; use its data sheet values (εr, tan δ, resistivity, viscosity, gassing). (Iteh Standards)
• Reactor specifics: for liquid-immersed shunt reactors, keep to the transformer series (IEC 60076, esp. Part 3) and the design/application guidance in IEC 60076-14 when using high-temperature insulation systems with alternative liquids. (Iteh Standards, IEC Webstore)

2) Re-distribute the electric field (the core design change with esters)

Why: Esters have a higher relative permittivity (typically ~3.1–3.3) than mineral oil (~2.2–2.3). In an oil-paper system this shifts more stress into the solid and reduces stress in the liquid, changing your optimum duct/pressboard proportions and grading. Model this; don’t assume mineral-oil clearances are optimal. (MDPI, Engineering ToolBox)

What to do (workflow):

• Update εr in your 2-D/3-D FEM (oil ducts, spacers, end-insulation, leads/brazes).
• Target more homogeneous field across oil + paper; avoid local peaks in paper edge zones and at conductor steps. (This is a known benefit of the ester εr shift, but only if you re-tune geometry.) (MDPI)
• Use larger radii on energized edges and grading parts (rule-of-thumb ≥0.5 mm on small features, larger on end-structures) and, where needed, add pressboard screens to move peaks away from long oil paths. (dtjs.com.au)

3) Partial-discharge (PD) control with esters

• PD patterns & PDIV can differ from mineral oil; some studies show broader discharge activity and sensitivity to gas-space pressure and temperature. Maintain a positive nitrogen cushion and consider PD under hot conditions. (mv-lub.com, eCIGRE)
• Engineer PD-free design margins with the ester in your model, then verify by induced-voltage + PD tests per IEC 60076-3. For EHV tanks, UHF PD sensors/windows must be compatible with ester liquids (materials/ seals). (Iteh Standards, cigre.cz)

4) Moisture management (different rules than mineral oil)

• Esters hold far more water than mineral oil; this is good for drying the paper (lower paper moisture) but means ppm numbers look “high” in the liquid. Control water activity, not just ppm; keep the system dry (vacuum-fill, hot-oil circulation) and verify breakdown per IEC 60156 / D1816. (ABB Group, Vaisala)
• For maintenance and acceptance limits, follow IEC 62975 (natural ester guide—many principles carry over) and IEEE C57.155 for DGA in natural & synthetic esters (different gas fingerprints than mineral oil). (Iteh Standards, NormSplash)

5) Materials, interfaces & auxiliaries

• Compatibility: confirm gaskets, paints, cable terminations, tap-changer seals with the ester supplier; use materials approved for IEC 61099 fluids. (NBR/HNBR/FKM are typically used; verify swelling data.) (Iteh Standards)
• Bushings & leads: re-check end-winding and bushing field grading with the higher εr; long oil paths may be less stressed, but solid interfaces (paper/pressboard, RIP/RIS) can see higher E-field without re-grading. Use the bushing maker’s ester models where available. (MDPI)

6) Thermal & flow implications that touch dielectric margins

• Esters have higher viscosity than mineral oil; confirm oil velocities in narrow ducts and hotspot margins. Lower velocities can slightly raise local temperatures, which in turn lowers PDIV—factor this into your PD margin. (CIGRE-UK, eCIGRE)
• If you adopt high-temperature insulation (e.g., paper upgrades) together with ester, apply IEC 60076-14 and validate temperature-rise tests accordingly. (IEC Webstore)

7) Test & qualification plan (recommended for a 400 kV reactor)

8) Bottom line—practical tips distilled

• Re-run your field model with ester εr and re-grade end insulation; don’t copy mineral-oil dimensions blindly. (MDPI)
• Aim for rounded electrodes (≥0.5 mm radius), pressboard shields, and balanced oil/solid gaps to keep peak stress out of the paper edges. (dtjs.com.au)
• Keep a dry, sealed system (vacuum processing, positive cushion). Judge moisture by water activity; verify breakdown per IEC 60156/D1816. (ABB Group, Vaisala)
• Use IEC 60071-1/-2 to set insulation levels and IEC 60076-3 to prove them; ester does not change test levels. (Iteh Standards)
• Specify the liquid to IEC 61099 and set operations/diagnostics with IEC 62975 (maintenance principles) and IEEE C57.155 (DGA) adapted for esters. (Iteh Standards, NormSplash)

Thanks, Mr Ahmed Ragab Mahmoud, for a detailed design philosophy.

There is an unwritten design tip that the dielectric design should be based on one step higher BIL. A 1300 kVp 400 kV class Shunt Reactor should be designed for one step higher BIL,say 1425 kVp ?

What is your opinion about this?