Oil-Immersed Transformers (S11-M Series)
Oil-immersed transformers use mineral oil or natural ester (vegetable oil) as both a cooling medium and electrical insulation. The core and windings are submerged in oil inside a sealed tank.
Where they excel:
- Outdoor installations where a concrete pad or pole mount is available
- Projects where initial cost is a primary concern — oil-filled transformers are generally 30-50% less expensive than dry-type of equivalent rating
- High-capacity applications up to 2500kVA and beyond
- Rural distribution networks where maintenance access is limited — the simpler construction is more forgiving
Key specifications (S11-M series):
- Capacity range: 30kVA to 2500kVA
- Primary voltage: up to 10kV / 20kV (dual-voltage design available)
- Cooling: ONAN (Oil Natural Air Natural) — self-cooling
- No-load loss: typically 130W to 2600W depending on rating
- Load loss: typically 600W to 24000W depending on rating
The downside:
- Fire risk — mineral oil is flammable. Indoor installation is restricted by fire codes in most countries
- Oil containment is required — you need an oil sump or bund wall to contain leaks
- Regular oil testing is recommended (dielectric strength, moisture content, dissolved gas analysis)
- Higher ongoing maintenance compared to dry-type
Dry-Type Transformers (SCBH15 Amorphous Alloy Series)
Dry-type transformers use cast resin (epoxy) insulation instead of oil. The windings are vacuum-cast in epoxy resin, which provides excellent electrical insulation and mechanical protection. The SCBH15 series uses an amorphous alloy core that reduces no-load losses dramatically.
The amorphous alloy advantage: The SCBH15 series uses a metallic glass core instead of conventional grain-oriented silicon steel. The core material is manufactured by rapid solidification (cooling at roughly 1 million degrees Celsius per second), creating a non-crystalline atomic structure with very low magnetic losses.
Key specifications (SCBH15 series):
- Capacity range: 100kVA to 2500kVA
- Primary voltage: 10kV / 11kV / 10.5kV
- No-load loss reduction: 60-80% lower than conventional silicon steel core transformers
- Insulation class: F or H (selectable)
- Temperature rise: 100K (F-class) or 125K (H-class)
The downside:
- Higher initial cost — typically 30-50% more than an oil-immersed equivalent
- More sensitive to overloading — dry-type transformers have less thermal mass than oil-filled units
- Requires clean, well-ventilated installation space
- Higher audible noise level compared to oil-immersed (typically 55-65 dB vs 45-55 dB)
Direct Comparison
| Aspect | S11-M Oil-Immersed | SCBH15 Dry-Type (Amorphous) |
| Capacity range | 30-2500 kVA | 100-2500 kVA |
| Primary voltage | 10kV / 20kV | 10kV / 11kV / 10.5kV |
| No-load loss | Standard (silicon steel core) | 60-80% lower (amorphous core) |
| Fire safety | Moderate — oil is flammable | Excellent — no oil, self-extinguishing |
| Indoor installation | Restricted by fire codes | Approved for all indoor installations |
| Maintenance | Periodic oil testing required | Minimal — cleaning only |
| Installation location | Outdoor preferred | Indoor preferred |
| Relative cost | 1.0 (baseline) | 1.3 – 1.5 |
| Typical service life | 20-25 years | 25-30 years |
Lifetime Cost Analysis
Here is something worth considering: although dry-type transformers cost more upfront, the total cost of ownership over 20 years can be lower in many scenarios.
For a 1000kVA transformer operating continuously, the annual energy saving from using an amorphous alloy core (SCBH15) versus conventional silicon steel (S11-M) is approximately 10,000-15,000 kWh in core loss alone. At an industrial electricity rate of $0.10/kWh, that is $1,000-$1,500 per year in savings.
When to Choose Which
Choose oil-immersed (S11-M) when:
- The transformer will be installed outdoors on a pole or pad
- Budget is the primary constraint
- Fire safety regulations permit oil-filled equipment
- Maintenance staff are available for periodic oil testing
Choose dry-type (SCBH15) when:
- The transformer will be installed indoors or in a fire-sensitive area
- Minimal maintenance is required
- Energy efficiency is a priority (the amorphous core pays back over time)
- High humidity or condensation is present in the installation environment
- Environmental regulations restrict oil-filled equipment