When you see or hear the word "transformer," depending on when you were born, what you do for a living and the kind of entertainment you gravitate toward, you will most likely think of either a giant, colorful robot or a key component of any electrical power grid. Even if you don't know what a transformer does, you've probably seen them and, if you're indoors as you read this, you are most likely within a couple hundred feet at most of a transformer.

Like virtually all components of modern power-delivery systems, transformers operate in an internal environment that is characterized by the generation of considerable amounts of heat. In addition, it is important to limit the flow of electricity within a transformer to the working parts that require it. That means transformers require both a coolant and some sort of insulator to function optimally.

As a result of these considerations, transformer oil is a critical element in electrical power systems, as certain types of oil have properties that contribute to the safe and smooth operation of these devices. Since a transformer has no moving parts, it may surprise you that they need oil at all, but some of the larger models contain several thousand gallons.

The job of a transformer is to transform the voltage entering the transformer via a wire to a greater or lesser value, depending on the needs of the part of the power grid in which the transformer sits. Generally, when electrical power leaves the plant at which it is generated, the voltage is increased ("stepped up") as it is conveyed to high-voltage transmission lines, easily identified by the tall towers stretching across long lengths of countryside.

At points along the way, wires leave the high-voltage (up to 750,000 V) lines, and transformers at substations reduce ("step down") the voltage for delivery into homes, offices and so on. Other transformers closer to the point of electricity delivery reduce the voltage further, with 120 V being the standard voltage obtained at electrical outlets in the United States.

A transformer can be thought of schematically as a hollowed-out rectangular box made of iron, a material that is strongly magnetized. A wire enters one side carrying electricity and is wrapped around that side of the transformer a number of times. The same arrangement is seen on the other side, but with a different number of wire turns around the transformer.

Moving charges (current, represented by I) generate magnetic fields, which in turn induce currents of their own. This relationship results in the expression:

Where the subscripts p and s denote the primary and secondary coils. Thus voltage changes are controlled by changing the number of turns.

• Note that transformers cannot generate additional power (P). Since P = IV, any increase in voltage in a transformer necessitates a corresponding drop in current and conversely.

Some transformers have only one coil and operate using a "tap connection" wire that connects to this coil. These are called autotransformers.

Instrument transformers are not used in power grids, instead being used to test and standardize equipment such as voltmeters and wattmeters (which measure electrical power in watts, or W). Potential transformers (PT) are used to drop down voltage, while current transformers (CT) step down current.

The main job of transformer oil is to protect the primary transformer apparatus, meaning the wires and iron core. It also acts as an insulator (also called a dielectric material, or just a dielectric) by keeping damaging chemical reactions, chiefly oxidation, from reaching the wires.

Another purpose of transformer oil is heat dissipation. Although there are technically no moving parts, the constantly shifting magnetic and electric fields in the transformer (which relies on alternating current, or AC) create forces that result in considerable heat generation. If this could not be absorbed by the typically vast amount of oil bathing the transformer, damage, including dangerous and even explosive outcomes, can result.

The potential damage to the transformer from oxidation is not to the iron core itself, which may surprise you if you recognize that oxidation where iron is concerned is what results in rust. Instead, it is the cellulose paper that surrounds the transformer that is subject to oxidative damage, and the transformer oil serves as a physical barrier to this process occurring.

The information in the previous section can be distilled into distinct electrical, chemical and physical properties that transformer oil should possess in order to be maximally effective.

• Electrical properties: Dielectric strength, or the ability to serve as an effective insulator, is the main concern in this area. An oil should have a known (specific) level of resistance, which is voltage divided by current (R = V/I) and is sensitive to temperature changes in the transformer. Finally, the oil's dielectric dissipation factor determines how much current inevitably "leaks" out of the system.
  
  
• Chemical properties: Water content in oil is undesirable as it impedes the oil's dielectric properties. Acidity and sludge content must also be minimized.
  
  
• Physical properties: High interfacial tension between the oil and water boundary is desirable, as is a high flash point (the temperature at which oil becomes volatile, or inflammable) and a low pour point (the temperature at which oil begins to flow freely). 

There are two main types of transformer oil in use today: Paraffin-based transformer oil and naphtha-based transformer oil.

Paraffin-based oil is not as easily oxidized as naphtha-based oil is, in theory producing less sludge. However, whatever sludge naphtha-based oil generates is more easily removed than the sludge from paraffin-based oil, because it is more soluble. When sludge builds up at the bottom of a transformer container, it interferes with its operation.

Naphtha-based oil does not contain dissolved wax, as does the paraffin-based type. This wax can increase the pour point and potentially cause issues, but in warmer climates where the temperature never gets very low, this is not an issue.

Despite the apparent superiority of naphtha-based oil, paraffin oil remains the most commonly used type of oil in transformers worldwide.

One nettlesome aspect of electrical equipment that runs 24 hours a day, seven days a week is the need for continual testing and maintenance to ensure both safety and proper functioning of the system in which the electrical elements lie. Transformer oil is no different in this regard.

Transformers are labeled when they are tested so that the date of the next scheduled test is made explicit, rather like the sticker a car gets after an oil change as a reminder. The oil to be tested is taken from the bottom of the transformer.

Transformer oil passes inspection in one of two ways. It should be able to withstand 45 kV for one minute in a testing cup with a space of 4 mm between electrodes placed in the oil. It should also be able to withstand 25 KW for one minute in the same type of cup with a 2.5 mm gap separating the electrodes. A failure to withstand the voltage occurs when the oil's dielectric strength is exceeded and a spark can "jump" between electrodes.