Efficiency, Losses and Heat

An ideal transformer would have no losses, and would therefore be 100% efficient.
In practice energy is dissipated due both to the resistance of the windings (known as load loss), and to magnetic effects primarily attributable to the core (known as iron loss). Transformers are in general highly efficient, and large power transformers (around 100 MVA and larger) may attain an efficiency as high as 99.75%. Small transformers such as a plug-in used to power small consumer electronics may be less than 85% efficient.

Efficiency Dry-Type Transformers.

Transformers reduce the voltage of the electricity supplied by your utility to a level suitable for use by the electric equipment in your facility. Since all of the electricity used by your company passes through a transformer, even a small efficiency improvement will result in significant electricity savings. High-efficiency transformers are now available that can reduce your facility's total electricity use by approximately 1 percent. That's good for your company; it's also good for the environment. Reduced electricity use provides cost savings for your company; it also reduces air emissions from electricity generation.

Two types of energy losses occur in transformers: Load and No-Load losses.

Load losses result from resistance in the copper or aluminum windings. Load losses (also called winding losses) vary with the square of the electrical current (or load) flowing through the windings. At low loads (e.g. under 30 percent loading), core losses account for the majority of losses, but as the load increases, winding losses quickly dominate and account for 50 to 90 percent of transformer losses at full load. Winding losses can be reduced through improved conductor design, including proper materials selection and increases in the amount of copper conductor employed.
No-load losses result from resistance in the transformer's laminated steel core. These losses (also called core losses) occur whenever a transformer is energized and remain essentially constant regardless of how much electric power is flowing through it. To reduce core losses, high-efficiency transformers are designed with a better grade of core steel and with thinner core laminations than standard-efficiency models. As well, new transformer core designs are emerging that use amorphous metal instead of the traditional silicon steel. These amorphous core transformers, available from major transformer manufacturers including GE, ABB and Howard Transformers, offer up to 80 percent lower core losses than conventional transformers.
Total transformer losses are a combination of the core and winding losses. Unfortunately, some efforts to reduce winding losses increase core losses and vice versa. For example, increasing the amount of conductor used reduces the winding losses, but it may necessitate using a larger core, which would increase core losses. Manufacturers are developing techniques that optimize these losses based on the expected loading.

The loses arise from:

• Winding resistance: Current flowing through the windings causes resistive heating of the conductors.
• Eddy currents: Induced currents circulate in the core and cause it resistive heating.
• Stray losses: Not all the magnetic field produced by the primary is intercepted by the secondary. A portion of the leakage flux may induce eddy currents within nearby conductive object such as the transformers support structure, and be converted to heat. The familiar hum or buzzing noise heard near transformers is a result of stray fields causing components of the tank to vibrate, and is also from magnetostriction vibration of the core.
• Hysteresis losses: Each time the magnetic field is reversed, a small amount of energy is lost to hysteresis in the magnetic core. The level of hysteretic is affected by the core material.
• Mechanical losses: The alternating magnetic field causes fluctuating electromagnetic forces between the coils of wire, the core and any nearby metalwork, causing vibrations and noise which consume power.
• Magnetostriction: The flux in the core causes it to physically expand and contract slightly with the alternating magnetic field, an effect known as magnetostriction. This in turn causes losses due to friction heating in susceptible ferromagnetic cores.

Efficiency gains can be achieved by using materials with lower resistively or greater diameters. For example, transformer coils made with low resistively conductors, such as copper, can have considerably lower load losses than those made with other material.

Heat

All transformers must have some circulation of coolant to remove the waste heat produced by losses. Small transformers up to a few kilowatts in size usually are adequately cooled by air circulation. Larger dry type transformers may have cooling fans. Some dry transformers are enclosed in pressurized tanks and are cooled by nitrogen or sulfur hexafluoride gas.
The windings of high-power or high-voltage transformers are immersed in transformer oil - a highly-refined mineral oil that is stable at high temperatures. Large transformers to be used indoors must use a non-flammable liquid. Formerly, polychlorinated biphenyl (PCB) was used as it was not a fire hazard in indoor power transformers and it is highly stable. Due to the stability of PCB and its environmental accumulation, it is no longer permitted in new equipment. Today, nontoxic, stable silicone-based oils or fluorinated hydrocarbons may be used, where the expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Other less-flammable fluids such as canola oil may be used but all fire resistant fluids have some drawbacks in performance, cost, or toxicity compared with mineral oil. The oil cools the transformer, and provides part of the electrical insulation between internal live parts. It has to be stable at high temperatures so that a small short or arc will not cause a breakdown or fire. The oil-filled tank may have radiators through which the oil circulates by natural convection. Very large or high-power transformers (with capacities of millions of watts) may have cooling fans, oil pumps and even oil to water heat exchangers. Oil-filled transformers undergo prolonged drying processes, using vapor-phase heat transfer, electrical self-heating, the application of a vacuum, or combinations of these, to ensure that the transformer is completely free of water vapor before the cooling oil is introduced. This helps prevent electrical breakdown under load. Oil-filled power transformers may be equipped with Buchholz relays - safety devices sensing gas buildup inside the transformer (a side effect of an electric arc inside the windings) and switching off the transformer.
Experimental power transformers in the 2 MVA range have been built with superconducting windings which eliminates the load losses, but not the core steel loss. These are cooled by liquid nitrogen or helium.

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