High Voltage Bushing Selection Standards for Power transformer
1. Structure Forms and Classification of Bushings
The structure forms and classification of bushings are shown in the table below:
| Serial No. | Classification Feature | Category | |
| 1 | Main insulation structure | Capacitive Type | Resin-impregnated paper Oil-impregnated paper |
| Non-capacitive Type | Gas insulation Liquid insulation Casting resin Composite insulation |
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| 2 | External Insulation Material | Porcelain Silicone Rubber |
|
| 3 | Filling Material Between Capacitor Core and External Insulation Sleeve | Oil-filled Type Gas-filled Type Foamed Type Oil-paste Type Oil-gas Type |
|
| 4 | Application Medium | Oil-Oil Oil-Air Oil-SF₆ SF₆-Air SF₆-SF₆ |
|
| 5 | Application Site | AC DC |
|
2. Selection Principles for Bushings
2.1 Basic Selection Principles
2.1.1 The selection of bushings should meet the performance specifications of transformers, such as: maximum equipment voltage, maximum operating current, insulation level, and installation methods, satisfying relevant requirements for safe operation of power grids.
2.1.2 The selection of bushings should also consider other factors, such as:
Operating environment: altitude, pollution level, ambient temperature, working pressure, arrangement method;
Transformer structure: lead-out method, bushing installation method, total installation height with current transformers;
Bushing structure: current carrying method, internal insulation form (oil-impregnated paper or resin-impregnated paper), external insulation sleeve material (porcelain or silicone rubber);
Bushing supplier, safety reliability, operational performance and other factors.
2.1.3 The insulation level of bushings should be higher than that of the transformer main body.
2.2 Selection Based on Transformer Rated Voltage Level
2.2.1 When the rated voltage of bushings exceeds 40.5kV, the main insulation structure of bushings should preferably be of condenser type.
2.2.2 When the rated voltage of bushings does not exceed 40.5kV, the main insulation structure of bushings may be either pure porcelain (composite) type or condenser type, depending on specific conditions.
2.3 Selection Based on Current Carrying Method of Bushings
2.3.1 When the rated current of bushings is less than 630A, the current carrying method should preferably be cable-through type.
2.3.2 When the rated current of bushings is not less than 630A or the voltage is not less than 220kV, the current carrying method should preferably be conductor rod type.
2.4 Selection Based on Transformer Operating Conditions
2.4.1 When the transformer operating location has normal environmental conditions, standard specification bushings provided by the bushing supplier should be directly selected.
2.4.2 When the transformer operating location is at an altitude greater than 1000m, bushings with external insulation dimensions calibrated according to GB/T4109 should be selected. For parts of bushings immersed in oil or SF6 medium, their breakdown field strength and flashover voltage are not affected by altitude, so insulation distances do not require calibration.
The internal insulation level of bushings is unrelated to altitude effects and does not require calibration. (Note: Due to limitations of breakdown strength and flashover voltage in the immersed medium parts, bushings used in high-altitude areas cannot be verified by testing at lower altitudes to confirm whether the increased arcing distance is sufficient. Therefore, bushing suppliers should demonstrate that the increased external insulation arcing distance of bushings is adequate.)
2.4.3 The maximum phase voltage of power grid systems may exceed Um/√3. When this condition does not exceed 8 hours cumulatively within any 24-hour period and 125 hours annually, bushings should be able to operate at the following voltage values:
For systems where operating voltage may exceed the values mentioned above, bushings with higher Um values should be selected.
2.4.4 For transformers with higher seismic performance requirements, dry-type bushings are recommended.
2.5 Selection Based on Transformer Insulation Medium Type
2.5.1 When the transformer's internal insulation medium uses transformer oil and is directly connected to overhead lines externally, oil-air structure bushings should be selected.
2.5.2 When the transformer's internal insulation medium uses transformer oil and is directly connected to external GIS, oil-SF6 structure dry-type bushings should be selected.
2.5.3 When the transformer's internal insulation medium uses SF6 gas and the external insulation is air, SF6-air structure dry-type bushings should be selected.
2.5.4 When both the internal and external insulation media of the transformer use transformer oil, oil-oil structure bushings should be selected.
2.6 Selection for Converter Transformer Valve Applications
For valve-side AC/DC bushings, resin-impregnated paper type AC/DC bushings or SF6-filled oil-paper capacitance type AC/DC bushings are recommended.
2.7 Selection for Oil-Immersed Smoothing Reactor Applications
For oil-immersed smoothing reactors, resin-impregnated paper type DC bushings or SF6-filled oil-paper capacitance type DC bushings are recommended for the valve hall side.
2.8 Selection for Online Monitoring Applications
When implementing online monitoring for bushings, bushings with voltage taps should be selected.
