1. Key Features & Applications

• Utility Networks: e.g., 13.8 kV -> 480 V
• Industrial Plants: 13.8 kV -> 480 V
• Commercial Buildings: Reliable power for data centers, hospitals
• Renewables: Grid interface (e.g., 34.5 kV collection systems)

Typical Ratings

• Voltage: 5 kV – 35 kV (ANSI/IEEE)
• Power: 500 kVA – 20 MVA
• Cooling: ONAN, ONAF, KNAN

2. Design & Construction

A. Core & Windings

• Core: Grain-oriented silicon steel
• Insulation: Kraft paper + oil
• Windings: MV-rated primary (e.g., 15 kV, 95 kV BIL), LV secondary (e.g., 480 V)

B. Tank & Cooling

• Sealed tank: Free-breathing or conservator
• Cooling: Radiators/fins, nitrogen blanket

C. Bushings & Accessories

• Tap Changers: NLTC (manual), OLTC (automatic)
• Monitoring: Sudden pressure relay, Buchholz relay, temperature gauges

3. US Standards & Compliance

• ANSI/IEEE C57.12.00 – General requirements
• IEEE C57.12.90 – Test codes
• NEMA TR-1 – Transformer standards
• DOE 2016 – Efficiency rules
• EPA & NFPA 70 – Environmental & fire safety

4. Oil-Filled vs. Dry-Type Comparison

5. Maintenance Overview

A. Common Issues

• Oil degradation, overheating, bushing failures, internal faults

B. Best Practices

• DGA testing (dissolved gas analysis)
• Oil filtration, infrared scans, leak checks
• Resistance testing (windings & bushings)

6. Future Trends

• Biodegradable ester fluids
• IoT-based smart monitoring
• Modular, space-saving substation designs

In Summary

Medium voltage oil-filled transformers are essential for reliable, efficient power delivery across the U.S. With proper maintenance and evolving smart technologies, they remain a top choice for utilities, industries, and renewable integrations.