Why Bushing Installation Is Critical in Large Power Transformers
In large power transformers, especially 100 MVA power transformers used in data centers or utility substations, transformer bushing installation is a critical part of transformer installation. Bushings are not merely connection components; they serve as high-voltage insulation interfaces, mechanical stress points, and common sources of installation-related failures.
Improper installation can lead to oil leakage, partial discharge, or bushing cracking/explosion. Therefore, strict control during transformer installation is essential to ensure operational safety and longevity.
Transformer Bushing Installation Requirements
Vertical bushings: deviation ≤ 30° from vertical
Horizontal bushings: deviation ≤ 15° from horizontal
Exceeding these limits may result in internal stress imbalance, uneven oil distribution, and reduced insulation performance. Adhering to transformer installation guidelines prevents long-term insulation risks.
Pre-Installation Safety Preparation
Pressure Release
Open any butterfly valve (~1/3 opening) to safely release nitrogen and ensure the transformer tank internal pressure equals zero.
Bushing Elevation Base Orientation Check
Ensure the markings at the bottom correspond with the elevation base flange markings on the transformer tank.
Ensure the flange of the connecting pipe on the elevation base matches the small oil-collecting pipe.
Following these steps ensures a safe and guideline-compliant transformer installation.
Tools and Materials Required
- Soft lifting rope – typically 20 m long, diameter 8–10 mm. (If using steel wire ropes, wrap with cloth tape to prevent damage to porcelain or metal parts.)
- Lifting eye screws – for inclined bushing installation.
- Long traction rope with M8 bolts – for threading through the bushing central tube, and for lifting lead connectors or guide rods.
- Torque wrench for hex bolts – across flats 16 mm (M10) and 13 mm (M8), torque range 10–80 Nm.
- Torque wrench for cylindrical head hex bolts – across flats 6 mm (M8), 5 mm (M6), and 4 mm (M5), torque range 5–30 Nm.
- Adjustable wrench – jaw width up to 42 mm, torque 10–80 Nm.
- Soft padding – for supporting bushings during handling and installation.
To prevent thread seizure or damage to wire inserts in threaded holes:
- Threads in contact with transformer oil: use anhydrous petroleum jelly, grease No.28, or other oil-safe lubricants.
- Grounding bolt threads: use grease No.28 or equivalent.
- External terminal and busbar fastening bolts: use grease No.28 or equivalent.
- Following this transformer installation checklist ensures proper mechanical and electrical connections.
Lifting and Handling of Bushings
For HV bushings (145 kV and above), use dual lifting points (double hook method), control the lifting angle gradually, and avoid impact on porcelain.
If placed on the ground, support at the same points as in packaging and use soft padding (rubber or wood).
Step-by-Step Bushing Installation Procedure
1. Unpack & Inspect
Remove bushings from packaging and place them on a dedicated rack. Clean and inspect the porcelain surface for cracks or scratches, check oil levels, and ensure there is no leakage. (Note: Petroleum jelly on threads may liquefy under high temperatures.)
2. Insulation Test
Conduct insulation tests, including insulation resistance, end-screen insulation, tanδ, capacitance, and oil performance. Proceed only if all tests pass.
3. Lifting with Double Hooks
Attach hook 1 to the flange lifting ring (3–4 skirts from the lead end) and hook 2 to the porcelain under the flange or lifting bracket. Lift both simultaneously, gradually raising hook 1 while lowering hook 2 until the bushing aligns with the elevation base angle.
4. Lead Wire & Sealing
Thread lead wires through the conductor tube, insert pins, then reinstall the top cap, grading shield, and terminal board. Ensure proper sealing at the top. Handle carefully to prevent porcelain damage.
5. Bolt Tightening
Tighten bolts diagonally and gradually to prevent uneven flange stress. Do not exceed the recommended torque (20–25 N·m) to avoid cracking porcelain.
| Step | Action |
|---|---|
| 1 | Unpack and inspect bushings |
| 2 | Conduct insulation tests |
| 3 | Lift with double hooks |
| 4 | Thread lead wires, reinstall top cap |
| 5 | Tighten bolts diagonally, control torque |
| 6 | Verify oil level and alignment |
High Voltage Bushing Installation
Use a dedicated bushing installation frame and perform a full inspection. Bushings have four lifting eyes on the flange; attach the main rope to bear the weight.
Key points:
Inspection & Cleaning
Inspect for damage, loose fasteners, and clean all dust and dirt. Retest sealing if possible.
Position & Oil Adjustment
Place bushing on stand and adjust oil conservator to mid-upper level (~1/2–2/3 of oil gauge).
- Oil too high → internal pressure increases, may cause leakage.
- Oil too low → prevents observation.
Adjust oil via flange drain plug or conservator filling plug.
Lead Connection
Insert the high-voltage lead end into the threaded conductor rod; heat the phosphor bronze filler to fill and weld the joint.
Lifting & Alignment
Thread a nylon rope (≥2× bushing length, sufficient strength) through the conductor tube to the bushing tail. Lift to the transformer hole center; attach the rope end to the lead connector via M12 thread. Align oil gauge and gradually lower while pulling lead upward until the bushing rests on the transformer tank elevation base.
Top Structure Sealing
Dual-seal top: screw positioning nuts, insert cylindrical pins, remove nylon rope, screw lead connector into conductor head, and tighten with cross-head screws.
Improved top structure: directly insert pins, remove nylon rope, reinstall conductor head, tighten hex bolts, and ensure proper sealing. All rubber gaskets must be correctly positioned.
Medium and low voltage bushings are simpler than HV bushings, requiring only direct terminal connection with clean contact surfaces and proper torque control.
Common Bushing Installation Mistakes
Testing Before Energization
Let the bushing rest for at least 24 hours. Clean the surface dust and moisture to avoid external discharge affecting partial discharge (PD) measurements. Use measurement terminals to check tanδ and partial discharge. Remove protective caps, but do not disassemble or rotate terminals. Connect the measurement leads to the terminals and conduct tests per procedure. After testing, reinstall protective caps to ensure effective grounding.
Notes: Do not remove measurement leads during live monitoring. Do not remove protective caps during operation. Restore protective caps before energization. Bushings must rest vertically (≤30° deviation) for at least 24 hours before applying voltage.
Conclusion
For large 100 MVA transformers, transformer bushing installation is high-risk and high-impact. Proper control of installation angle, torque, sealing, and electrical testing directly determines transformer lifespan, operational safety, and maintenance costs. Following transformer installation rules, guidelines, and checklists ensures reliable transformer installation and safe operation.
