*2.1. Transformer*

In a standard electrical feeding substation (ESS), a three-phase distribution transformer with two LV windings, star, and delta, and a MV delta winding, is typically used. It is manufactured so there is no phase displacement between the MV secondary winding and the LV delta winding and hence the star LV secondary winding leads the MV winding by 30◦. Each LV winding is then connected to a converter's bridge [10].

This transformer typology, as all electrical machines, has losses in the windings (in losses), due to the current circulation, and in the iron (no-load losses) due to magnetic hysteresis and eddy currents. Although the latter are a small fraction, their contribution in energy is not negligible compared to load losses because the transformer is always connected to the network and then subjected to the grid voltage almost constant over time: thus their no load losses remain unchanged. Except for densely populated urban centers, the average power delivered by a MV/LV transformer, during a week, is less than 40% of the nominal power. This value is reduced to 10% at night. A cabin transformer generally is oversized to compensate for, in the future, any increase of the output power, taking into account that the average useful life exceeds thirty years. Load losses, by following the same trend, are therefore always lower than the nominal one. All this explains the interest in industry and regulations, regarding the realization of machines with lower iron losses. In the joint overlap regions, the losses increase around 50% due to interlaminar flux and deviation of flux from rolling direction.

In the case of converter transformer, under normal operating conditions, the current in the windings has a stepped waveform. For a 12-pulse operating condition, the converters create harmonic of order 12k ± 1, where k is integer, at the HV side. The presence of harmonics is one of the most severe aspect of the converter transformer. Harmonics increase eddy losses in the windings and stray losses in structural elements of the converter transformer. In addition, due the asymmetries in valve firing angles in the converter, the currents in the transformer windings have a DC component. In order to minimize excessive losses and noise, an accurate design of the magnetic circuit is needed.

Transformers distribution (with the high voltage (HV) less than 24 kV and oil cooling) are classified according to IEC 50464-1 based on the rated power and divided into subclasses according to the value of no-load losses (five classes from A0 to E0) and load losses (four classes from Ak to Dk). The state of the art is represented by machines in class CkA0.

Regulation 548/2014 of the European directive in 2009 defined the minimum e fficiency requirements and deadlines for new distribution transformers placed on the European market: class AkA0 from July 2021 [11]. Currently in Europe, the power transformers core is made with regular grain-oriented (RGO) silicon steel. This technology is su fficiently mature so that the achievement of greater e fficiency classes may be obtained only by decreasing the current density in the coils and the magnetic induction in the cores, increasing the volume of the transformer. Furthermore, the production process has a low degree of automation. Considering this background, the request for high e fficiency transformers could increase significantly in the future; therefore, the excellent market perspective for these machines is evident.

A really interesting solution is the amorphous metal transformer (AMT). It was introduced in the US market (in the mid-1980s), and in Southeast Asia (Japan and China) during the last decade. The amorphous metal is a metallic alloy of iron, boron, and silicon (Fe-B-Si) made by solidifying alloy melts at rates rapid enough to prevent the metal crystallization [12,13]. Such rapid solidification leaves a vitrified solid with a random (amorphous) atomic structure. The high heat extraction rates constrain the solid in the form of a thin ribbon, about 25 μm thick. Due to the presence of boron, amorphous metal has a reduced saturation induction than RGO steel. As result, amorphous core transformers often have a larger core cross-sectional area, resulting in larger coils and transformer footprint. The most significant characteristic of an amorphous metal in a transformer is that it yields a much lower core loss than even the best grades of RGO steel, by up to 70% [14].

Since the amorphous ribbon does not have a mechanical sti ffness, the transformer core no longer has the function of holding up the coils. As a result, the structural design of this machine is significantly di fferent compared to a transformer with RGO silicon steels, as shown in Figure 1a.

**Figure 1.** Three-legged transformer core made by amorphous alloy (**a**); transformer continuous core [15] during the automated manufacturing process (**b**).

Due to their significant di fferent mechanical properties, another transformer manufacturing technique is also needed. Since the material is thin, the application of amorphous metal is restricted to wound transformer cores. Until now, the solution has been a wound core with distributed gaps: the ends of each ribbon are lapped with each other. The flux concentrations in the neighborhood of the joint gaps cause additional iron losses.

By exploiting the mechanical characteristics (flexibility) of ribbon [15] proposes an innovative assembly process for wound ferromagnetic core in which the joint gaps are eliminated, as in Figure 1b. All this, in addition to further improvement of the e fficiency, will ensure a high standard product.

The technological innovation consists in being able to wind up a ribbon of magnetic material directly into the preformed coils. In particular the originality consists in providing a removable automated system, constituted by a series of rolls, on which the thin sheet of amorphous material is wound (with a thin layer of thermosetting resin deposit), mounted on a motorized guide with an optimized shape. The shape is a closed loop (rectangular with rounded corners or elliptical) formed around the coils.

The production process is completely di fferent from the one currently used for the production of core laminations, and the solution developed will allow to introduce a high degree of automation, so that the productivity of its plants can be significantly increased.

Typically, an AMT is always more expensive than a silicon steel unit but can be more economical in many power systems. To specify cost-e ffective transformer performance, utility engineers commonly use a "loss-evaluation" method. This approach considers transformer loading patterns, energy costs, inflation, interest rates, and other economic factors to calculate the net present value of a watt of electric power. The combination of the initial cost of the transformer with its cost of operation, is summarized by the total owning cost (*TOC*) [16]:

$$T\mathbf{O}\mathbf{C} = B\_P + \left(A \cdot \mathbf{C}\_L\right) + \left(B \cdot L\_L\right) \tag{1}$$

where *BP* is the bid price, *A* is the core loss factor, *CL* are the core losses (e.g., losses occurring in the magnetic core, due to alternating magnetization), *B* is the Load Loss Factor, and *LL* are the load losses (e.g., losses when load currents flow). In the case of energy costs su fficiently high, AMTs make economic as well as environmental sense.
