transformer engineering design and practice 2_phần 1

9 Cooling Systems The magnetic circuit and windings are the principal sources of losses and resulting temperature rise in various parts of a transformer. Core loss, copper loss in windings (I2R loss), stray loss in windings and stray loss due to leakage/high current field are mainly responsible for heat generation within the transformer. Sometimes loose electrical connections inside the transformer, leading to a high contact resistance, cause higher temperatures. Excessive temperatures due to heating of curb bolts, which are in the path of stray field, can damage gaskets (refer to Chapter 5). The heat generated due to all these losses must be dissipated without allowing the core, winding and structural parts to reach a temperature which will cause deterioration of insulation. If the insulation is subjected to temperatures higher than the allowed value for a long time, it looses insulating properties; in other words the insulation gets aged, severely affecting the transformer life. There are two principle characteristics of insulation: dielectric strength and mechanical strength. The dielectric strength of insulation aged in oil remains high up to a certain temperature after which it drops rapidly. At this point the insulation material becomes brittle and looses its mechanical strength. Thus, it is primarily the mechanical strength which gets affected by the higher temperatures and aging, which in turn affects the dielectric strength. Hence, the dielectric strength alone cannot always be depended upon for judging the effect of temperature on the insulation [1]. Accurate estimation of temperatures on all surfaces is very critical in the design of transformers to decide the operating flux density in core and current densities in windings/connections. It helps in checking the adequacy of cooling arrangements provided for the core and windings. It also helps in ensuring reliable operation of the transformer since the insulation life can be estimated under overload conditions and corrective actions can be taken in advance. 367 Copyright © 2004 by Marcel Dekker, Inc. 368 Chapter 9 The values of maximum oil and winding temperatures depend on the ambient temperature, transformer design, loading conditions and cooling provided. The limits for ambient temperature and the corresponding limits for oil temperature rise and winding temperature rise are specified in the international standards. As the ambient temperature varies from one country to another, the limits could be different for different countries. For example in IEC 60076–2 (second edition: 1993), a maximum ambient temperature of 40°C is specified with a limit on top oil temperature rise of 60°C. In a country where the maximum ambient temperature is 50°C, the top oil temperature rise limit may be correspondingly reduced to 50°C. If the installation site is more than 1000 m above the sea level, the allowable temperature rise for transformers is reduced as per the guidelines given in the standards because of the fact that air density reduces with the increase in altitude lowering the effectiveness of cooling. Altitude basically affects the convective heat transfer (because of lower buoyancy effect) and not the radiation. A corresponding reverse correction is applied when the altitude of factory location is above 1000 m and the altitude of installation site is below 1000 m. In oil cooled transformers, the oil provides a medium for both cooling and insulation. Heat from core, windings and structural components is dissipated by means of the oil circulation. The heat is finally transmitted either to atmospheric air or water. In the subsequent sections, modes of heat transfer and their application in different cooling configurations in a transformer are discussed. 9.1 Modes of Heat Transfer The heat transfer mechanism in a transformer takes place by three modes, viz. conduction, convection and radiation. In the oil cooled transformers, convection plays the most important role and conduction the least important. Rigorous mathematical treatment for expressing these modes of heat transfer is quite difficult and hence designers mostly rely on empirical formulae. 9.1.1 Conduction Almost all the types of transformers are either oil or gas filled, and heat flows from the core and windings into the cooling medium. From the core, heat can flow directly, but from the winding it flows through the insulation provided on the winding conductor. In large transformers, at least one side of insulated conductors is exposed to the cooling medium, and the heat flows through a small thickness of the conductor insulation. But in small transformers the heat may have to flow through several layers of copper and insulation before reaching the cooling medium. The temperature drop across the insulation due to the conduction heat transfer mechanism can be calculated by the basic thermal law: Copyright © 2004 by Marcel Dekker, Inc. Cooling Systems 369 Δθ=Q×RT (9.1) where Q is heat flow (power loss) in W and RT is thermal resistance in °C/W. The thermal resistance is given by (9.2) where ti is the insulation thickness in m, A is cross-sectional area in m2, and k is thermal conductivity in W/(m °C). If q denotes heat flux per unit transfer area, the temperature drop across the insulation can be rewritten as (9.3) It should be noted that the thermal conductivity of oil-impregnated paper insulation is temperature dependent and its proper value should be taken in the calculations [2]. 