I. Core and Insulation Fundamentals

• Core Material: Typically made of silicon steel (for low hysteresis loss) or advanced materials like nanocrystalline steel (for high precision).

• Function: Step down high primary voltages (e.g., 115 kV) to safe secondary levels (e.g., 120 V) for metering and protection.

• Insulation Goal: Prevent electrical breakdown between windings and ground, while dissipating heat.

II. Oil-Immersed Voltage Transformers

Design and Construction

• Insulation Medium: Mineral oil (or synthetic fluids like silicone oil) fills the transformer tank, providing:

• Electrical insulation between windings and the case.

• Thermal conductivity to dissipate heat generated during operation.

• Enclosure: Heavy-duty steel or cast-iron tank, sealed to prevent moisture ingress. Bushings (porcelain or composite) protrude for high-voltage connections.

• Cooling: Natural convection of oil (self-cooled) or forced cooling in large units.

Advantages

1. Superior Insulation Strength:

• Oil has a high dielectric strength (up to 30–50 kV/mm), enabling reliable operation at very high voltages (e.g., 34.5 kV to 345 kV and above).

• Self-healing properties: Oil can quench small internal arcs or discharges.

2. Thermal Performance:

• Efficient heat dissipation allows continuous operation under heavy loads without overheating.

3. Long Lifespan:

• Properly maintained units (e.g., regular oil testing and drying) can last 30–50 years, making them suitable for long-term infrastructure.

4. Environmental Resistance:

• Sealed design protects against moisture, dust, and salt fog, ideal for coastal or industrial areas.

Limitations

1. Fire and Environmental Risks:

• Mineral oil is flammable and poses a spill hazard. Regulatory restrictions (e.g., EPA rules in the U.S.) apply to installation and disposal.

2. Weight and Size:

• Bulky and heavy (e.g., a 115 kV oil-immersed VT may weigh 500–1,000 kg), requiring heavy equipment for installation.

3. Maintenance Intensive:

• Regular oil testing, filtering, and replacement are needed to prevent degradation from moisture or contaminants.

4. Cost:

• Higher upfront cost due to tank, oil, and sealed enclosure.

Typical application scenarios

• High-Voltage Substations: 69 kV and above systems, where reliable insulation and thermal management are critical (e.g., transmission grids).

• Harsh Environments: Coastal regions (salt resistance), industrial zones (pollution tolerance), or high-humidity areas.

• Large-Scale Power Infrastructure: Hydroelectric, thermal, or nuclear power plants.

III. Dry-Type Voltage Transformers

Design and Construction

• Insulation Medium: No liquid; uses solid insulation materials like:

• Resin Casting: Windings are encapsulated in epoxy or polyurethane resin, providing mechanical strength and electrical isolation.

• Air Insulation: Simple designs (low voltage) rely on air gaps and insulation coatings (e.g., varnish).

• Enclosure: Lightweight aluminum or polymer cases, often with ribbed surfaces for natural cooling. Composite or porcelain bushings may be used for high-voltage connections.

Advantages

1. Fire and Environmental Safety:

• Non-flammable and spill-free, meeting strict environmental regulations (e.g., indoor or urban installations).

2. Lightweight and Compact:

• Up to 50% lighter than oil-filled counterparts at the same voltage class (e.g., a 34.5 kV dry-type VT may weigh 100–200 kg).

3. Low Maintenance:

• No oil to test or replace; suitable for remote or hard-to-access locations.

4. Cost-Effective for Low/Medium Voltages:

• Lower upfront and lifecycle costs for systems below 35 kV.

Limitations

1. Voltage and Power Constraints:

• Practical voltage limit ~35 kV (some advanced resin designs reach 69 kV, but with reduced capacity).

• Lower thermal conductivity than oil, limiting continuous power handling (e.g., 50–200 VA ratings vs. 1,000+ VA for oil-filled units).

2. Environmental Sensitivity:

• Resin can degrade in prolonged exposure to UV radiation or extreme temperatures (>90°C), requiring UV-stabilized coatings.

• Less tolerant of contamination (e.g., salt or dust) without periodic cleaning.

3. Mechanical Vulnerability:

• Resin cores are brittle; rough handling during installation can cause cracks.

Typical application scenarios

• Distribution Grids: 12 kV to 35 kV systems in urban or suburban areas (e.g., pole-mounted or pad-mounted transformers).

• Renewable Energy Systems: Solar farms, wind turbines, or microgrids where fire safety and low maintenance are priorities.

• Indoor/Outdoor Hybrid Settings: Industrial plants with outdoor switchyards but strict safety codes (e.g., refineries, data centers).

IV. Key Performance Comparison

V. Selection Criteria for Outdoor Applications

Choose Oil-Immersed When:

• High Voltage (≥34.5 kV) and/or high power (≥500 VA) are required.

• Harsh Environments (e.g., coastal, industrial) demand robust, moisture-resistant insulation.

• Long Lifespan and minimal replacement frequency are priorities (e.g., in remote substations).

Choose Dry-Type When:

• Low to Medium Voltage (≤35 kV) and moderate power (≤200 VA) suffice.

• Fire Safety or environmental regulations prohibit oil use (e.g., near residential areas).

• Easy Installation and Low Maintenance are critical (e.g., temporary or mobile substations).

VI. Emerging Trends

• Composite Insulation Materials: Advanced resins with improved thermal stability (e.g., silicone-based composites) are extending dry-type VTs to 69 kV and higher.

• Gas-Insulated VTs: SF₆ or nitrogen-insulated designs offer oil-like performance without flammability, though SF₆ faces environmental restrictions.

• Digital/Virtual VTs: Non-conventional VTs (e.g., optical or capacitive voltage dividers) are gaining traction in smart grids, but traditional oil and dry-type designs remain dominant for legacy systems.

Conclusion

• Oil-Immersed VTs are the workhorse for high-voltage transmission, offering unmatched insulation and thermal performance in harsh environments.

• Dry-Type VTs excel in distribution networks, prioritizing safety, lightweight design, and low maintenance.