AMORPHOUS METAL DISTRIBUTION TRANSFORMER

Distribution transformers are used to distribute the electrical power in residential, commercial and industrial areas. Distribution transformers are energized for twenty four hours with wide variation in load; therefore they are designed to have low no-load losses. It is generally designed for maximum efficiency at about half full load. In order that the all-day efficiency is high, iron loss is made less by selecting a lesser value of flux density. In other words distribution transformers are generally designed for a lesser value of flux density. Two types of losses are inherent in the running of distribution transformers: no-load losses that occur in the transformer cores due to hysteresis and eddy current losses which are constant and present as soon as the transformer is energized and load losses that occur in the transformer’s electrical circuit due to resistive losses that are a function of loading conditions. The main no-load loss is core loss, which is associated with the time varying nature of the mag netizing force and results from hysteresis and eddy currents in the core materials. Core losses are dependent upon the excitation voltage and can increase sharply if the rated voltage of the transformer is exceeded. Hysteresis losses can be reduced by selecting low core losses material (such as amorphous metal), while eddy currents can be lowered by reducing lamination thickness. The problem has been overcome to some extent with the development of amorphous metal strips. This is achieved by compacting number of thin ribbons. This strip is commonly known as `Power Core’. Amorphous strips are four times harder than CRGO steel . The brittleness property of amorphous metal has also made it un-friendly to the transformer manufacturers. The manufacturers of amorphous core distribution transformers are very limited in the world because of two reasons, one is its high material cost and another is its brittleness property. Because of limitation of its brittleness property, in amorphous core transformers manufacturers are using square or rectangular cross section of the core.

Amorphous cores are usually produced as wounded, one-side cutting ones, due to mechanical properties of amorphous ribbons. This solution ensures the correct location of air gaps inside a core and simplifies electric windings assembling as well. Amorphous transformers are produced as 1-phase or 3-phase units, with 3-limbs or 5-limbs core construction. The capacity of currently produced amorphous transformers is limited up to 10 MVA.

The cross-section of amorphous cores is larger in comparison to silicon steel ones, due to lower saturation induction of amorphous ribbons. It results in the increase of transformer dimensions and weight. An essential part of the design of a transformer consists of the determination of the ferromagnetic material cross section core and the conductor’s cross-section area. These areas are determined from estimates of suitable values for the peak flux density the winding space factor, the stacking factor, and the full-load RMS current density in the windings. This current density depends on the mode the transformers will be operated, if intermittent or in a continuous form. The space factor is the ratio between the total conductor cross-section area and the core window area. The stacking factor is defined by the ratio between the ferromagnetic material crosssection area and the total core cross-section area

Usually, the thickness of a sheet of grain oriented silicon is 0.9 mm or higher. With the amorphous alloy this value is smaller than conventional silicon iron. This thinness, combined with its uneven surface, gives the amorphous material a space factor of only 80% compared to 95% achieved with silicon iron. The Designing of various types of core is discussed.

What are Amorphous Core Transformers (AMT’s)

The cores of conventional transformers consist of stacks of laminations that are made from silicon steel with an almost uniform crystalline structure (CRGO). In transformers with amorphous cores, a ribbon of steel is wound to form the core. The big benefit of amorphous transformers is that amorphous steel has lower hysteresis losses. Simply put this means that less energy is wasted as heat during magnetisation and de-magnetisation of the core

Amorphous metals are made of alloys that have no atomic order. They are made by rapid cooling of molten metal’s that prevents crystallisation and leaves a vitrified structure in the form of thin strips. Due to the lack of systematic structure, this type of metal has also been given the name “The Metallic Glasses”.

An amorphous metal transformer (AMT) is a type of energy efficient transformer found on electric grids. The magnetic core of this transformer is made with a ferromagnetic amorphous metal. The typical material (Metglas) is an alloy of iron with boron, silicon, and phosphorus in the form of thin (e.g. 25 μm) foils. These materials have high magnetic susceptibility, very low coercively and high electrical resistance. The high resistance and thin foils lead to low losses by eddy currents when subjected to alternating magnetic fields. On the downside amorphous alloys have a lower saturation induction and often a higher magnetostriction compared to conventional crystalline iron-silicon electrical steel.

In a transformer the no load loss is dominated by the core loss. With an amorphous core, this can be 70–80% lower than with traditional crystalline materials. The loss under heavy load is dominated by the resistance of the copper windings and thus called copper loss. Here the lower saturation magnetization of amorphous cores tend to result in a lower efficiency at full load. Using more copper and core material it is possible to compensate for this. So high efficiency AMTs can be more efficient at low and high load, though at a larger size. The more expensive amorphous core material, the more difficult handling and the need for more copper windings make an AMT more expensive than a traditional transformer.

Amorphous Metal

The amorphous metal used by ABB is a metallic alloy of iron, boron and silicon (Fe-B-Si) produced by solidifying alloy melts at rates rapid enough to prevent crystallization of the metal. Such rapid solidification leaves a vitrified solid with a random (amorphous) atomic structure, essentially as in the liquid phase. This differs from the atomic structure of conventional regular grain-oriented (RGO) silicon steel (a Fe-Si alloy), which has an organized crystalline structure. The largest volume usage of amorphous metal is in the cores of electrical distribution transformers. These materials offer, in concert, excellent magnetic characteristics and economy in production costs.In fact, the advent of Fe-B-Si amorphous metal alloys in the mid-1980s has been the most important advancement in materials for distribution transformers in the second half of the 20th century.

The disordered structure of amorphous steel and the ordered crystalline structure of regular grain-oriented steel.

Solidification rates of 106 K/s are necessary to produce Fe-B-Si amorphous metals. The high heat extraction rates constrain the solid in the form of a thin ribbon, about 25 μm thick. Since the material is thin, the application of amorphous metal is restricted to wound transformer cores. Amorphous metal cores have been in use for over 20 years in liquid-filled transformers, and this technology is now being applied to dry type transformers.