9.1.2 Radiation Any body, at a raised temperature compared to its surroundings, radiates heat energy in the form of waves. The heat dissipation from a transformer tank occurs by means of both radiation and natural convection. The cooling of radiators also occurs by radiation, but it is far less as compared to that by convection. Because of closeness of radiator fins, the entire radiator surface does not participate in the heat transfer mechanism by radiation. Thus, the effective area for radiation can be taken as the outside envelope surface of the radiator. Therefore, for the case of tank with radiators connected to it, actual radiating surface area is that area on which a tightly stretched string would lie. The emissivity of the radiating surface affects the radiation. The heat transfer in watts by radiation is expressed by the Stephan-Boltzmann law: (9.4) where η=5.67×10-8W/(m2 °K4) is the Stephan-Boltzmann constant, E is surface emissivity factor, AR is surface area for radiation in m2, Ts is average temperature of radiating surface in °K, and Ta is ambient air temperature in °K. Surface emissivity is a property, which depends on several factors like surface finish, type of paint applied on the surface, etc. When the emissivity factor is less than unity, the effective radiating surface is correspondingly less (as indicated by the above equation). For tank and radiators painted with grey colour having emissivity of 0.95, the effective radiating area is usually assumed to be that of outside envelope without introducing much error. Copyright © 2004 by Marcel Dekker, Inc. 370 Chapter 9 9.1.3 Convection The oil, being a liquid, has one important mechanical property that its volume changes with temperature and pressure [3]. The change of volume with temperature provides the essential convective or thermosiphon cooling. The change of volume with pressure affects the amount of transferred vibrations from the core to tank. The heat dissipation from the core and windings occurs mainly due to convection. When a heated surface is immersed in a fluid, heat flows from the surface to the cooling medium. Due to increase in the fluid temperature, its density (or specific gravity) reduces. The fluid (oil) in oil-cooled transformers, rises upwards and transfers its heat to outside ambient through tank and radiators. The rising oil is replaced by the colder oil from the bottom, and thus the continuous oil circulation occurs. The convective heat transfer is expressed by the relationship: Q=hA(Tsurface-Tfluid) (9.5) where Q is heat flow in W, h is heat transfer coefficient in W/(m2 °C), A is surface area in m2, and temperatures Tsurface and Tfluid are in °C. Since h depends on both geometry as well as fluid properties, its estimation is very difficult. However, a lot of empirical correlations are available, which can be used in majority of design calculations. In one such correlation, the heat dissipated per unit surface area is expressed as equal to a constant multiplied by temperature rise raised to an empirical coefficient. The heat dissipation from the transformer tank to ambient air occurs similarly but the warmed air after cooling does not come back and its place is occupied by new quantity of fresh air. In the case of tank, heat dissipation by convection and radiation mechanisms are comparable since the surface area available for the convective cooling is same as that for the radiation cooling. The heat dissipated by the tank through the convection and radiation is also usually calculated by empirical relations in which the resultant effect of both the mechanisms is taken into account. 9.2 Cooling Arrangements 9.2.1 ONAN/OA cooling In small rating transformers, the tank surface area may be able to dissipate heat directly to the atmosphere; while the bigger rating transformers usually require much larger dissipating surface in the form of radiators/tubes mounted directly on the tank or mounted on a separate structure. If the number of radiators is small, they are preferably mounted directly on the tank so that it results in smaller overall dimensions. Copyright © 2004 by Marcel Dekker, Inc. Cooling Systems 371 Figure 9.1 ONAN cooling When number of radiators is large, they are mounted on a separate structure and the arrangement is called as radiator bank. The radiators are mounted on headers, which are supported from the ground. In this case, strict dimensional control of pipes and other fittings is required in order to avoid oil leakages. Oil is kept in circulation by the gravitational buoyancy in the closed-loop cooling system as shown in figure 9.1. The heat developed in active parts is passed on to the surrounding oil through the surface transfer (convection) mechanism. The oil temperature increases and its specific gravity drops, due to which it flows upwards and then into the coolers. The oil heat gets dissipated along the colder surfaces of the coolers which increases its specific gravity, and it flows downwards and enters the transformer tank from the inlet at the bottom level. Since the heat dissipation from the oil to atmospheric air is by natural means (the circulation mechanism for oil is the natural thermosiphon flow in the cooling equipment and windings), the cooling is termed as ONAN (Oil Natural and Air Natural) or OA type of cooling. In the arrangement consisting of radiator banks, higher thermal head can be achieved by adjusting the height of support structures. The thermal head can be defined as the difference between the centers of gravity of fluids in the tank and radiator bank. Although it is difficult to get higher thermal head for the case of tank mounted radiators, reasonable amount of thermal head is achieved by the arrangement shown in figure 9.2. When the radiators are mounted at higher height, the buoyancy effect on the cooling-loop increases resulting in increase of the rate of oil flow and heat dissipation in the cooling equipment. However, it is to be noted that the increase in flow rate results in increased frictional pressure loss, thereby offsetting the thermal head gained by the height difference. Copyright © 2004 by Marcel Dekker, Inc. ... - tailieumienphi.vn