Optimum design & operation of power transformers  Fulltext 

[ Ahmad Ebrahimi ] - University of Shahid Rajaee, Faculty of Electrical & Electronic Engineering Country: Islamic Republic of Iran

خلاصه مقاله:

Reducing Building factor and loss in laminated cores during ac magnetization is the aim of steel manufacturers as well as the wish of the user. A microprocessor controlled system for magnetizing and measuring the characteristics of magnetic materials was employed to calculate the losses and the other magnetic properties of electrical steel materials. The power loss of the transformer cores is usually higher than nominal loss of the core materials, and measured in the Epstein square or the single strip tester. The ratio of the core loss to the nominal loss is termed the building factor, B.F. The B.F. ranges from 1.2 to 1.8 or more depending on the magnetic properties of the core material and the design of a magnetic circuit. The academic as well as economic importance of the B.F. has been highlighted as the nominal loss of the core material improved.
Laser scribing, surface scratching with a ball point pen, creating lines using spark ablation, creating scratches with special small cylinders, or stress relief each have a large impact on the better behaviour and the homogenisation of the passing flux and as a result decreasing of the loss
in the created core. In the design of the shape and the size of the core for instance choosing a suitable width to fit transformer windows and well sized angles for cutting sheets for T Joints and corners are worthy factors to be considered when reducing the B.F. and are also very important issues in increasing efficiency. Choosing a core with a suitable permeability for the middle limb in order to equal it's reluctance to that of the ending limb will homogenize the flux density in all parts of the core, minimizing the related loss and thus increasing efficiency. It must be considered that maximum efficiency usually occurs when iron loss equals the copper loss and the load has a suitable and standard value. The full load power loss can also be reduced by working on core creation designs. At last choosing a suitable usage for the transformer, considering the fact that the most efficiency can be reached through a particular percentage of the full load leads us to our aim of increasing efficiency. For example some transformers have their maximum efficiency under a mean of their full load, whereas power transformers show full efficiency in taking values near the whole full load.

ZDKH : A high permeability material. C.G.O.: A conventional grain oriented material. Building factor (B.F.): Actual loss / Nominal loss BN: Normal Flux density transfer from lamination to lamination during the magnetization process. BL : Longitudinal flux density through the laminations.