What should be considered when selecting and using bushings?

When selecting and using bushings, certain considerations need to be taken into account. Bushings, as current-carrying components of transformers, directly impact the insulation performance of transformers. Here are the key points to be aware of during selection and usage:

a. Ambient Medium Temperature: For oil-immersed transformers, the temperature of the bushings in the oil is related to the temperature rise of the upper oil layer. In the case of self-cooling and forced-air cooling, the temperature rise of the upper oil layer is 55K, thus allowing a temperature rise of 15K for the lower terminal. In the case of strong oil-air cooling, the upper oil layer temperature rise is generally 40K, allowing an increased temperature rise of 25K for the lower terminal. In the case of strong oil-water cooling, the upper oil layer temperature rise is lower, at 30K, allowing a temperature rise of 35K for the lower terminal.

b. Electrical Characteristics: Bushings connected to Gas Insulated Switchgear (GIS) must withstand very fast transient overvoltages (VFTO) and prevent SF infiltration into the oil. To ensure local discharge in the transformer, bushings should also ensure local discharge; oil-paper capacitor bushings should have small capacitor tap bushings for monitoring local discharge. The upper part of oil-filled bushings should have a vent plug to ensure it is filled with oil. When installing external discharge gaps on bushings, attention should be paid to the surge withstand capability. Large current oil-paper capacitor bushings must maintain FR and dielectric loss to ensure temperature rise and insulation stability. The rated current of the bushing should be 20% higher than the maximum current of the transformer to maintain a certain excess capacity. When cables pass through the middle of the bushing, the cable should not loosen and magnetically interact with the copper tube inside the bushing to prevent overheating.

c. Mechanical Endurance of External Terminals: External terminals of bushings should possess a certain mechanical load-bearing capacity.

d. External Umbrella Spacing and Number of Umbrellas: Using larger umbrellas can increase the leakage distance, but the leakage ratio calculation uses U as the denominator, not the rated voltage. Increasing the number of umbrellas should not reduce the spacing between umbrellas to increase the leakage ratio; instead, the ratio of creepage distance to clearance distance should be less than 3.5 to 4.

e. When using two bushings in parallel, the two lower terminals under the cover of the bushings should not be connected in parallel; they should only be connected externally.

f. Magnetic Shielding at the Cover of Large Current Bushings: The cover of large current bushings should be magnetically shielded.

g. Installation Angle of Bushings: The inclination angle of bushings should generally not exceed 30°; when installed at 90°, the insulation strength decreases.

h. Usage of Low Voltage High Current Bushings in Enclosed Busbars: When used at the rated current, the temperature rise should be reduced due to the higher temperature inside enclosed busbars.

i. Leakage Distance Increase for Bushings at High Altitudes: Bushings used at high altitudes should have an increased leakage distance.

j. For bushings with U₄ ≥72.5kV, consideration should be given to fitting three bushing-type current transformers on the middle flange.

LuShan, est. 1975, is a Chinese professional manufacturer specializing in power transformers and reactors for 49 years. Leading products are single-phase transformer, three-phase transformers, DC inductors, AC reactors, filtering reactor, expoxy resin high-voltage transformer and intermediate, high-frequency products. Our transformers and reactors are widely used in 10 application areas: rapid transit, construction machinery, renewable energy, intelligent manufacturing, medical equipment, coal mine explosion prevention , excitation system, vacuum sintering, central air conditioning.

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