Oil-Immersed Transformer Selection Guide: Understanding Performance Differences Through Model Numbers

As the “heart” of power systems, oil-immersed transformers dominate the industry with their unique structural design. Transformer oil not only provides excellent insulation but also enables efficient heat dissipation through natural convection. This makes oil-immersed transformers particularly suitable for high-capacity, continuous operation scenarios. Compared to dry-type transformers, oil-immersed models offer smaller size, lower cost, and stronger overload capacity at equivalent ratings, making them the most widely used transformers in power systems.

Decoding Model Numbers: The Technical Language Behind Letters and Digits

1. Voltage Level Identification

• S11-M-100/10: The “10” indicates 10kV rated voltage on the high-voltage side
• S13-200/35: “35” represents 35kV voltage class
• Common voltage classes: 10kV, 35kV, 66kV, 110kV, etc.

2. Capacity Code Interpretation

• 50kVA: Small distribution transformer for small commercial/industrial use
• 315kVA: Medium transformer commonly used in residential complexes
• 2000kVA+: Large industrial transformers

3. Special Feature Markers

• R: Rolled core construction (e.g., S11-M.R)
• Z: On-load tap changer capability
• F: Fully sealed construction (e.g., SFZ11)

Energy Efficiency Classes and Model Evolution

China’s transformer efficiency standards have undergone multiple upgrades:

• S7 series: Obsolete high-energy-consumption products
• S9 series: Mainstream products around 2000
• S11/S13 series: Current high-efficiency models
• SH15 series: Amorphous alloy ultra-low loss transformers

From Class III to Class I efficiency, Class I transformers may cost 15-20% more initially but reduce operational losses by over 30%, typically recovering the price difference within 3-5 years.

Golden Rules for Selection

1. Capacity Selection: Choose 120% of actual load, avoiding both oversizing and insufficient capacity for future needs
2. Voltage Matching: Ensure system voltage compatibility with transformer tap changers
3. Environmental Adaptation:
   • Select fully sealed type (e.g., SF series) for outdoor use
   • Choose fire-resistant oil transformers for high fire-risk areas
   • Special low-temperature oil required for extreme cold regions
4. Efficiency Trade-offs: Prioritize Class I efficiency for annual operation exceeding 4,000 hours

Key Maintenance Points

1. Oil Quality Monitoring: Perform annual dissolved gas analysis
2. Temperature Control: Keep top oil temperature below 85°C
3. Breather Maintenance: Replace silica gel when 2/3 discolored
4. Regular Testing: Include winding resistance, insulation resistance measurements

Market Trends

1. Smart Features: Online monitoring systems for real-time data on oil temperature, level, and partial discharge
2. Eco-Friendly: Adoption of biodegradable transformer oils
3. Compact Designs: New structures like 3D wound cores reducing footprint

When selecting oil-immersed transformers, provide suppliers with these parameters: installation environment, load characteristics, voltage class, capacity requirements, and efficiency needs to receive optimal model recommendations. High-quality oil-immersed transformers can last over 25 years, with proper selection and maintenance being key to long-term reliable operation.