transformer engineering design and practice 2_phần 2

10 Structural Design 10.1 Importance of Structural Design The tank of a transformer is a closed structure which is made by steel plates. It behaves like a plate structure. Stiffeners are usually provided on all the sides and also on the top cover of the tank to reduce stresses and deflections in plates under various types of loads. The transformer tanks are designed for a pressure higher than the operating one, as specified by the standards. The tank design and fabrication are complicated due to limitations imposed by transportation (weight and size), requirement that the oil quantity should be optimum, etc. Apart from pressure and vacuum loads, the transformer structure has to withstand other loads such as lifting, jacking, haulage, etc. Depending on the location of transformer installation, the strength of the transformer structure against a seismic load may also need to be ascertained. Design of the transformer tank becomes complicated due to number of accessories and fittings connected or mounted on it. These include: conservator and radiator mounting arrangements, cooler pipes, turrets which house bushings, support arrangement for control box housing controls for fans and pumps, support structures for tap changer drive mechanism, valves for sampling/draining/ filtration, cable trays or conduits for auxiliary wiring, inspection covers for getting access to important parts inside the transformer such as bushings (for making connections) and tap changer, cable box, bus duct termination, etc. Certain simplifying assumptions are done while analyzing the strength of the tank with all these fittings under various loading conditions. The stress analysis of a transformer structure can be done by mainly two methods, viz. analytical methods and numerical methods. The analytical methods are used for determining the stiffening requirements to limit stresses and 389 Copyright © 2004 by Marcel Dekker, Inc. 390 Chapter 10 deflections for simple tank constructions. The tank shapes are usually complex and the application of analytical methods is difficult. For example, if the tank is not rectangular and if there are many pockets (extruding structures) or openings, numerical methods such as FEM are used to determine the stresses and deflections under various loading conditions. 10.2 Different Types of Loads and Tests 10.2.1 Loads The transformer tank should be capable of withstanding the following loads: Lifting and jacking:The tank is designed to facilitate handling of the transformer. For this purpose, lifting lugs and jacking pads (as shown in figure 10.1 (a)) are provided on the tank. Lifting lugs, provided towards the top of the tank, are used to lift the structure by a crane. Jacking pads provided towards the base of the tank, are used for handling the transformer in the absence of crane, especially at the site. Generally, four jacking pads/lifting lugs are used. Figure 10.1 Jacking pad and lifting bollard Copyright © 2004 by Marcel Dekker, Inc. Structural Design 391 Lifting lugs are used for distribution transformers, where the loads are less. Lifting bollards are used for medium and large power transformers as shown in figure 10.1 (b). Ride-over (transport) lugs are provided for the purpose of supporting the transformer on a floorless wagon during transport. Haulage load:For local movements of the transformer at the place of installation, rollers and haulage lugs are provided. The haulage lugs are provided on the lower portion of the tank, whereas the rollers are provided under the base plate. Usually, four rollers are provided but for large transformers six or eight rollers may be provided. In place of rollers, a solid under-base is sometimes provided to facilitate skidding over rails or pipes. Seismic and wind load:The transformer has to be designed for a specified seismic acceleration and wind load. Seismic and wind loads are very important design considerations for bushings, supporting structures of conservator and radiators, etc. It is very difficult, if not impossible, to conduct the seismic test on a transformer. Seismic tests on bushings are usually specified and can be done. Special care has to be taken for bushings because they have high cantilever load. Transient pressure rise: When an internal fault takes place in an oil filled transformer, a large volume of decomposed gases may get generated due to arcing. Under these conditions, the tank structure has to withstand a rapid rise of pressure if the pressure relief device does not act in such a short time. If the tank is not designed with adequate factor of safety, it may rupture leading to fire hazard and serious environmental impact due to outflow of oil. The tank should be designed in such a way that it should be in an elastic limit under the pressure rise conditions. The tank should not be too rigid or too flexible, otherwise it may burst. Special devices such as sudden pressure relays are used which can act quickly under such transient pressure rise conditions. 