Under normal operating conditions, the electromagnetic forces acting on the transformer windings are quite modest. However, if a short-circuit fault occurs, the winding currents can increase between 10- and 30-fold, resulting in forces 100–900 times the normal because the forces increase as the square of the current. The windings and supporting structure must be designed to withstand these fault current forces without permanent distortion of the wind- ings or supports. Because current protection devices are usually installed, the fault currents are interrupted after a few cycles, however, that is still long enough to do some damage if the supporting structure is inadequate.
Faults can be caused by falling trees that hit power lines, providing a direct current path to the ground or by animals or birds bridging across two lines belonging to different phases, causing a line-to-line short. These rarely occur, but their probability increases over the 20–50-year lifetime of a transformer so that sufficient mechanical strength to withstand such events is required.
The coils are generally supported with pressure rings at the ends. These are thick rings made of pressboard or another material that cover the wind- ing ends. The center opening allows the core to pass through. The size of the rings is between 30 and 180 mm for large power transformers. Since all the windings are not necessarily of the same height, some pressboard or wood blocking is required between the tops of the windings and the rings. Additional blocking is usually placed between the ring and the top yoke and clamping structure to provide clearance between the high winding voltages and the grounded core and metallic clamp (Figure 1.12).
The top and bottom clamps are joined by vertical tie plates, sometimes called flitch plates, which pass along the sides of the core. The tie plates are solidly attached at both ends so that they pull the top and bottom clamps together by means of tightening bolts, thus compressing the windings. The compressive forces are transmitted along the windings via the key spacers, which must be strong enough to accommodate these forces. The clamps and tie plates are usually made of structural steel. Axial forces, which tend to
elongate the windings when a fault occurs, will put the tie plates in tension. The tie plates must also be strong enough to carry the gravitational load when the core and coils are lifted as a unit because the lifting hooks are attached to the clamps. The tie plates are about 10 mm thick and are of varying widths depending on the expected short-circuit forces and transformer weight. The width is often subdivided to reduce eddy current losses. Figure 1.16 shows a top view of the clamping structure.
The radial fault forces are countered inwardly by the sticks separating the oil barriers and by additional support next to the core. The windings themselves, particularly the innermost winding, are often made of hardened copper or bonded cable to provide additional resistance to the inward radial forces. The outermost winding is usually subjected to an outer radial force, putting the wires or cables in tension. The material must be strong enough to resist these tensile forces, as there is no supporting structure on the outside to counter these forces. A measure of the material’s strength is its proof stress—the stress required to produce a permanent elongation of 0.2% (sometimes 0.1% is used). Copper of a specified proof stress can be ordered from the wire or cable supplier.
The leads are also acted on by extra forces during a fault. These forces are produced by the stray flux from the coils or from nearby leads, interacting with the lead’s current. The leads are therefore braced by wood or pressboard supports that extend from the clamps. This lead support structure can be
Figure 1.16
(See color insert following page 338.) Top view of a clamping structure for a three-phase transformer.
quite complicated, especially if there are many leads and interconnections, so it is usually custom-made for each unit. An example of such a structure is shown in Figure 1.15.
The assembled coil, core, clamps, and lead structure are placed in a trans- former tank. The tank serves many functions: It contains the oil for an oil- filled unit and protects not only the coils and other transformer components but also personnel from the high voltages present inside the grounded tank. If it is made of soft (magnetic) steel, it keeps stray flux from reaching outside the tank. The tank is usually airtight so that air cannot enter and oxidize the oil.
Besides being a containment vessel, the tank also has numerous attach- ments such as bushings for getting the electrical power into and out of the unit, an electronic control and monitoring cabinet for recording and trans- ferring sensor information to remote processors or receiving remote control signals, and radiators with or without fans to provide cooling. On certain units, there is a separate tank compartment for tap-changing equipment. Some units have a conservator attached to the tank cover or to the top of the radiators. This is a large, usually cylindrical structure that contains oil that is contiguous with the main tank oil. The conservator also has an air space, which is separated from the oil by a sealed diaphragm. Thus, as the tank oil expands and contracts due to temperature changes, the flexible dia- phragm accommodates these volume changes while maintaining a sealed oil environment. Figure 1.17 shows a large power transformer with a cylindrical
Figure 1.17
conservator visible on top. Bushings mounted on top of the tank are visible and the radiators with fans are also shown. Technicians are working on the control box.