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Views: 0 Author: Zisheng Electric Publish Time: 2026-09-08 Origin: Zisheng Electric
Transformer bushings look simple on a general arrangement drawing, but they sit at one of the most exposed electrical and mechanical interfaces on the complete unit. For EPC contractors and utility purchasers, transformer bushing selection should be resolved before the tank layout and external connections are released. The bushing has to carry current, withstand system voltage and impulse stress, maintain external insulation in the site environment, seal the tank, support terminal loads and match the customer’s cable or bus connection.
A specification that states only “porcelain bushing, IEC compliant” leaves too much unresolved. The engineer still needs the highest voltage for equipment, insulation level, rated current, creepage requirement, terminal arrangement, conductor force, mounting angle, oil-side clearances and interface details. A mismatch may not be found until the transformer reaches site, when changing the bushing can affect the cover layout, lead routing and delivery schedule.
Bushing section showing terminal, external insulation, earthed tank flange and oil-side lead.
A bushing passes an energized conductor through an earthed barrier. On an oil-immersed transformer, that barrier is normally the tank cover or turret. The external section must maintain insulation through air and across its surface. The internal section must maintain clearance in oil and control the electric field around the conductor and transformer lead.
IEC 60137:2017 specifies bushing characteristics and tests for its defined AC applications. State the contract edition and applicable amendments. Bushing compliance does not establish compliance of the assembled transformer: separately coordinate winding insulation, terminal clearances and dielectric tests using the project specification and IEC 60076-3.
Design input | Main risk if it is missing | Engineering check |
|---|---|---|
System voltage and highest voltage for equipment | Incorrect insulation class or test level | Use the project single-line diagram and insulation coordination schedule, not nominal voltage alone |
Rated and emergency current | Excessive conductor or terminal heating | Check continuous load, overload duty, terminal material and connection resistance |
Lightning impulse and power-frequency withstand levels | Insufficient internal or external insulation margin | Coordinate the bushing rating with the transformer winding insulation level |
Pollution, altitude and installation environment | Surface flashover or unexpected derating | Define creepage distance, shed profile, material and altitude correction requirements |
Cable, busbar or overhead-line connection | Terminal mismatch and excessive mechanical load | Confirm palm size, hole pattern, connector type, direction and allowable cantilever load |
Mounting and oil-side geometry | Local electric-field concentration or lead interference | Review mounting angle, under-cover length, shield position and lead clearances |
The nominal system voltage is not enough to order a bushing. The design review needs the highest voltage for equipment, usually written as Um, together with the required lightning impulse withstand and separate-source power-frequency withstand levels. These values must agree with the transformer datasheet and the insulation coordination study.
A common procurement error is copying a bushing description from an earlier project that used the same nominal voltage. The new utility may apply a different equipment class or insulation level. Neutral terminals also require a deliberate decision. A neutral that is solidly earthed may use a reduced insulation level only when the system design and project specification permit it. The supplier should not assume a reduced neutral bushing from the vector group alone.
The bushing’s oil end, transformer lead and nearby earthed metal form one electric-field system. A physically interchangeable bushing is not automatically electrically interchangeable. A longer or differently shielded oil end changes clearances and field distribution. The design review should check the bushing outline drawing against the cover, turret, lead exit and clamping arrangement before releasing the tank drawing.
Selection workflow linking system insulation requirements, bushing ratings and assembled transformer checks.
The calculated winding current is the starting point. Selection also has to consider emergency loading, ambient temperature, enclosure conditions and the temperature rise of the terminal connection. A bushing with a current rating only marginally above nameplate current may be technically valid under reference conditions but leave little margin when the transformer operates in a hot terminal box or carries planned overload.
Connection hardware matters. An undersized palm, poor plating compatibility, limited contact area or an unsupported cable can create a hot joint even when the bushing conductor is correctly rated. The procurement package should state terminal material, plating, palm dimensions, hole pattern and the mating connector supplied by each party.
For higher-voltage units, the bushing turret may also carry current transformers. Their ratios, accuracy classes, core purposes, secondary terminal routing and shorting arrangements need to match the protection schematic. Capacitance-graded bushings may include a test or voltage tap. Its normal operating condition, protective cap and earthing arrangement must be clear in the operation manual. Leaving a test tap improperly earthed can damage the tap insulation.
Creepage distance is measured along the external insulating surface. It is selected from the pollution severity and insulation design, not from country name alone. An inland substation, a coastal industrial site and a cement plant in the same country can require different external insulation solutions.
