The cooling equipment collects the warm oil at the top of the tank and returns, cooled, oil lower down on the side. The cooling arrangement can be seen as two oil circuits with an indirect interaction, one inner and one outer circuit. The inner circuit transfers the loss energy from the heat producing surfaces to the oil. In the outer circuit the oil transfer the heat to a secondary cooling medium. The ambient air normally cools transformers.
It is possible to build air coolers with forced air circulation more compactly than cooler with natural draught. However, such a cooler also has fairly high impedance to the oil circulation in the internal circuit, which necessitates the oil to be pumped through the cooler. For built in transformers, e.g., deep underground power caverns or in some industrial application oil to water heat exchanger are used; as in such case, sufficient air for cooling may not be available. The system also permits small physical dimensions. The disadvantage of the compact design is that auxiliary power must always be available.
Present standards give clear definitions of the different types of cooling together with special designations. The examples below is taken from IEC:
ONAN – Oil natural – air natural ONAF – Oil natural – air forced OFAN – Oil forced – air natural OFAF – Oil forced – air forced OFWF – Oil forced – water forced
On the oil side the oil can be directed to the heat producing surfaces by the oil pumps: OD – Oil directed
A given transformer can have a combination of cooling types to permit a change in the type of cooling e.g. ONAN/ONAF etc.
Pumps as well as fans sometimes suffer breakdowns. It must be possible to exchange such components without emptying the transformer or even taking it out of service. All cooling circuits should therefore be provided with necessary valves for shutting off each separate oil circuit.
The letter O is used for mineral oil and insulating liquid with fire point ≤ 300 °C. The letter K is used for insulating liquid with fire point > 300 °C.
The letter L is used for insulating liquid with no measurable fire point.
7.2.1. Radiators (SVLØ READY)
Radiators are available in various patters. They almost consist basically of number of flat passages of edge-welded plates connecting a top and bottom header. It is possible to make the radiators slightly higher than the tank so that the top header has a swan-necked shape, this has the added benefit that it also improves the oil circulation by increasing the thermal head developed in the radiator. The radiators have a venting plug in the top header and a draining plug in the bottom header. Radiators are almost flanged direct to the transformer tank or to the headers in freestanding cooler banks through a butterfly valve permitting individual radiators to be shut off and even easily removed.
A separate, self-supporting rack, forming a freestanding cooler bank entails the cost for a separate mounting pad and fitting of the pipe work. One advantage is that the conservator may be placed on the bank. This simplifies the orientation of bushings with regard to clearances.
Because of their construction it is difficult to prepare the surface adequately and to apply paint protection to radiators under site condition. It is therefore recommended to have sheet-steel radiators hot dip galvanized.
7.2.2. Corrugated tanks (GS READY)
The corrugated tank is both tank and cooling surface for small and medium distribution transformers. The tank consists of cover, corrugated tank walls and bottom box.
The standard tank design is hermetically sealed type where the corrugated walls are of such a flexible construction that they expand and contract depending on the changes in the oil volume, due to temperature variations during operation. Number and depth of corrugations on each side of the tank can be optimised to meet the cooling and dynamic pressure variation requirements.
7.2.3. Fans (SVLØ READY)
For larger units it is possible to suspend fans below or on the side of radiators to provide a forced draught, and achieve ONAF cooling arrangement. This might enable the transformer loading capacity to be increased by some 25%. The radiators have to be grouped in such a way to obtain coverage by the fans.
The cooling fans will generate a low frequency noise, which will be added to the noise from the transformer itself. The intensity depends on fan size, rotation speed and the design of the fan blade. Fans are available in a rich several of designs. In basic it is a standard squirrel cage motor totally enclosed with a propeller mounted direct to the shaft and in a casing provided with necessary wire guards.
Fan is also a part of a forced air heat exchanger
7.2.4. Forced oil, forced air heat exchangers (SVLØ READY)
For large transformers it became very space consuming to remove the heat with natural circulation through radiators. The space requirement of compact coolers is drastically lower than for simple radiator batteries. For space reasons it may be economical to use compact coolers with appreciable flow resistance, which requires forced circulation of the oil by pumps, and strong fans for air blowing.
Such OFAF heat exchangers usually consist of round tubes with thin swaged cooling fins or elements of flat corrugated tubes. Some designs are very compact with narrow ducts on the airside and high fan pressure.
When using several coolers for OFAF cooled transformers it is preferred to arrange two or more individual coolers in series instead of parallel in order to reach a higher temperature drop. On large transformer the cooling equipment is separated into several groups or circuits, and these can be activated or deactivated as required, controlled by thermostats or may in some more advanced way.
The mounting of coolers for forced cooling with pumps and fans have several options for the orientation and grouping on the tank, depending on local conditions. In certain cases separate racks are also an option.
7.2.5. Oil-water coolers (SVLØ READY)
Water-cooling may be an attractive alternative when transformers are installed in a cave station, or in harsh industrial environments as steel works etc. But the choice has to be evaluated against to pump the cooling oil out in the air environment to an air cooler. An obvious risk with water-cooling in a temperate climate is freezing. In hotter climates it may more critical to have cooling water in sufficient quantities.
Water coolers are compact. They are almost conventional cylindrical tubular heat exchangers, with removable tubes. Such heat exchangers are very common and represent classical technology, and are supplied for quite many different purposes in technology. More recent designs, for instance membrane type flat heat exchangers, have hardly been used.
A central problem is properties of the cooling water with regard to corrosion of the materials in water tubes. The risk of water leakage into the oil system must be watched meticulously. The installation shall be arranged so that the pressure on oil side is higher than the pressure on water side, but this may not always be possible to safeguard.
Transformer handbook. Draft. Rev. 02Q Page 90 of 197
Corrosion can be a problem. Cooling water sample analysis are recommended in order to select the most suitable materials in the cooler. Sophisticated materials as for instance titanium tube coolers may be used.
Deposition has to be avoided by ensuring that an adequate rate of water flow is maintained, but allowing this to become excessive will lead to tube erosion.
Double-tube cooler may be applied. With such an arrangement the oil and water circuits are separated by an interspaced so that any fluid leakage will be collected in this space and will raise an alarm.
7.2.6. Oil pumps
Circulation pumps for oil cooling equipment are special compact, totally sealed models. The motor is immersed in the transformer oil, and there are no stuffing boxes.
The sound level of these pumps is low, compared to the transformer sound level.