The electrical losses in a transformer appear in the form of heat in the core and coils. This heat must be transferred away from hot areas without allowing the windings to reach a temperature that will cause deterioration of the insulation.
Cooling Circuits
The cooling of a transformer is required to conserve the life of the insulation. The life of the insulation is a function of temperature and time. The heat generated in the core and coils must be transferred through the insulation to the surrounding air or cooling fluid and then through the enclosure to the outside environment. This path is similar to an electrical circuit with resistance in series; each step impedes the flow of heat. This relationship is shown in Figure 34.
The more efficient (lower resistance) each step, the more heat can be transferred from the core and coils. This allows the core and coils to be cooler. When designing a cooling system, the designer must look at a number of parameters that affect this heat transfer. Following are some of the more important ones:
• The insulation must be able to provide good dielectric and mechanical strength, but be thin enough to allow for the fast transfer of heat.
• The cooling medium (air or oil) must move past the heat source so that the heat may be removed from the source quickly.
• The enclosure must be mechanically strong, but thin enough to transfer heat through it rapidly.
• External devices such as fins and radiators may be added to speed up the transfer of heat to the environment.
• Fans may be attached to the radiators to cool the fluid. This will increase the flow of fluid through the radiators.
• Pumps may be added to the external circuit to enhance flow in a liquid-filled transformer.
• The pumps and fans may be controlled with a feedback circuit consisting of thermal sensitive elements in the oil, air and windings.
Engineering Encyclopedia Electrical Power Transformers
Figure 34. Transformer Cooling Circuit Temperature Gradient
In an electric circuit having resistance, a difference of potential causes a current to flow through the circuit. Similarly, a difference of temperature between windings assembly and oil causes the heat to flow from the windings (hotter area) into the oil (cooler area), then again from the oil (hotter area) to the tank wall (cooler area). This temperature difference is known as the temperature gradient. Thick coil insulation will not transfer heat to the oil as well as thin insulation. Therefore, the gradient is dependent on the coil construction for the transfer of heat from the coils. The temperature gradient will vary as the total amount of heat generated in the windings varies, or as the copper loss varies. The temperature gradient may be considered to be composed of two parts:
• Point of maximum, or "hot spot", temperature to the average temperature of entire winding
• From average temperature to the hot oil
The hot-spot temperature is seldom known for it can be measured only by means of measuring devices and sensors embedded in the windings. This is generally done by heaters and current transformer sized in proportion to the loading of the transformer.
Methods Used for Cooling
The methods used by manufacturers to cool transformers vary depending on transformer type, size and application. The important principle is that the cooling medium efficiently transfers the heat from the core and coils to the outside air. The cooling medium for power transformers is the insulating dielectric fluid (mineral oil) which is used to transfer the heat. In liquid-filled power transformers, heat from the core and coils is transferred through the fluid, to the tank wall, then conducted through the tank wall, and radiated to the ambient air. For small transformers the tank surface is usually adequate to dissipate the heat, especially when the surface area is increased by making the tank taller than actually needed for enclosure of the transformer. See Figure 35.
For medium-sized transformers, the rate of heat dissipation can be improved by adding cooling tubes or fins (radiators) to the tank (Figures 36 and 37). These are typically flattened external vertical tubes welded into horizontal headers, that are in turn welded or bolted into the tank wall. The increased rate of cooling results from the increased surface area and thinner tube material being exposed to the air. The action that occurs inside the cooling tubes is referred to as thermosiphon flow. The oil next to the windings heats, and thus rises to the top of the tank where it moves into the upper cooling header and tubes. In the tubes, the oil cools and sinks to the bottom, where it returns to the tank through the bottom header, ready to begin the cycle again (Figure 36). The temperature of the transformer will continue to rise until the rate at which the cooling system dissipates the heat is equal to the rate at which the heat is generated when this condition is reached for a given steady load. The transformer is said to have reached a stable condition and it will operate continuously and indefinitely at this constant temperature and load. This is provided the maximum temperature reached is not high enough to injure insulation.
