Analysis of the Relationship Between Oil Quantity and Capacity in Oil-Immersed Transformers

　　The oil quantity in oil-immersed transformers is closely related to their capacity, but this relationship is not a simple linear proportion. Below is an analysis from four dimensions: design principles, influencing factors, calculation methods, and practical applications.

　　I. Design Principles and Core Relationships

　　1. Core Functions of Oil Quantity

　　Insulating Medium: Transformer oil (e.g., mineral oil, natural ester, or synthetic ester) fills the gaps between windings and the core, providing electrical insulation (breakdown voltage ≥35 kV/2.5 mm).

　　Heat Dissipation Carrier: It carries away heat through natural convection or forced oil circulation (e.g., oil temperature rise in a 1000 kVA transformer must be controlled ≤65 K).

　　Protective Function: It isolates air to prevent winding oxidation and suppresses partial discharge.

　　2. Nonlinear Relationship Between Capacity and Oil Quantity

　　Capacity (kVA): Reflects the transformer's ability to transmit electrical energy and is directly related to the winding cross-sectional area and core size.

　　Oil Quantity (L): Depends on the internal volume of the transformer (tank size), heat dissipation requirements, and insulation grade.

　　Typical Ratios: For small-to-medium transformers (e.g., 100–2500 kVA), oil quantity is approximately 5%–15% of capacity; for large transformers (e.g., >10 MVA), this ratio may drop to 3%–8% due to improved heat dissipation efficiency.

　　II. Key Factors Influencing Oil Quantity

　　1. Transformer Capacity and Voltage Level

　　As capacity increases, winding volume and core size grow, but tank design may become more compact (e.g., using corrugated tank technology), leading to a decrease in the oil quantity ratio.

　　Higher voltage levels require greater insulation distances, potentially increasing the oil quantity ratio (e.g., 110 kV transformers may have a 2%–3% higher oil quantity ratio than 35 kV transformers).

　　2. Cooling Methods

　　Oil-Immersed Natural Air Natural (ONAN): Relies on natural convection, requiring a larger tank size and higher oil quantity ratio (e.g., 800–1200 L for a 1000 kVA ONAN transformer).

　　Forced Oil Circulation (OFAF/ODAF): Uses oil pumps to accelerate heat dissipation, allowing for a smaller tank and lower oil quantity ratio (e.g., 20%–30% reduction in oil quantity for the same capacity compared to ONAN).

　　3. Insulating Materials and Processes

　　Using oils with high dielectric constants (e.g., natural ester oils) or optimizing winding structures (e.g., reducing interlayer insulation paper thickness) can reduce oil quantity requirements.

　　Sealed designs (e.g., fully sealed structures) reduce oil evaporation but do not change the initial oil filling quantity.

　　III. Calculation Methods and Examples of Oil Quantity

　　1. Empirical Formula Estimation

　　Small-to-Medium Transformers: Oil quantity (L) ≈ Capacity (kVA) × (0.05–0.15)

　　Example: For an 800 kVA transformer, oil quantity ≈ 800 × 0.1 = 80 L (actual may be 600–1000 L due to heat dissipation redundancy requirements).

　　Large Transformers: Requires calculation based on heat dissipation design (e.g., radiator area) and oil circulation efficiency, with data typically provided by the manufacturer.

　　2. Manufacturer Standards Reference

　　S11 Series (10 kV Level):

　　110 kV Level (e.g., SFZ11 Series):

　　A 50 MVA transformer has an oil quantity of approximately 12–15 tons (ratio ≈2.4%–3%) due to high-voltage insulation requirements.

　　IV. Practical Application Considerations

　　1. Redundancy Design for Oil Quantity

　　Actual oil filling quantities are typically 10%–20% higher than theoretical calculations to compensate for volume expansion due to oil temperature changes (e.g., oil volume expands by ~7% when temperature rises from 20°C to 90°C).

　　2. Oil Level Monitoring and Maintenance

　　Install oil level gauges (e.g., magnetic flap level indicators) and temperature controllers to ensure oil levels remain within upper and lower limits (usually 1/4 to 3/4 of tank height).

　　Regularly top up or replace oil (as per DL/T 572 standard, replacement is recommended after 15 years of operation).

　　3. Environmental Protection and Safety

　　Waste oil must be treated as hazardous waste (e.g., handed over to qualified recycling units) to avoid environmental pollution.

　　In high-altitude areas (e.g., >1000 m above sea level), oil quantity must be increased (by ~1% for every 100 m increase in altitude) to compensate for reduced heat dissipation efficiency due to lower air density.

　　V. Conclusions and Recommendations

　　1. Core Conclusions

　　The oil quantity in oil-immersed transformers is weakly positively correlated with capacity but is significantly influenced by cooling methods, voltage levels, and design processes.

　　Users should rely on manufacturer-provided oil quantity data during selection and consider operational environments (e.g., temperature, altitude) and redundancy requirements.

　　2. Recommended Measures

　　Design Phase: Prioritize efficient cooling technologies (e.g., forced oil circulation) to reduce oil quantity and lower costs.

　　Operation and Maintenance Phase: Install intelligent oil level monitoring systems and combine with infrared thermography to detect abnormal oil levels or local overheating in advance.

　　Environmental Upgrades: Gradually replace mineral oil with natural ester or synthetic ester oils to enhance biodegradability (e.g., natural ester oils have a degradation rate >98%).

　　By reasonably designing the matching relationship between oil quantity and capacity, a balance can be achieved among safety, economy, and environmental protection for transformers.