*4.1. Design of the Transformer*

In order to improve the system reliability, the transformer TR1 can be replaced by two identical transformers having half power (i.e., 1100 kVA), in parallel connection. The main aim of the activity was therefore the design of a 1100 kVA AMT transformer, having one primary and two secondary windings.

First of all, starting from the architecture depicted in Figure 1a, it has been made the analytical winding size, then moving to the CAD modelling and finally the FEM analysis. Main parameters of the 1100 kVA rectifier transformer with amorphous alloy core have been finally summarized in the next Table 5. The design is compliance with EN-60146–1-3. The rated voltage and currents in primary and secondary windings of the transformer are 15 kV/0.4 kV and 42 A/538 A, respectively.

The operating flux density in the transformer core is fixed around 1.6 T to mitigate the effects of harmonics and dc component of the winding current. Due to the shape of the amorphous alloy strip, the cross section of the core is generally made rectangular, and the winding is also made rectangular. Delta and star LV windings are insulated from one another. MV windings are wrapped between the LV windings. Usually, LV windings are foil wound and MV may be windings disc wound. Both are made by aluminum.

As remarked, the detailed 3D FEM analysis has been carried out in order to validate the design of the transformer by using Simcenter Magnet software [19].


**Table 5.** Main parameters of 1100 kVA amorphous core transformer (AMT) transformer.

### *4.2. Application to the Tramway System*

After designing the AMT transformer, it is important to evaluate which help this latter can guarantee on the full amount of energy consumption evaluation, as in the previous Table 4. Tables 2 and 5 show that both load and no-load losses are reduced. In detail, load losses are decreased from 17.5 kW to 12 kW in the case of two AMT transformers connected in parallel, while a significant reduction concerns the no-load losses, moving from 4.5 kW to 1.2 kW. Although these values are negligible with respect to the load losses amount, we must consider that they occur on the full number of hours per day, while load losses are present only in the average period of use. This is because, it is therefore needed to calculate through a weighted average the effects of the contributions of losses. In particular, we can consider what is currently shown in Equation (2).

$$L\_{tot} = n\_{l^\*} n\_{d^\*} (t\_{\mathbb{C}L} \cdot \text{C}\_{L} + t\_{\mathbb{L}L} \cdot \text{L}\_{L}) \tag{2}$$

where *Ltot* is the full amount of losses per year given by utilization of standard transformers, *nt* is the number of the transformers, *nd* is the operative number of days per year, *CL* are the core power losses calculated for the time duration *tCL*, i.e., equal to 24 h/day, *LL* are the full load losses, calculated for the average time duration *tLL*. Moving from the previous experimental evaluation of losses, for which 606 MWh/y were measured (see Table 4), and by using data of standard transformers, i.e., having *CL* and *LL* respectively equal to 4.5 kW and 17 kW, it was then possible to obtain the average time duration *tLL* by utilization of Equation (2). It was finally considered 10 installed transformers, and 8760 operative hours per year. In particular, we obtained:

$$t\_{LL} = \frac{L\_{tot}}{n\_l \cdot n\_d \cdot L\_L} - \frac{t\_{CL} \cdot C\_L}{L\_L} = 3.4 \, h/day \tag{3}$$

Moving from this, relation Equation (2) was newly adopted, by changing *CL* and *LL* to the values of the AMT transformers, e.g., 1.2 kW and 12 kW, respectively (see Table 4), and by taking unmodified the previously time durations. With the new sizing for the transformer, and by taking unmodified the architecture of Figure 3, losses changes from 606 MWh/y to 254 MWh/y; thus, having a 58% reduction. Therefore, by taking the same requests for traction (e.g., 2756 MWh/y) and auxiliary loads (738 MWh/y), the total MV energy request is reduced to 3748 MWh/y.

In conclusion, the total MV energy shows an 8% reduction, and losses are about 7% of the total MV energy demand. The amount of losses are therefore considerably reduced compared to before. This is visible also from Equation (1). In fact, *TOC* reduction is about 20% than for the standard transformer.

It is finally questionable if also the TR2 transformer, aimed to feed auxiliary loads, could be replaced with the introduction of a second AMT transformer. However, its reduced power (i.e., 200 kVA with respect to 2.2 MVA) would make a negligible reduction in the LV aux energy spent, if an equivalent improvement of efficiency as for the TR1 would be considered, in the order of a few MW per year.
