SHORT CIRCUIT FORCES ON POWER TRANSFORMERS BASIC INFORMATION
What Are The Short Circuit Forces Acting On Power Transformers?
Forces exist between current-carrying conductors when they are in an alternating-current field. These forces are determined using :
F = B I sin x
where
F = force on conductor
B = local leakage flux density
x = angle between the leakage flux and the load current. In transformers, sin x is almost
always equal to 1.
Thus
B = uI
and therefore
F directly proportional to I^2
Since the leakage flux field is between windings and has a rather high density, the forces under shor tcircuit conditions can be quite high. This is a special area of transformer design. Complex computer programs are needed to obtain a reasonable representation of the field in different parts of the windings.
Considerable research activity has been directed toward the study of mechanical stresses in the windings and the withstand criteria for different types of conductors and support systems.
Between any two windings in a transformer, there are three possible sets of forces:
• Radial repulsion forces due to currents flowing in opposition in the two windings
• Axial repulsion forces due to currents in opposition when the electromagnetic centers of the two windings are not aligned
• Axial compression forces in each winding due to currents flowing in the same direction in adjacent
conductors
The most onerous forces are usually radial between windings. Outer windings rarely fail from hoop stress, but inner windings can suffer from one or the other of two failure modes:
• Forced buckling, where the conductor between support sticks collapses due to inward bending into the oil-duct space
• Free buckling, where the conductors bulge outwards as well as inwards at a few specific points on the circumference of the winding
Forced buckling can be prevented by ensuring that the winding is tightly wound and is adequately supported by packing it back to the core. Free buckling can be prevented by ensuring that the winding is of sufficient mechanical strength to be self-supporting, without relying on packing back to the core.
POWER TRANSFORMERS LOAD LOSSES BASICS AND TUTORIALS
LOAD LOSSES OF POWER TRANSFORMERS BASIC INFORMATION
What Are The Load Losses Of Power Transformers?
The term load losses represents the losses in the transformer that result from the flow of load current in the windings. Load losses are composed of the following elements:
• Resistance losses as the current flows through the resistance of the conductors and leads
• Eddy losses caused by the leakage field. These are a function of the second power of the leakage field density and the second power of the conductor dimensions normal to the field.
• Stray losses: The leakage field exists in parts of the core, steel structural members, and tank walls. Losses and heating result in these steel parts.
Again, the leakage field caused by flow of the load current in the windings is involved, and the eddy and stray losses can be appreciable in large transformers. In order to reduce load loss, it is not sufficient to reduce the winding resistance by increasing the cross-section of the conductor, as eddy losses in the conductor will increase faster than joule heating losses decrease.
When the current is too great for a single conductor to be used for the winding without excessive eddy loss, a number of strands must be used in parallel. Because the parallel components are joined at the ends of the coil, steps must be taken to circumvent the induction of different EMFs (electromotive force) in the strands due to different loops of strands linking with the leakage flux, which would involve circulating currents and further loss.
Different forms of conductor transposition have been devised for this purpose. Ideally, each conductor element should occupy every possible position in the array of strands such that all elements have the same resistance and the same induced EMF.
Conductor transposition, however, involves some sacrifice of winding space. If the winding depth is small, one transposition halfway through the winding is sufficient; or in the case of a two-layer winding, the transposition can be located at the junction of the layers.
Windings of greater depth need three or more transpositions. An example of a continuously transposed conductor (CTC) cable, shown in Figure 1.10, is widely used in the industry. CTC cables are manufactured using transposing machines and are usually paper-insulated as part of the transposing operation.
Stray losses can be a constraint on high-reactance designs. Losses can be controlled by using a combination of magnetic shunts and/or conducting shields to channel the flow of leakage flux external to the windings into low-loss paths.
What Are The Load Losses Of Power Transformers?
The term load losses represents the losses in the transformer that result from the flow of load current in the windings. Load losses are composed of the following elements:
• Resistance losses as the current flows through the resistance of the conductors and leads
• Eddy losses caused by the leakage field. These are a function of the second power of the leakage field density and the second power of the conductor dimensions normal to the field.
• Stray losses: The leakage field exists in parts of the core, steel structural members, and tank walls. Losses and heating result in these steel parts.
When the current is too great for a single conductor to be used for the winding without excessive eddy loss, a number of strands must be used in parallel. Because the parallel components are joined at the ends of the coil, steps must be taken to circumvent the induction of different EMFs (electromotive force) in the strands due to different loops of strands linking with the leakage flux, which would involve circulating currents and further loss.
Different forms of conductor transposition have been devised for this purpose. Ideally, each conductor element should occupy every possible position in the array of strands such that all elements have the same resistance and the same induced EMF.
Conductor transposition, however, involves some sacrifice of winding space. If the winding depth is small, one transposition halfway through the winding is sufficient; or in the case of a two-layer winding, the transposition can be located at the junction of the layers.
Windings of greater depth need three or more transpositions. An example of a continuously transposed conductor (CTC) cable, shown in Figure 1.10, is widely used in the industry. CTC cables are manufactured using transposing machines and are usually paper-insulated as part of the transposing operation.
Stray losses can be a constraint on high-reactance designs. Losses can be controlled by using a combination of magnetic shunts and/or conducting shields to channel the flow of leakage flux external to the windings into low-loss paths.
Subscribe to:
Comments (Atom)