How to calculate transformer load loss

In the power system, transformer is one of the important devices for power transmission and distribution, and its operating efficiency directly affects the energy utilization rate of the whole system. Transformer will produce certain losses during operation, mainly including no-load loss and load loss. Among them, the load loss changes with the transformer load, which is usually of great significance in design, selection and economic operation analysis. This paper will focus on the definition, composition and calculation method of transformer load loss.

1. What is transformer load loss?

The load loss of transformer (also called short-circuit loss) refers to the energy loss caused by winding resistance at rated current when the transformer is running with load. This part of the loss is proportional to the square of the load current, so it is also called "copper loss". Load loss mainly occurs in the primary and secondary windings of the transformer, which is one of the main sources of transformer heat, and has a direct impact on the efficiency, temperature rise and service life of the transformer.

Second, the composition of load loss

Load loss mainly includes the following aspects:

1. Basic copper loss: This is due to Joule heat generated by the resistance of the winding itself when the current passes.
2. Additional loss: including eddy current loss and proximity effect loss in the conductor, which is usually more obvious in high-frequency or large-capacity transformers.
3. Lead loss: the loss of internal leads and connecting parts of the transformer under high current.
4. Loss of structural parts: eddy current loss caused by magnetic leakage of metal components such as clamps and oil tanks.

Third, the calculation method of load loss

1. Experimental measurement method

Before the transformer leaves the factory, a short-circuit test is usually carried out to measure the load loss. In the experiment, the low voltage side is short-circuited, and the rated frequency voltage is applied to the high voltage side, so that the rated current flows in the winding. At this time, the measured power loss is the load loss. This method is accurate and reliable, and is suitable for the performance evaluation of actual transformers.

2. Theoretical calculation method

In the design stage or when the experiment is impossible, the load loss can be calculated by theoretical formula:

-The basic copper loss can be calculated by the formula:

[
P_{cu} = I^2 cdot R
]

Where ( I ) is rated current and ( R ) is winding DC resistance.

-The additional loss is usually estimated as a percentage of the basic copper loss, for example, 5%~10% of the basic copper loss.

Therefore, the total load loss can be expressed as:

[
P_{k} = P_{cu}+P_{ additional} = i 2r+ alpha i 2r = i 2r (1+ alpha).
]

Where ( alpha ) is the additional loss coefficient, which is usually between 0.05 and 0.1.

3. Standard specification method

According to the national standards GB/T 6451-2015 and GB/T 1094.1-2013, there are clear methods and standards for testing and calculating load loss, which should be strictly followed in the design and manufacturing process.

Four, the factors affecting the load loss

1. Load current: The greater the load current, the higher the copper loss; The load loss is proportional to the square of current.
2. Winding material and cross-sectional area: Using high conductivity materials (such as oxygen-free copper) and increasing conductor cross-sectional area can effectively reduce resistance and thus reduce loss.
3. Temperature: The winding resistance increases with the increase of temperature, so the load loss will increase in high temperature environment.
4. Frequency and harmonics: High-frequency or non-sinusoidal current will cause eddy current loss and skin effect, resulting in increased additional loss.

V. Significance and Measures of Reducing Load Loss

Reducing load loss can not only improve the operation efficiency of transformer and reduce energy waste, but also help to prolong the service life of equipment and reduce the operation cost. Specific measures include:

-Use highly conductive materials;
-Optimizing winding design and reducing resistance;
-Improve the cooling system and control the temperature rise;
-Reasonable selection of transformer capacity to avoid light load or overload operation;
-Use energy-saving transformers, such as amorphous alloy transformers.

VI. Conclusion

To sum up, the calculation of transformer load loss is an important basis for evaluating transformer performance and economic operation. Accurate calculation and effective control of load loss can not only improve the overall efficiency of power system, but also achieve the goal of energy saving and emission reduction. In practical application, the design and operation mode of transformer should be comprehensively judged and optimized by combining experimental test, theoretical analysis and standard specification.