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Oil Immersed Transformer: Impedance
Classification:
Industry News
Release time:
2025-03-10
As the core equipment of the power system, the impedance and leakage reactance of transformers play an important role in design and application. This article discusses in depth the origin of leakage reactance, the evolution of impedance and its impact on modern power systems. From the early leakage flux, which was regarded as a disadvantage, to now becoming a key parameter for limiting fault currents and optimizing economy, the transformation of transformer design concepts reflects the combination of technological progress and actual needs. By analyzing the determinants of impedance and their relationship with core size and manufacturing cost, this article reveals how to find a balance between performance and economy, providing readers with a comprehensive insight into transformer design.
The relationship between voltage conversion and leakage reactance
The voltage conversion between the primary and secondary windings is not ideal. First, not all the magnetic flux generated by the primary winding passes through the secondary winding, thereby giving the transformer a leakage reactance. Early transformer designers believed that the transformer leakage reactance was a disadvantage and should be minimized to meet normal economic constraints.
The evolution of leakage reactance and the significance of impedance
As power stations and transmission systems expand and become more complex, leakage reactance, or in actual terms, is called impedance, because transformer windings also have resistance - gradually being used as a useful parameter that can limit the fault current. Transformer impedance is generally expressed as the percentage of voltage drop at the full load current of the transformer, which is the habitual representation of impedance by transformer designers. For example, an impedance of 10% means that the voltage drop at the full load current is 10% of the open circuit voltage. In other words, regardless of other impedances of the system, the voltage drop of the transformer is equal to the total voltage of the system at 10 times the full load current.
Source of leakage reactance
The transformer leakage reactance originates from the magnetic flux generated by one winding that does not pass through the other winding completely. In fact, most large power transformers have so little resistance value that there is almost no difference between reactance and impedance.
Transformation from disadvantages to advantages
For a long time in the past, the reactance or impedance of a transformer has been simply considered a disadvantage that incurs voltage regulation, and this disadvantage originates from the inevitable leakage flux. Transformer impedance is a very useful parameter today, which enables system designers to determine the level of failure of the system and meet the economic limits of switching devices and other connected devices. Therefore, today's transformer designers no longer pursue the lowest possible transformer impedance, but meet the maximum and minimum impedance values proposed by system designers in order to suit the economics of system design (of course, there may be such a situation, that is, designers always do not consider the change of impedance value with tap position according to outdated manufacturing tolerances, but if these aspects are considered comprehensively, this problem can generally be properly solved, and this point will be discussed in detail later).
Impedance determinants and economic benefits
It is necessary to carefully analyze the determinants of impedance and how these factors affect the economic benefits of the transformer. This relationship must be quite simple in nature, because the reactance is generated by the leakage flux, so by minimizing the leakage flux and making the core section as large as possible, a lower reactance can be obtained. On the contrary, if a higher reactance can be allowed, the core can be made smaller. From this, it can be easily seen that the total size of the transformer depends on the core size. The large core size means that the transformer size is also large. Large size must be high in construction. Small core size will reduce the cost of the transformer. Therefore, transformers with low reactance values are costly, while transformers with high reactance values are relatively costly. Nevertheless, for a transformer of specific size, there is still a set of optional reactance values within the above limit range, and the cost varies greatly within this set of values.
Factors influencing reactance
In the range of reactance, the influence of various factors. For example, the cross-sectional size of a large iron core usually refers to increasing the size of the iron core frame and increasing the axial height of the winding, which can reduce the reactance. Otherwise, reducing the size of the iron core frame and reducing the winding height will increase the reactance. In fact, however, the task faced by the designer is not the case, because any changes in the main parameters will affect other parameters, thereby impressing the reactance.
The relationship between capacity and reactance
It should be noted that because the capacity factor occurs in the molecule of the reactance percentage expression, the reactance refers to increasing with the increase in the rated capacity of the transformer. This is of little significance to most transformers, because the required reactance finger can be obtained by properly adjusting the physical dimensions of the windings. But it is very important for large generator transformers. Because the size and weight of the generator transformer must meet the transportation requirements. It is in this case that single-phase transformers need to be considered.
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