UHV Bushing Selection: Field Grading, Moisture and Altitude
Identify voltage duty, field grading, moisture control, external insulation and altitude corrections in the project specification.
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.
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In short
A 1100 kV UHV bushing must withstand lightning impulse levels around 2400 kV, keep partial discharge in the single-digit pC range at more than 900 kV phase-to-earth and remain mechanically stable at a length of more than 13 m. Three engineering levers make this possible: optimised condenser grading derived from field simulation, deep vacuum drying and degassing that holds residual moisture below 10 ppm in oil, and altitude-corrected external insulation to IEC 60071-2 for sites up to 4300 m and beyond.
- At 1100 kV, field grading inside the condenser core, not material strength alone, decides partial discharge performance.
- Moisture is the main enemy: residual water raises tan delta and can trigger thermal instability at UHV.
- External insulation at 4300 m needs an altitude factor of about 1.69 for lightning impulse (IEC 60071-2, m = 1).
- A composite housing may reduce mass and change fracture behaviour; confirm both against the proposed UHV assembly and its mechanical tests.
- High-altitude field verification complements laboratory tests that are performed near sea level.
At UHV, coordinate condenser grading, moisture control, external insulation and mechanical interfaces for the specific voltage and site conditions.
01Why is a 1100 kV bushing so difficult to design?
A 1100 kV bushing has to carry more than 635 kV phase-to-earth continuously while passing type tests at roughly twice that level in power frequency and nearly four times in lightning impulse. At the same time it must be long enough for external flashover distance, often exceeding 13 m, yet stiff enough to resist wind, conductor tension and seismic loads. Every millimetre of screen position and every ppm of moisture affects the result.
Typical project specifications for 1100 kV transformer bushings call for lightning impulse withstand around 2400 kV, switching impulse around 1950 kV and power-frequency withstand around 1200 kV, together with low partial discharge and tan delta during long-duration induced tests on the transformer.
| Parameter | 800 kV class (Um 800 kV) | 1100 kV class (Um 1100 kV) |
|---|---|---|
| Phase-to-earth operating voltage | about 462 kV | about 635 kV |
| Lightning impulse withstand (typical spec) | about 2100 kV | about 2400 kV |
| Switching impulse withstand (typical spec) | about 1550 kV | about 1950 kV |
| PD acceptance | Agree voltage point and limit | Agree voltage point and limit |
| Tan delta acceptance | Agree type, temperature and clause | Agree type, temperature and clause |
| Altitude factor at 4300 m (LI, m = 1) | about 1.69 | about 1.69 |
| Typical overall length | about 10 m to 12 m | more than 13 m |
02How is the electric field controlled inside a UHV condenser core?
The field is controlled by dozens of concentric aluminium foil screens whose lengths and radial spacing are optimised so that each layer carries a near-equal share of voltage, both radially through the insulation and axially along the surface. Finite element simulation identifies stress concentrations at screen edges, the flange and the top terminal, where grading rings or shields are then added.
Review screen spacing and edge geometry in the approved field study, then compare the core capacitance and dielectric test results with that design.
- Radial grading: equal voltage per layer through the paper thickness.
- Axial grading: stepped screen lengths keep surface stress uniform along the air and oil sides.
- Shielding: corona rings at the top terminal and flange limit external discharge.
03Why does moisture control matter so much at UHV?
Moisture raises dielectric loss and lowers breakdown strength. At UHV the dielectric loss scales with the square of voltage, so a small rise in tan delta caused by moisture can push the core toward thermal instability, where heating raises losses faster than the bushing can dissipate them. Deep vacuum drying of the core and degassed, dried oil are therefore mandatory.
Request moisture measurements and the corresponding tan delta results for the actual core and oil system, with the test conditions recorded.
“Buyer check: document insulation level, altitude correction and moisture-control evidence for the project and selected model.”
04How does high altitude change UHV bushing design?
Lower air density at high altitude reduces the flashover voltage of external insulation. IEC 60071-2 corrects the required withstand with the factor Ka = e^(m x H / 8150), where H is altitude in metres. With m = 1 for lightning impulse, a site at 4300 m needs about 1.69 times the standard withstand, which lengthens the arcing distance and increases shed count on the air-side housing.
State the site altitude and required external insulation on the drawing; request evidence that the proposed housing and grading arrangement cover those conditions.
Applicable EONOVA Equipment
Conclusion
Verify the transport frame, lifting points, permitted mounting angle and pre-energization capacitance and tan delta checks for the exact bushing.
Talk to an EONOVA engineerFrequently Asked Questions
Can RIP technology be used for 1100 kV bushings?
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RIP is a possible design route at UHV, subject to the specific AC or DC duty and proven model. Request the complete-bushing drawings, applicable tests, manufacturing records and permitted mounting angle rather than assuming every RIP design covers 1100 kV.
Why are UHV bushings tested at high-altitude sites?
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Most high-voltage laboratories are near sea level, where air density is higher. High-altitude testing confirms that external insulation, corona rings and housing materials perform as designed under low air pressure, strong UV and large temperature swings, which cannot be fully reproduced by applying a correction factor alone.
How are 13-metre UHV bushings transported and installed?
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They are shipped in rigid frames with shock recorders, often horizontally with oil-side support, and lifted with a dedicated tilting fixture to avoid bending stress. Installation follows the supplier procedure for lifting points, vacuum or oil filling and tan delta and capacitance checks before energisation.
Standards & Sources
- [1]IEC 60071-2: Insulation co-ordination: Application guidelines
International Electrotechnical Commission (TC 28)
Supports: Altitude correction factor Ka = e^(m x H / 8150) for external insulation.
- [2]IEC 60137:2017: Insulated bushings for alternating voltages above 1000 V
International Electrotechnical Commission (TC 36)
Supports: AC complete-bushing scope and test categories; confirm numerical limits by type and clause.
- [3]CIGRE TB 456: Background of technical specifications for substation equipment exceeding 800 kV AC
CIGRE Working Group A3.22
Supports: Insulation levels and specification practice for UHV AC substation equipment.
- [4]IEC 60060-1: High-voltage test techniques: General definitions and test requirements
International Electrotechnical Commission (TC 42)
Supports: Impulse waveform definitions and atmospheric correction used in UHV testing.
Selection guidance for buyers. Confirm specifications and test evidence for the quoted model and supplying entity.