When additional cooling is needed to allow a transformer to operate at its nameplate rating and temperature rise, fans can be used in conjunction with the radiators to provide forced air cooling. See Figures 38 and 39. For some larger designs, the fans are mounted on the side within the upper half of the radiators so that more fans can be mounted and blow more volume of air across the tube surfaces to obtain maximum air flow over the areas of highest temperature. The typical fan assembly consists of a fractional horsepower motor (either three- phase or single-phase) using a non-metallic fan blade (e.g. polyester). The motor is then mounted on a steel wireform bracket, which also serves as a guard for the fan blade.
Engineering Encyclopedia Electrical Power Transformers
Figure 35. Cooling of Core and Coils by Natural Oil Circulation or Thermosiphon Flow
Radiator
Tubes or Fins (Partial View of all Fins or Tubes) Oil Circulation inside radiator Tubes or Fins Internal Oil Circulation Transformer Tank Sides Core & Winding Assembly Cooler Oil Hot Oil Heat radiated off by convection Oil Level Pressure Relief Device Cut Off Valve Cut Off Valve Gas Space
Figure 36. Oil-Immersed, Self-Cooled (Arrows Show Flow of Hot Oil)
Engineering Encyclopedia Electrical Power Transformers
Core & Winding Assembly
Radiator
Fans & Motor
Oil Circulation inside radiator Tubes or Fins Internal Oil Circulation Transformer Tank Sides Pressure Relief Device Heat radiated off by convection fans speed up convection Hot Oil Cooler Oil Cut Off Valve
Cut Off Valve Oil Level
Engineering Encyclopedia Electrical Power Transformers
Cooling Classes
To provide a degree of standardization for transformer cooling systems, the American National Standards Institute has identified and published (ANSI/IEEE C57.12.00-1987) cooling classes for liquid immersed transformers.
In selecting the type of cooling to be used for a transformer, the designer applies the principle that the minimum amount of cooling required by a transformer is the amount needed to allow its operation at rated conditions and rated temperature rise. In accordance with ANSI/IEEE C57.12.00-1987, the standard rated temperature rise for liquid filled transformers is 55°C and 65°C with winding hottest-spot temperature rise of 65°C and 80°C. The designer first determines if self-cooling will be sufficient to allow the transformer to operate at rating without overheating. When self-cooling is insufficient, additional cooling is added using the available methods of radiators, fans and/or pumps. Ultimately, the use of additional cooling methods provides the transformer with an increased kVA capability.
The transformer nameplate gives the kVA rating, rated temperature rise and cooling class. The cooling classes and class codes, as identified by ANSI/IEEE standards are:
Transformer Cooling Class Codes Transformer
Type Class Code Method of Cooling
Liquid OA Liquid-immersed, self-cooled
Liquid OA/FA Liquid-immersed, self-cooled/forced-air cooled Liquid OA/FA/FA Liquid-immersed, self-cooled/forced-air cooled
/forced-air cooled (2 banks of fans)
Liquid OA/FA/FOA Liquid-immersed, self-cooled/forced-air cooled /forced-liquid cooled/forced-air-forced-liquid cooled
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Self-Cooled
Liquid completely covers the core and coils of a self-cooled, liquid-immersed transformer. The oil inside the transformer has a natural circulation of the oil due to temperature changes in oil at the top and bottom of the tank. Cooling occurs dues to the natural circulation of air over the cooling surface of the tank sides and/or radiators (Figures 36 and 37). Self-cooled (OA is the preferred cooling system for most liquid-immersed transformers installed in Saudi Aramco electrical distribution systems. Unless a particular installation specifically allow the use of other methods, this method is the one that should be specified. By selecting a transformer whose OA/kVA rating is large enough to supply all normal loads and to allow for a 10 % load growth factor, the possibility of transformer failure and the need for transformer maintenance are reduced to normal maintenance activities.
Self-Cooled and Forced-Air Cooled (OA/FA)
When transformers became large enough and the design will not allow the self-cooling method to dissipate heat fast enough a method of forcing more air across the radiators became necessary. This gives rise to forced-air-cooling. It is always used in conjunction with self- cooled method, therefore OA/FA. (See Figures 38, 39, and 40.)
