In the whole life cycle of a transformer, loss is one of the core indicators for measuring its economic efficiency. An oil-immersed transformer usually operates for 20 to 30 years, and the electricity cost incurred by losses during this period is likely to exceed the purchase cost of the equipment itself. Therefore, understanding the essence of no-load loss and load loss and mastering the methods to reduce losses are crucial for optimizing equipment selection and saving operating costs.

I. No-Load Loss: The Persistent "Fixed Cost"

1. What is No-Load Loss?

No-load loss refers to the loss generated when the secondary side of a transformer is open-circuited (with no load connected) and the rated voltage is applied to the primary side. As long as the transformer is connected to the power supply, this part of the loss persists continuously, even if it supplies no power to any equipment.

No-load loss mainly stems from core loss (iron loss), including:

• Hysteresis loss: Heat generated by the friction of magnetic domains when the iron core is repeatedly magnetized in an alternating magnetic field
• Eddy current loss: Thermal loss produced when alternating magnetic flux induces circulating currents inside the iron core, which flow through the core resistance

2. Measurement of No-Load Loss

No-load loss is determined by a no-load test: apply the rated voltage to the primary side and measure the input power, with the obtained value being the no-load loss. This value mainly depends on the core material, magnetic flux density and core structure.

II. Load Loss: The "Variable Cost" That Changes with the Load

1. What is Load Loss?

Load loss refers to the loss generated by a transformer when it outputs the rated current. This part of the loss changes with the load current and acts as the "variable cost" during operation.

Load loss mainly includes:

• Winding resistance loss (copper loss): Thermal loss caused by resistance when current flows through the winding conductors
• Additional loss: Including conductor eddy current loss induced by the skin effect and proximity effect, as well as stray loss generated by leakage flux in metal structural components such as oil tank walls and clamps

2. Measurement of Load Loss

Load loss is measured by a short-circuit test: short-circuit the secondary side, apply a voltage to the primary side to make the current reach the rated value, and measure the input power. The obtained value is the load loss (converted to a reference temperature, usually 75℃ or 85℃).

III. Total Loss and Energy Efficiency Grade

Total transformer loss = No-load loss + Load loss. Among them, no-load loss exists constantly around the clock, while load loss changes with the actual load rate.

IV. How to Reduce No-Load Loss?

• Select high-quality silicon steel sheets: Using high magnetic permeability grain-oriented silicon steel or amorphous alloy cores can significantly reduce hysteresis loss and eddy current loss.
• Optimize the core structure: Adopt processes such as stepped joints, hole-free lashing and laser scribing to reduce local losses.
• Select the capacity reasonably: Avoid the "over-sized transformer for light load" situation, select the appropriate capacity according to actual load requirements, or adopt parallel operation of multiple transformers and cut off some transformers during light load operation.

V. How to Reduce Load Loss?

• Use high-conductivity conductors: Copper windings have 30-40% lower load loss than aluminum windings.
• Optimize winding design: Increase the cross-sectional area of conductors and adopt transposed conductors to suppress the skin effect and proximity effect.
• Control additional loss: Use non-magnetic materials or install magnetic shielding at parts affected by leakage flux to reduce stray loss.
• Ensure good heat dissipation: Winding resistance increases with the rise of temperature, so keeping the heat dissipation system unobstructed can effectively reduce losses.

VI. Conclusion

• No-load loss and load loss represent the "fixed cost" and "variable cost" of a transformer respectively. Reducing losses runs through the entire life cycle of a transformer, from material selection and structural optimization to capacity matching and operation management. While paying attention to the initial purchase cost, focusing on the whole life cycle cost and selecting high-efficiency oil-immersed transformers that meet the actual working conditions is the key path to achieve energy conservation, consumption reduction and lower operating costs.