Determining a transformer bushing typically involves selecting or designing a bushing that meets the electrical, mechanical, and thermal requirements of the transformer. It’s not about a single formula, but rather a design process based on key parameters. Below is a structured breakdown:
1. Electrical Rating Calculation
This defines the voltage insulation level and current carrying capacity of the bushing.
a. Voltage Rating
- Must match or exceed the transformer’s system voltage and Basic Insulation Level (BIL).
- BIL is chosen based on IEC/IEEE standards for system voltage.
| System Voltage (kV) | Typical BIL (kV) |
|---|---|
| 66 | 325 |
| 132 | 550 |
| 220 | 1050 |
| 400 | 1425 |
b. Current Rating
- Based on load current of transformer winding.
- Bushing conductor must have sufficient cross-section for thermal limits (e.g., 1200 A, 2000 A, etc.).
Use:I = S / (√3 × V)
Where:
- I = current (A)
- S = apparent power (VA)
- V = line voltage (V)
2. Thermal Considerations
- Bushing must dissipate heat generated by load current.
- Important for high-current bushings.
- Use manufacturer’s thermal curve or simulation to ensure temperature rise stays within limits.
3. Creepage Distance Calculation (for outdoor bushings)
- Determines length of the insulating surface to avoid flashover.
Required Creepage Distance (mm) =Pollution Severity Factor × System Voltage (kV)
| Pollution Level | Creepage Factor (mm/kV) |
|---|---|
| Light | 16 |
| Medium | 20 |
| Heavy | 25 |
| Very Heavy | 31 |
Example:
For a 220 kV system in a heavily polluted area:Creepage distance = 220 × 25 = 5500 mm
4. Mechanical Strength
Ensure mechanical strength is adequate to withstand:
- Short-circuit forces
- Terminal weight and wind loading (especially for outdoor bushings)
5. Type of Bushing (Based on Insulation Type)
Choose based on application, environment, and voltage:
- OIP (Oil-Impregnated Paper)
- RIP (Resin-Impregnated Paper)
- Porcelain or Polymer Composite housing
- Dry-type for indoor use
6. Mounting Dimensions
- Based on transformer tank design.
- Must match standardized flange sizes (e.g., per IEC 60137 or ANSI standards).
- Determine oil-side and air-side lengths, depending on transformer height and clearances.
Summary Checklist for Bushing Calculation
| Parameter | Consideration |
|---|---|
| Voltage Rating | ≥ Transformer system voltage and BIL |
| Current Rating | ≥ Transformer full-load current |
| Creepage Distance | Based on pollution class |
| Insulation Type | OIP, RIP, RBP, etc. |
| Thermal Design | Acceptable temperature rise |
| Mechanical Requirements | Short-circuit and wind force withstand |
| Mounting & Dimensions | Flange size, air/oil side length |
