Choosing Oil-to-SF6 and Wall Bushings for Equipment Interfaces
Check oil and gas interfaces, sealing, wall penetration, insulation level and the applicable test plan against equipment drawings.
Selection note: use this guide to compare technical requirements. Confirm catalog ranges, typical values and standard clauses against the model, edition and agreed test conditions; project acceptance follows the approved drawings and technical agreement.
On this page
- 01When should engineers specify Oil-to-SF6, Oil-to-Oil, GIS Lead-Out or Wall Bushings?
- 02How does an Oil-to-SF6 bushing prevent SF6 gas from leaking into transformer oil?
- 03Why are dry RIP wall bushings preferred for horizontal and HVDC valve hall penetrations?
- 04What mechanical and pollution ratings are required for GIS lead-out bushings?
- 05Frequently asked questions
- 06Standards & sources
In short
Select the bushing by its two insulating media, mounting angle and equipment drawings; agree sealing and test scope for each interface.
- Oil-to-SF6 bushings can connect a transformer turret directly to GIS; compare site layouts before claiming a footprint saving.
- Because SF6 pressure (0.35 to 0.50 MPa) exceeds transformer oil pressure, a vented dual-seal flange barrier is mandatory to prevent gas-in-oil alarms.
- Some dry RIP wall-bushing designs permit horizontal mounting without a core oil conservator; confirm the complete wall-penetration and fire-separation design.
- GIS lead-out bushings combine an SF6 gas barrier on the bottom with a 31 mm/kV HTV silicone shed housing on the outdoor side and up to 5000 N cantilever rating.
- Confirm the pressure and sealing boundary for each oil, gas or air interface on the complete bushing assembly.
While standard oil-to-air bushings serve open-air switchyards, modern underground substations, offshore wind platforms, pumped-storage caverns and HVDC converter stations rely on direct equipment-to-equipment and through-wall bushings. Governed by IEC 60137, IEC 62271-203 (for GIS interfaces) and IEC 62199 (for DC wall bushings), these specialized bushings must seal two different insulating media while grading extreme electric fields in a compact envelope. This engineering guide explains how to specify Oil-to-SF6, Oil-to-Oil, GIS lead-out and AC/DC wall bushings.
01When should engineers specify Oil-to-SF6, Oil-to-Oil, GIS Lead-Out or Wall Bushings?
Select the bushing by the two media it separates. An Oil-to-SF6 bushing (ETGD) joins a transformer tank directly to an SF6 GIS duct; an Oil-to-Oil bushing (ETOD) links a transformer main tank to an oil-filled cable box; a GIS lead-out bushing (FETA/CET) transitions from pressurized SF6 to outdoor air; and a wall bushing (CET / FET wall type) carries high-voltage conductors horizontally or inclined through a building or valve hall wall.
Oil-to-SF6 interfaces can support a direct transformer-to-GIS layout. Compare the actual drawings, clearances, maintenance access and sealing requirements before estimating site space or pollution exposure.
- ETGD Oil-to-SF6 (66 kV to 1200 kV, up to 5000 A): underground substations, offshore platforms, hydropower caverns.
- ETOD Oil-to-Oil (66 kV to 800 kV, up to 8000 A): oil-filled cable boxes, tap-changer compartments, locomotive transformers.
- FETA / CET GIS Lead-Out (66 kV to 1200 kV, up to 8000 A): SF6-to-air overhead line takeoff.
- CET / FET RIP Wall Bushing (12 kV to 1100 kV AC, plus or minus 12 kV to 1200 kV DC): indoor substations and HVDC valve halls.
02How does an Oil-to-SF6 bushing prevent SF6 gas from leaking into transformer oil?
Separate gas and oil with the approved barrier and seals; compare their pressure duties and define how any inter-seal cavity will be monitored and tested.
