Transformer Cooling

When the transformer is under load, the energy loss in the magnetic core and the windings turns into heat which can damage insulation. It is important to keep a close watch on the temperature inside the equipment to prevent overheating and possible fires at substations. The main part of transformer cooling in this case is transformer oil. Output of heat from the windings and the core is gradual: from the hot metal to the oil and then to the environment.

The process of transformer cooling can be divided into two component: internal and external cooling. Each component requires its own technical implementation.  In the first case, the entire system (tubes, channels etc) are located inside the transformer and are invisible from the outside. In the second case, all elements (filters, fans, pumps etc) are placed externally and can be serviced systematically.

Transformer Cooling Methods

• Transformers up to 16000 KVA are cooled by natural oil cooling.The tanks of such transformers are smooth, with cooling tubes and tube radiators. The oil transmits the heat from the core and the coils into the environment.

The natural liquid motion is caused by the difference of density at higher and lower temperatures, driving the convection process. The heat concentrated at the top of the radiator, is dissipated externally partly by radiation and partly by convection of air.

The maximum temperature of the oil under nominal load must not exceed +95oC.

Oil heating with forced draught and forced oil circulation is used for transformer units operating at higher voltages.

The system includes external radiator tubes with air cooling  from fans installed on transformer tank walls. The fans suck the air from below and direct it to the top part of the tubes, cooling the hot oil.

If the nominal load is below 100%, this cooling system can operate with the fans off. In this case, the main condition is to maintain temperature below 55oC when the external temperature is above zero, and below 45 oC when the temperature is subzero.

This arrangement can be used for cooling of transformer up to 100 000 KVA.

• Another method used with transformers from 63 000 KVA and above is transformer cooling with forced draught and forced circulation of oil through air coolers.

The system is based on air-oil coolers. They consist of thin ribbed tubes with electric pumps. The pumps push the fluid along the pipes at a high rate. With forced air flow and large surface area, such coolers are both compact and highly efficient.

• For high voltage transformers, directed circulation is used, when oil is supplied via tubes to the hottest parts of the windings.

The system can be operated both manually and automatically, with the capability to engage the system simultaneously with the transformer, to intensify the cooling process by connecting auxiliary cooler (when nominal load is achieved) or a backup cooler (in case the primary system fails).

• The last method is oil-water cooling with forced oil circulation. The system is rather complex and is used in transformer operating at 160 000 KVA and above.  This system is very similar to the previously described in terms of design. The difference is that the radiator tubes contain water, with the oil in between the tubes.

A certain complication is the large amount of water necessary for the system function.

Water drain and heating are possible for the cold season to prevent freezing.

The system is also equipped with temperature, oil flow rate and fluid pressure sensors, as well as sound alarms.

Oil-water cooling of transformer can keep the temperature of the top oil layers at 50-55 oC. Water supply is stopped when the temperature drops to 10  oC.

Required Parameters of Transformer Oil

Transformer oil is not only a transformer cooling agent, but also a dielectric. Its purity and quality must comply with the regulations.

The main quality parameters of transformer oil are:

• Absence of sulfur and acid, as these may damage transformer insulation;
• Efficient dissipation of heat from the core and the windings. The oil must have high heat capacity and heat conductivity. Viscosity of the oil is also important. It changes with temperature. High power transformer are filled with oil types that can change viscosity quickly with the changing temperature;
• High dielectric strength. The dielectric strength of air is normally 30 kV/cm, but for transformer oil this parameters is 150 kV/cm. Dielectric strength is influenced by moisture content. The oil must be as dry as possible. Dust and other contaminants, such as gases, can also reduce the oil’s dielectric strength;
• High oil flashpoint, significantly higher than the safe operating temperature of the transformer, to prevent ignition. The lower limit of transformer ignition is 150oC. The dielectric liquid must then have flashpoint at least 180  oC. Transformer oil is a flammable material, so fire safety measures must be taken.

Transformer Oil Purification and Regeneration Methods

The oil in the transformer tank must be changed from time to time to perform is main function as a dielectric and a medium for cooling the transformer. However, even new oil is saturated with oxygen, moisture and solid particles due to incorrect storage or transportation.

Modern industrial technologies allow purification, drying and degassing of oil, both used and new oil, before it is used in electrical equipment.

GlobeCore develops equipment for purification of transformer oil to remove particulate matter and other impurities, such as moisture, gases, resins etc.

One of such units is the CMM-0.6 mobile oil processing unit. It consists of a vacuum chamber, oil and vacuum pumps, a heater and a filter section with disposable filter cartridges. The primary purpose of the machine is to purify the oil with vacuum, but it can also pump the oil from one tank to another and heat it.

After processing with the CMM-0.6, the oil has a dielectric strength of 60 kV and contains no solid particles larger than 5 micron.

The unit is compact and mobile to be used on site. It is powered by a regular 220V power source.

The unit is commissioned by GlobeCore specialists, who also instruct the client from the moment of purchase to initial operation. The unit is built using high quality components for reliability and long service life.

GlobeCore unit will benefit transformer servicing and maintenance companies.