• Transformer loss refers to the process of converting electrical energy into other forms of energy (mainly heat) during transformer operation.
• Loss is an important indicator of transformer performance evaluation, which directly affects the efficiency and economy of the transformer.
No load loss
No-load loss refers to the power loss of the transformer when the primary side is combined with the rated voltage under the condition of no load (that is, the secondary side is open). No-load loss mainly includes iron loss and a small amount of other losses, such as dielectric loss and excitation current loss. It reflects the energy loss of the transformer under no-load conditions.
Iron loss, also known as core loss or core loss, is the energy loss caused by the alternating magnetic field in the core material during transformer operation. The iron loss mainly includes two parts: hysteresis loss and eddy current loss.
Hysteresis loss:
Hysteresis loss is the energy loss in the process of magnetization and demagnetization of the core material under the alternating magnetic field. When the direction of the magnetic field changes, the magnetic domains in the core material need to be rearranged, and there will be energy loss in this process. The hysteresis loss is related to the area of the hysteresis loop of the core material, the operating frequency and the magnetic flux density
Eddy current loss:
Eddy current loss is the heat loss generated when the current (eddy current) induced by the iron core material flows inside the iron core under the alternating magnetic field. The alternating magnetic field induces eddy current in the iron core, which forms a loop in the iron core material and generates heat, resulting in energy loss. Eddy current loss is related to the resistivity of the iron core material, the thickness of the laminate and the working frequency.
Dielectric Losses is the energy loss caused by the insulating material in the transformer under the action of alternating electric field. Insulation materials in transformers are often used to isolate conductive parts to prevent short circuits and other electrical failures. The dielectric loss mainly occurs inside the insulating material due to the dielectric properties of the material and the alternating electric field.
Insulation loss:
This is the main part of the dielectric loss. Insulation materials are used in transformers to isolate windings, iron cores and other conductive parts. The alternating electric field inside the insulating material will cause the polarization of the dielectric and the current leakage, and these processes will lead to energy loss. Common insulating materials include transformer oil, paper and resin.
Capacitance effect loss:
The winding and insulation structure of the transformer may form parasitic capacitance. Under the alternating electric field, the charging and discharging process of these capacitors will also cause energy loss.
Partial discharge at high voltage
Under high voltage conditions, Partial Discharge may occur inside or on the surface of the insulation material, which is a local electrical breakdown phenomenon that leads to energy loss and gradual deterioration of the insulation material.
Dielectric losses are usually small, but may appear relatively significant in high voltage transformers. The loss is related to the dielectric constant of the insulation material, the tangent of the loss Angle (tanδ) and the working voltage and frequency of the transformer.
The dielectric loss in the no-load loss of the transformer is relatively small, usually between 0.5% and 2%, but in the high-voltage transformer requires special attention and control.
The excitation current loss is due to the I²R loss in the primary winding caused by the excitation current required to establish the magnetic field in the core. Even if there is no load, the primary side still needs a certain current to maintain the magnetization of the core, and this part of the current produces heat loss on the winding resistance.
Stray Losses are losses in a transformer caused by leakage flux. Leakage flux refers to those that do not centrally leak through the transformer core, but through other paths (such as transformer structural components, clips, tank walls, etc.). Because these leakage flux induce eddy currents in the conductive material, the energy loss is caused, and this part of the loss is called stray loss.
Clamping Losses: Eddy current losses induced by leakage flux in transformer clips (such as core clips, support structures).
Tank Wall Losses: Eddy current losses induced by leakage flux in the tank wall of a transformer. Because fuel tanks are usually made of metal, eddy current losses can be significant in these metal structures.
Lead and end-frame Losses: Eddy current losses induced by leakage flux in transformer leads, End frames, and other metal structures.
Stray losses are generally a small fraction of the total loss of a transformer, but can become significant in high-capacity transformers. Therefore, in transformer design, it is important to reduce the influence of leakage flux and optimize the design of core and structural components to reduce stray loss.
Load loss
• Load loss refers to the electrical energy loss caused by the load current passing through the transformer winding under the load operation condition.
The copper loss of a transformer is the loss of resistance in the winding of the transformer (copper or aluminum) due to the current flowing through the conductor. It is a major component of load loss and increases significantly as the load current increases.
DC Resistance Loss
Loss caused by the DC resistance of the winding. The formula is 
Where I is the load current and R is the winding resistance.
• Leakage Magnetic Losses
Eddy current loss in structural components (such as clips, steel plates, box walls, etc.) due to the leakage flux of the transformer.
• Additional Copper Losses
Additional loss due to uneven current distribution due to skin effects and proximity effects. These effects are more pronounced at high loads or frequencies.
• Mechanical Losses
Loss due to mechanical vibration and noise inside the transformer. This part of the loss in the total loss accounted for a small proportion.
• Cooling Equipment Losses
In oil-immersed transformers, the fans and oil pumps used for cooling consume electrical energy, and these devices generate losses during operation.
Temperature and loss
• No load loss
The effect of temperature on no-load loss is small, mainly affecting the resistivity of the core material, but the change is little. The main component iron loss (hysteresis loss and eddy current loss) is not sensitive to temperature change.
• Load loss
Temperature has a great influence on the load loss, mainly because the resistivity of the conductor in the load loss increases significantly with the change of temperature. As a result, the winding resistance increases with the increase of temperature, so that the copper loss generated by the current through the winding increases significantly. Since copper loss is the main component of load loss, the influence of temperature increase on load loss is more obvious.
Where,
I= load current
R= resistance
ρ= resistivity
L= length of wire
S= cross-sectional area of the wire
The standard reference temperature of the transformer’s no-load loss and load loss is to ensure the consistency of the test and performance evaluation. International and national standards usually specify reference temperatures for these tests.
| Reference temperature | IEC | IEEE | CSA |
| No load loss | No calibration required | 20℃ | 85℃ |
| On load loss | 75℃ | 85℃ | 85℃ |
The main reason for correcting the loss of the transformer to the standard reference temperature is to ensure comparability and consistency of the test results under different conditions.
Temperature effect
Transformer losses, especially load losses, are significantly affected by temperature. The winding resistance increases with increasing temperature, resulting in increased load loss. By correcting the loss value to a standard reference temperature, the effect of temperature changes can be eliminated, making the results comparable under different test conditions
standardization
Using a uniform reference temperature, such as 75 or 85°C, ensures consistency and standardization of transformer test results across manufacturers, models and test times
Efficiency of transformer
• The efficiency of the transformer refers to the ratio between the input power and the output power, usually expressed as a percentage, and the simplified understanding formula is as follows