10.2.2 Tests The following tests are conducted to check the strength of the transformer structure: Leak test: This test is meant to check whether the welded joints of the tank structure are leak-proof or not. The test is conducted by pressurizing the tank using air pressure. A soap solution is sprayed over all the welded joints under specified pressure conditions. Any leak due to weld defects (crack, pin hole, etc.) leads to bubble formation. Vacuum test: The leak test (done with pressurized air) is followed by the vacuum test. A specified vacuum is applied to the tank for at least an hour. The permanent deflections measured after removal of the vacuum should be within the limits (which depend on the size of tank) specified by the users/standards. The tank is then cleared for shot-blasting and painting processes. This test is important because oil filling is done under specified vacuum conditions (either at works or Copyright © 2004 by Marcel Dekker, Inc. 392 Chapter 10 site). In addition, the drying and impregnation may be done in the tank itself (e.g., in vapour phase drying process). The vacuum may be partial or full depending on the voltage class and size of the transformer, and the user specifications. Pressure test:This test is usually done after all the dielectric tests are completed in the manufacturer’s works. The accessories like bushings are removed and a pressure of 5 psi higher than the maximum operating pressure is generally applied to check the pressure withstand capability of the tank. All the welded joints are checked manually; if any oil leakage is noticed, the oil is drained and the defective welding is rectified. The gasket leaks, if any, are also rectified. Dye-penetration test: This test is conducted for load bearing members to detect weld defects. In this test, the surface to be tested is cleaned thoroughly and a dye (usually of pink colour) is applied to the weld surface. The dye is left there for some time, typically 30 minutes, and then it is wiped clean. During this period, if there are any weld defects in the surface being tested, the dye due to capillary action penetrates through. After this, another solution known as developer is sprayed on the surface. This developer brings out the dye that has penetrated inside and leaves the pink marks on the locations where the weld defects are present. This test is useful to detect weld integrity of load bearing members like jacking pads and lifting lugs/bollards. 10.3 Classification of Transformer Tanks Depending upon the position of joint between upper and lower parts of the tank, we have two types of tank construction, viz. conventional tank and bell tank. Conventional tank: This type of construction has a top cover as shown in figure 10.2 (a). Since the joint is usually above the top yoke level, it facilitates a proper placement of magnetic shunts on the tank wall for an effective stray loss control. The disadvantage is that the core and windings are not visible at site when the cover is removed. Hence, for inspection of core-winding assembly, a crane with higher capacity is required to remove the core-winding assembly from the tank. Bell tank: In this type of tank construction, shown in figure 10.2 (b), the joint between the two parts is at the bottom yoke level to facilitate the inspection of core-winding assembly at site after the bell is removed. Thus, it consists of a shallow bottom tank and a bell shaped top tank. The bell tank construction may not be convenient for a proper placement of magnetic shunts if the joint is at such a height from the bottom that it comes in the path of leakage field. This may lead to a bolt overheating problem (discussed in Chapter 5). For the above two types of tank, either plain or shaped tank can be used. Copyright © 2004 by Marcel Dekker, Inc. Structural Design 393 Figure 10.2 Types of tank Plain tank: The plain tank of rectangular shape is quite simple in construction. It is easy from design and manufacturing points of view since it facilitates standardization. The design of stiffeners is also quite simple. It usually leads to higher oil and steel quantity in high voltage transformers. If special detachable (bolted) bushing pockets are used for center-line lead HV winding arrangement, some saving in oil quantity can be achieved. This is usually done in large high voltage transformers. Shaped tank. In order to save oil quantity, tank is shaped so that its volume reduces. The tank shaping is mainly influenced by electrical clearances (between the high voltage leads and grounded tank), transport considerations, tap changer mounting arrangements, etc. The lower portion of the tank may be truncated in order to facilitate the loading of a large transformer on some specific type of wagon (in case of rail transport) and/or to reduce the oil quantity. The tank walls may be curved/stepped to reduce the tank size and volume. The shaped tank has the advantage that the curved portions of its walls give a stiffening effect. But the design of the shaped tank is more complex leading to higher engineering and manufacturing time. Also, it may not be conducive for putting magnetic shunts or eddy current shields on it for an effective stray loss control. The joint between the two parts of the tank can be either bolted or welded type, which gives the following two types of construction. Bolted constructiom The joint between top and bottom tanks can be of bolted type. The bolted construction, though preferred for easy serviceability, has the disadvantage of developing leaks if gaskets deteriorate over a period of time. The oil leakage problem can occur if there is unevenness in the plates which are bolted or if the gaskets are over-compressed. The bolted joint may lead to overheating hazard in large transformers. Copyright © 2004 by Marcel Dekker, Inc. ... - tailieumienphi.vn