If gas migrates into transformer oil, it can affect gas monitoring; oil entering a GIS chamber can contaminate insulating parts. Where an inter-seal sampling cavity is provided, agree its monitoring method and acceptable leakage with both equipment designers and record the commissioning result.
| Model & Type | Media Interface | Rated Voltage / Current | PF / LIWL Withstand | Cantilever / Weight |
|---|---|---|---|---|
| ETGD-252/1250 (Oil-SF6) | Transformer Oil to SF6 | 252 kV / 1250 A | 460 kV / 1050 kV | 4000 N / 190 kg |
| ETGD-550/1250～2500 (Oil-SF6) | Transformer Oil to SF6 | 550 kV / 1250～2500 A | 740 kV / 1675 kV | 4000～5000 N / 400～440 kg |
| ETOD-363/1250 (Oil-Oil) | Oil Tank to Oil Cable Box | 363 kV / 1250 A | 570 kV / 1175 kV | 4000 N / 190 kg |
| FETA-252/3150 (GIS Lead-Out) | SF6 Gas to Outdoor Air | 252 kV / 3150 A | 460 kV / 1050 kV | 5000 N / 430 kg |
03Why are dry RIP wall bushings preferred for horizontal and HVDC valve hall penetrations?
A dry RIP core avoids the oil reservoir associated with an OIP core, which can simplify an approved horizontal wall design. Confirm the proposed complete bushing, wall seal, mounting orientation, AC or DC duty and fire-protection assessment before selection.
A dry RIP wall bushing avoids an oil conservator in a horizontal installation. Confirm mounting angle, wall interface, insulation duty and complete-bushing tests for the proposed model.
“Buyer check: confirm both equipment interfaces, pressure boundaries, seals and acceptance tests on approved drawings.”
04What mechanical and pollution ratings are required for GIS lead-out bushings?
A GIS lead-out bushing (FETA/CET) supports overhead line conductors directly at its top terminal while containing pressurized SF6 at its base. Under IEC 60137 and IEC 62271-203, 252 kV to 550 kV units typically require 3150 N to 5000 N continuous cantilever withstand, a triple gas-seal base flange, and an HTV silicone composite housing with at least 31 mm/kV specific creepage distance for coastal or industrial sites.
Specify conductor load and site pollution, then check cantilever and creepage values on the offered GIS lead-out bushing drawing and report.
Applicable EONOVA Equipment
Conclusion
Confirm the two media and mounting duty first, then approve interface drawings, sealing checks and the model-specific test plan.
Talk to an EONOVA engineerFrequently Asked Questions
Who supplies the mating flange dimensions for an Oil-to-SF6 transformer-to-GIS bushing?
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The transformer and GIS designers must agree the mating interface. Send both flange and terminal drawings, pressure duties and sealing details to the bushing supplier for a coordinated drawing approval.
Can an Oil-to-SF6 RIP bushing be tested in air before installation into the GIS chamber?
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No. Because the SF6-side and oil-side ends are dimensioned for pressurized gas and mineral oil, their flashover distance in open air is far below the rated test voltage. Factory routine testing is performed with the ends immersed in a dedicated oil-and-SF6 test tank.
Do RIP wall bushings require periodic oil or gas maintenance?
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A dry RIP core does not require core oil sampling or gas replenishment. The complete wall bushing still needs the model-specific inspection plan, including housing and seal checks and any capacitance or tan delta measurements required by the operator.
Standards & Sources
- [1]IEC 60137:2017: Insulated bushings for alternating voltages above 1000 V
International Electrotechnical Commission (TC 36)
Supports: Dielectric, thermal and cantilever requirements for oil-to-gas, oil-to-oil and wall bushings.
- [2]IEC 62271-203: Gas-insulated metal-enclosed switchgear for rated voltages above 52 kV
International Electrotechnical Commission (TC 17)
Supports: SF6 gas tightness, pressure coordination and transformer-to-GIS interface rules.
- [3]IEC 62199: Bushings for d.c. application
International Electrotechnical Commission (TC 36)
Supports: DC withstand, polarity reversal and thermal-electrical stress requirements for HVDC wall bushings.
- [4]CIGRE TB 755: Transformer Bushing Reliability
CIGRE Working Group A2.43
Supports: Service experience of Oil-to-SF6 and dry-type RIP condenser bushings in substations.
Selection guidance for buyers. Confirm specifications and test evidence for the quoted model and supplying entity.