Salt, conductive dust and industrial deposits create a leakage path when the surface becomes wet. More creepage can help, but the shed profile and material are also important. Porcelain remains common and provides long service experience. Composite housings can reduce weight and offer strong pollution performance, but the project should confirm material, ageing requirements and handling rules.
Site condition | Effect on bushing | What the purchaser should confirm |
|---|---|---|
Coastal salt mist | Conductive surface contamination and corrosion of terminals | Pollution severity, creepage, shed profile and exposed metal finish |
Desert dust | Dry deposits that become conductive with dew or rain | Insulator profile, cleaning access and actual contamination assessment |
High altitude | Reduced air dielectric strength and cooling capability | Site altitude and required external-insulation correction |
Indoor cable box | Restricted ventilation and limited clearance | Box dimensions, cable bend radius, separable connector and heat dissipation |
Seismic or high-wind location | Higher mechanical loading at the flange and terminal | Seismic duty, conductor tension and support arrangement |
Insulator profile distinguishing the surface creepage path from an air-clearance check.
The bushing terminal should not be used as a structural support for heavy cables or rigid busbar. Cable weight, short-circuit forces, thermal expansion and installation misalignment can apply bending load to the top terminal and flange. The connection design should include flexible links or independent supports where necessary.
The purchaser should provide the cable schedule, termination drawing or busduct interface before finalizing the cover. The interface review should cover phase spacing, terminal orientation, cable entry direction, bend radius, enclosure size and safe working clearance. This review is especially important when the cable box, surge arrester bracket or busduct is supplied by another contractor.
The factory acceptance test (FAT) does not repeat every type test on every bushing. The purchaser should therefore review the bushing nameplate data, routine test certificate and type-test evidence required by the contract. On the assembled transformer, inspection should confirm identification, phase marking, installation angle, gasket seating, terminal tightness, oil level where applicable and absence of visible damage.
Electrical transformer tests also provide useful interface checks. Ratio and vector-group verification can reveal a lead connection error. Applied-voltage and induced-voltage tests challenge the transformer insulation system under the agreed test program. Any unusual discharge indication, leakage or physical movement around a bushing requires investigation before release.
Agree the inspection and test plan before FAT. Record bushing manufacturer, model and serial number against the installed phase, certificate number, drawing revision and applicable test results. Define witness/hold points and acceptance criteria in the contract. Record defects, corrective action and any required retest before closing the release dossier; a transformer test report does not replace a bushing routine-test certificate.
Consider a project datasheet that states only “33 kV transformer, outdoor porcelain bushings.” Before quotation, the supplier still cannot select a reliable bushing. The engineer needs the actual highest voltage for equipment and insulation levels, rated current on each winding, neutral treatment, pollution severity, altitude and connection type. If the high-voltage side connects to overhead conductors, the terminal loading and phase spacing are different from a screened cable box. If surge arresters are mounted on the transformer, their bracket position and lead length also affect the layout.
A useful technical offer would therefore list the proposed bushing standard, manufacturer or approved vendor class, Um, rated current, withstand levels, minimum creepage distance, housing material, terminal details and outline-drawing reference. Any deviation should be visible. This makes bid comparison practical: the purchaser compares actual electrical and mechanical interfaces instead of three offers that all say “IEC bushing” while using different assumptions.
Spare strategy also deserves attention. For a remote substation, the customer may request one spare high-voltage bushing. The spare must match not only the voltage and current but also the flange drilling, oil-end geometry, overall height, terminal palm and test-tap arrangement. Recording the exact model and outline drawing in the final documentation reduces replacement risk years later.
FAT documentation sequence for identifying installed bushings, checking evidence, inspecting interfaces and closing deviations.
Nominal voltage, Um and insulation withstand levels for every terminal
Rated current, overload duty and ambient conditions
Bushing technology, insulation material and applicable standard
Pollution severity, creepage distance, altitude and shed profile
Terminal palm, connector, plating, cable or busbar responsibility
Mounting angle, oil-end dimensions and transformer clearances
Mechanical terminal loads, seismic duty and conductor supports
Current transformers, test taps and monitoring accessories
Routine test certificates, type-test evidence and spare-bushing interchangeability
Packing, lifting, storage and site installation instructions
For a project-specific enquiry, send the single-line diagram, transformer datasheet, insulation coordination schedule, site altitude and pollution assessment, cable or busduct drawing and terminal-load requirements to Zisheng Electric. Ask the supplier to identify proposed bushing ratings, interface drawings, exclusions and deviations in the technical offer.
Supporting reading: 11 kV, 22 kV and 33 kV transformer guide; Oil-immersed transformer design and manufacturing; Transformer fault analysis; 110 kV/115 kV power transformers.
