Power Transformer Oil: Insulation, Cooling and Maintenance

When people think of transformers, the core and windings come to mind first, but in a class of equipment, there is another equally critical member, which is transformer oil. It quietly fills the fuel tank, wraps the windings and core, and provides electrical insulation while continuously removing the heat generated during operation. It can be said that whether a liquid-immersed transformer can operate stably for a long time depends largely on the condition of this layer of oil.

Insulation and cooling shoulder-to-shoulder

Liquid-immersed transformers immerse the entire active part in mineral oil or ester insulating fluid, which carries two things at the same time. The first is insulation, which, together with the main insulation, isolates windings of different potentials, windings from the core, and live parts from the ground, allowing high voltages to be safely confined in their respective positions. The second is heat transfer. The heat generated by the losses in the windings and core is first handed over to the oil, which then transfers the heat to the oil tank wall, radiator or heat exchanger, and finally dissipates into the surrounding environment. In contrast, dry transformers do not have this layer of oil and rely on solid insulation and air for insulation and cooling, with the active parts not immersed in any insulating fluid. Both routes have their own applicable scenarios, and the dual role of oil is the fundamental difference between liquid-immersed structures and dry ones.

Mineral oil and ester insulating fluids

Oil itself also has its own particularities. The common insulating fluid is mineral oil, but other insulating fluids such as esters can also be selected according to project requirements. Choosing which oil to use is not just an option on the purchase list; the type of insulating fluid directly affects fire resistance, environmental friendliness, compatibility with materials such as seals, and subsequent maintenance plans. Therefore, during the planning stage, the oil type, installation site, and fire protection requirements must be considered together. It will be too late to make a temporary decision after the equipment enters the site.

How heat is carried away

Oil divides tropical walks into several ways. The most basic is natural circulation, where the density of oil changes after heating, and it flows between the oil tank and the radiator by its own convection. When the power is greater and the heat dissipation requirements are higher, forced cooling is required, using a fan to blow air to the radiator, using an oil pump to promote oil circulation, and on special occasions, water coolers are used to exchange heat. These cooling accessories are only auxiliary means. After forced cooling is stopped, the equipment can no longer continue to operate at its original forced capacity and must return to the derated state. The water-cooled structure looks compact, but it brings new interface risks such as a whole set of heat exchangers, water quality, flow rate, leak detection and water-oil pressure difference, which must be thought out before choosing it.

Moisture and impurities are the greatest enemies

For operating oil, the most important things to be wary of are the moisture, particles, oxidation products, and sludge that get in. These things will gradually reduce the dielectric properties of the oil, reducing its insulation capacity, while hindering heat dissipation, allowing the heat that should have been taken away to accumulate in the windings and core. Once oil becomes dirty and damp, the problem is often chronic. No obvious abnormalities can be seen in the early stages, and it may have accumulated for some time by the time it is discovered. Therefore, oil management is essentially about keeping it clean and dry, and trying to delay its deterioration.

Which signals should you monitor

What to look at during operation can be made into a simple list. Oil temperature, oil level, pressure, whether there is leakage, and casing condition are all basic items for daily inspections. Dissolved gas analysis is also required when necessary. The composition of dissolved gases in the oil and the gas production rate can identify potential thermal faults and discharge activities. However, we cannot draw conclusions from a single piece of data. Gas combinations, gas production speed and historical trends must be viewed together to avoid misjudgment.

Sampling and testing must be moisture-proof and pollution-proof

During maintenance, liquid-immersed equipment typically needs to check for leaks and oil levels, cooling equipment, casing, tap changers, as well as insulation fluid tests, dissolved gas analysis, and temperature meter inspections. Here’s a detail that’s easy to overlook: the entire sampling and testing process must avoid introducing moisture and contamination into the oil. Failure to clean the sampling port, moisture in the container, or leaving the port open for too long during operation can all lead to a job that was originally intended to detect problems, but instead add new trouble to the equipment.

Oil storage tanks, respirators and protection

Those seemingly inconspicuous accessories outside the fuel tank are actually all around the oil. The oil storage cabinet leaves a buffer space for the thermal expansion, cooling and contraction of the oil. The respirator blocks some of the water when the oil tank is venting to the outside world. The gas relay, sudden pressure protection and pressure relief device will give an alarm or action when there is an abnormality inside. They each care about only one thing, and together they constitute complete protection for oil and equipment.

Transformer oil may seem like just a pool of liquid in the equipment, but it is actually an indispensable link in the insulation and heat dissipation chain. Only by understanding its role, choosing the right type of oil, keeping an eye on temperature, oil level and gas trends, and standardizing sampling and maintenance can liquid-immersed transformers maintain a stable design life.