How to Choose the Right Bushing by Voltage Class

When selecting a transformer bushing, looking at "voltage class" alone is far from enough. Correct selection must determine rated voltage, basic impulse level (BIL), rated current, creepage distance and mounting arrangement together. This article gives a systematic approach.

1. Rated voltage and BIL

Rated voltage sets the basic insulation requirement, while BIL sets lightning-impulse withstand capability. The mapping between system voltage and BIL differs by region, so follow your project specification.

2. Rated current

Calculate rated current from transformer rating and voltage, and leave a margin. Excessive current causing temperature rise is a common cause of bushing overheating failures.

3. Creepage distance

Determine specific creepage (mm/kV) from the pollution class of the site (IEC 60815 classes I–IV). Coastal, industrial and sandy areas need larger creepage distance.

4. Mounting arrangement

• Oil-to-air: one end in transformer oil, the other in air — the most common.
• Air-to-air (wall): air on both sides.
• Oil-to-oil: oil on both sides, for special designs.

5. Typical considerations by voltage class

• Medium voltage (≤ 36 kV): RBP or lightweight RIP; focus on current and mounting size.
• 66–110 kV: RIP is becoming mainstream; watch BIL and creepage distance.
• 220 kV and above: prefer RIP/RIS; emphasise partial discharge level, shielding design and test tap.

6. Other factors

• Ambient temperature and altitude (high altitude requires insulation and cooling correction).
• Seismic requirements.
• Whether test taps, current transformers (CT) and other accessories are needed.

Compile the above parameters into a complete bushing technical specification for the manufacturer. Ever Power provides full support from selection calculation to type testing based on your system parameters.