Transformer losses can be divided into two main components:
1. No-load losses and
2. Load losses.
Thermal ratings of a transformer are determined by the following 3 factors:
• The amount of heat produced in the windings and connections.
• The amount of heat produced in the iron core.
• How effectively the heat can be removed from the transformer?
When the thermal rating of the transformer is reached, At this point, the heat being produced must equal the heat being removed or dissipated called – thermal equilibrium. The efficiency of power transformers is high, especially, for large transformers at full load. However, losses are present in all transformers.
- Transformer No-load Losses:
- Iron/Core losses:
Iron losses are Eddy current loss and Hysteresis losses. Iron loss magnitude is quite small. The Iron losses depend upon the supply frequency, the maximum flux density of the core, Volume of the core.
At no load, Primary copper losses are very small and may be neglected. Hence at no load, all the input is utilized to supply Iron losses only. These types of losses are common to all types of transformers, regardless of transformer application or power rating.
When any transformer is operating at its rated voltage and frequency, it will be operating with its rated value of flux in the core. If the voltage rises while the frequency remains constant, or the frequency falls while the voltage remains constant, the core flux will increase. The core will heat up due to the effects of hysteresis and eddy currents in the core.
A voltage increase of 10% above the rated value will give a flux level of 10% above its rated value. If the flux level is 10% above normal, the iron has commenced to saturate.
As soon as iron begins to saturate, the heating, due to the eddy currents and hysteresis effects increases rapidly as shown in the below figure. For this reason, the voltage applied to a transformer should never be allowed to exceed the rated value by more than 10%.
Core losses in transformers, which account for the majority of distribution losses at low power, can be reduced by improved core materials and by reducing harmonics.
Non-linear loads, such as variable speed drives, computers, UPS systems, TV sets, and compact fluorescent lamps, cause harmonic currents on the network.
Harmonic voltages are generated by the harmonic load currents in the impedance of the network. Harmonics increase the load and no-load losses due to increased skin effect, eddy current, stray, and hysteresis losses.
The most important of these losses is that due to eddy current losses in the winding; it can be very large and consequently most calculation models ignore the other harmonic-induced losses.
2. Transformer Load Losses:
- Copper losses:
Copper losses are resistive and proportional to load current and are sometimes called “load losses” or “I2R losses”. As the transformer is loaded, heat is produced in the primary and secondary windings and connections due to I2R. At low loads, the quantity of heat produced will be small but as load increases, the amount of heat produced becomes significant. The windings will operate at or near their design temperature at full load.
The figure below shows the relationship between load current and the heat produced in transformer windings and connections.
Copper losses, which become more important at high power levels, can be reduced by several methods, including increasing the voltage of the distribution lines, shunt compensation, reduction of harmonics, load balance, and demand side management. Several methods can be employed to reduce the harmonics and consequently improve the transformer efficiency such as using filters, and zigzag transformers.