Saudi Aramco requires the use of this type of cooling system for specific types of installations and specific sizes of transformers. The specific type of installation is a transformer that serves a double-ended substation. This type of installation normally operates with the bus-tie breaker open. In this case, each transformer should be able to supply all the load, using its OA, kVA rating. Under certain abnormal conditions, the bus-tie breaker is closed, and one transformer supplies the loads on both buses. Consequently, the transformer must have a forced-air cooling system. The extra cooling allows the transformer to safely carry the increased kVA during the period of time the bus-tie breaker is closed.
Saudi Aramco also requires forced-air cooling systems on all transformers rated 2,500 kVA and above.
The effectiveness of the cooling system may be increased by forcing a current of air to blow away the heated air adjacent to the heated surfaces and replacing it with cool air in rapid motion. The air blast may be directed against the transformer enclosure and radiating surfaces, or may be forced through air ducts within the transformer.
Forced-Oil Circulation Cooling (Pumps)
When the normal rate of movement of the oil by thermosiphon is insufficient for the cooling required, oil pumps (Figures 41 and 42) can be used in conjunction with the radiators and fans to give a self-cooled/forced-air, forced-oil cooled system. For this design, the oil is collected at the bottom of the radiators and forced by the pumps at an accelerated rate past the core and coils, thereby increasing the thermal capability of the transformer. The pump is typically close-coupled with its motor and enclosed in a single housing that utilizes the transformer oil for its lubrication. An oil-flow indicator is provided to visually confirm that the pump is operating. Operation of the pump is typically controlled by one or more thermal sensors located in the transformer.
Forced-Oil-Cooled Process
The liquid completely covers the core and coil of these transformers. Cooling occurs in the following stages:
• Natural internal circulation of oil inside transformer due to changes in temperature of oil at top and bottom of tank
• Natural circulation of air over the cooling surface increases dissipation of heat in the oil (OA)
• Forced circulation of air over the cooling surface (FA)
• Forced circulation of oil through the transformer internal coil assembly and increased oil flow through the radiators. This increases heat dissipation to radiators and cooling with forced air over the cooling surface increases even more the dissipation of heat from the oil (FOA)
Saudi Aramco only permits use of FOA as a second stage of forced cooling on transformers that have OA ratings of 90 MVA and above.
Engineering Encyclopedia Electrical Power Transformers
Upper Filter Press Valve
Drain and Filter Press Valve with Sampling Valve Control Cabinet
Top Mounted Cooling Fans
Welded on Coolers
Oil-Pump # 1
(Located on back side) High Voltage
Bushing
Low Voltage Bushing
Cooling Fans & Motors Radiator Fins or Tubes Transformer Tank Oil Circulation Route Piping Manifold to each Pump
Oil-Pump # 2 Manifold Feeding Pumps from Radiator
Oil Flow Typical inside all Radiator Fins or Tubes
Heat radiated off by convection Core & Winding
Assembly
Cut Off Valve Cooler Oil Cut Off Valve
Hot Oil
Figure 41. Oil-immersed, Self-Cooled Forced-Air Cooled and 2-Stage Forced-Oil-Cooled Transformer
Engineering Encyclopedia Electrical Power Transformers
Figure 42. Oil Circulating Pump
Example:
The nameplate kVA and cooling class for a liquid immersed transformer might be given as: 1500/1725 kVA OA/FA
The first number given for the kVA rating (1500) corresponds to the first cooling class given (OA), and likewise, the second kVA rating (1725) corresponds to the second cooling class (FA). This means that the transformer has a rating of 1500 kVA when the fans are turned off and there is no forced cooling. This is known as the self-cooled rating (OA).
When the fans, mounted on the cooling radiators, are turned on and operating, the transformer has the higher rating of 1725 kVA. This is called the forced-air cooled rating (FO).
For example, when a large power transformer had forced-oil pumps with fans, then the rating would be approximately as follows:
OA FA FOA
12 MVA 16 MVA 20 MVA
The percentage increased kVA for force air and for force oil is totally dependent on the manufacturer's cooling system design.