# Selecting an Optimally Sized Transformer

In this study, the most important calculations (power flows, short circuit currents and etc.) are made to determine the right size for a transformer.

It is crucial to select optimal size equipment transformer is no exception. Undersized transformers would limit the operation of a factory and could result in lower productivity. While if they were oversized, it could increase expenses for the reserved plant power. In this study, the most important calculations are made to determine the right size for a transformer.

## Object Description

In this study, multiple scenarios are simulated to find the optimal size for a 110/6.3 kV power transformer. Four cases are analyzed:

• Case 1 (25MVA transformers)
• Case 2 (40MVA transformers)
• Case 3 (31.5MVA transformers)
• Case 4 (20MVA transformers, reserve not considered)

All simulation results are calculated according to the peak demand of each substation in the plant. The list of power demands is provided in Table 1.

In carried out calculations, the voltage level in the infinite busbar is assumed to be 115kV. It is also assumed that the short circuit current in the infinite busbar is equal to 11.6 kA. The network single-line diagram can be found in a downloadable PDF file in Annex no. 1. Calculations are performed with a power system modeling software EA-PSM. Learn more about the software.

## Case 1 – 25MVA Power Transformers

Case 1 represents the existing situation and is calculated for comparison purposes. Transformer characteristics used in calculations can be found in Table 2.

#### Maximum load operation mode (considering reserve power)

In maximum load operation mode (both transformers are operating) calculations showed that transformer TR1 loading reaches up to 26.7MVA (26.15MW and 5.56MVar), transformer TR2 loading reaches up to 18.4MVA (17.86MW and 4.30MVar). Based on the expected situation, transformer TR1 is loaded at 106.94% and transformer TR2 is loaded at 73.49%. The power reserve for transformer TR2 is 6.63 MVA and transformer TR1 is overloaded by 1.74 MVA. These results were reached by considering the 1.8 MW reserve power of substation 1. Calculation results are depicted in Figure 1.

#### Maximum load operation mode (reserve power not considered)

If this reserve power mentioned above is not included in the calculations, transformer TR1 loading does not exceed 25 MVA and it is loaded at 99.8% while transformer TR2 loading does not change. The full result for the described situation can be seen in Figure 2.

When comparing results, the reserve power of Substation 1 will not be included in Case 1 – 25MVA power transformers’ calculation results.

#### Repair/Maintenance mode

In repair/maintenance mode only one of two transformers is working. After installing new loads, the total apparent power without considering the reserve or CM8 and storage silo would be 44.45MVA. The operating transformer could only handle up to 25MVA of power. Consequently, the transformer would be 177.8% loaded. In this mode, the factory can only work at about half of its maximum power capacity.

## Case 2 – 40MVA Power Transformers

Case 2 is the situation that would occur after upgrading high to medium voltage transformers to 40MVA power. 40MVA transformer characteristics used in calculations can be found in Table 3.

In maximum load operation mode (both transformers are operating) calculations showed that transformer TR1 loading reaches up to 26.5MVA (26.07MW and 4.72MVar), transformer TR2 loading reaches up to 18.2MVA (17.82MW and 3.85MVar). Based on the expected situation, transformer TR1 is loaded 66.23% and transformer TR2 is loaded 45.57%. The power reserve for transformer TR1 is 13.51 MVA and for transformer, TR2 is 21.77 MVA. Calculation results are depicted in Figure 3.

#### Repair/Maintenance mode

In repair/maintenance mode only one of two transformers is working. After installing new loads and not considering reserve power, the total apparent power would be 43.46MVA. The operating 40MVA transformer would be loaded at 108.7%. Simulated situation results are depicted in Figure 4.

## Case 3 – 31.5MVA Transformers

Case 3 is the situation that would occur after upgrading high to medium voltage transformers to 31.5MVA power. 31.5MVA transformer characteristics used in calculations can be found in Table 4.

In maximum load operation mode (both transformers are operating) calculations showed that transformer TR1 loading reaches up to 26.6MVA (26.08MW and 5.24MVar), transformer TR2 loading reaches up to 18.2MVA (17.82MW and 4.08MVar). Based on the expected situation, transformer TR1 is loaded 84.44% and transformer TR2 is loaded 58.04%. The power reserve for transformer TR1 is 4.9 MVA and for transformer, TR2 is 13.3 MVA. Calculation results are depicted in Figure 5.

#### Repair/Maintenance mode

In repair/maintenance mode only one of two transformers is working. After installing new loads and not considering reserve power, the total apparent power would be 43.85MVA. The operating 31.5MVA transformer would be loaded at 139.2%. Case 3 emergency operation mode simulation results are shown in Figure 6.

## Case 4 – 20MVA Transformers

Case 4 is the situation that would occur after downgrading high to medium voltage transformers to 20MVA power. Because the transformers, in this case, are downgraded, the reserve power of CM8 and the new storage silo are not considered when carrying out calculations. 20MVA transformer characteristics used in the analysis can be found in Table 5.

In maximum load operation mode (both transformers are operating) calculations showed that transformer TR1 loading reaches up to 25.1MVA (24.59MW and 4.95MVar), transformer TR2 loading reaches up to 18.5MVA (17.85MW and 4.69MVar). Based on the expected situation, transformer TR1 is loaded 125.41% and transformer TR2 is loaded 92.25%. The power reserve for transformer TR2 is 1.55 MVA and transformer TR1 is overloaded by 5.08 MVA. Calculation results are depicted in Figure 7.

#### Repair/Maintenance mode

In repair/maintenance mode only one of two transformers is working. The operating transformer could only handle up to 20MVA of power, while the entire demanded apparent power would be equal to 44.86MVA. Therefore, the transformer would be 224.3% loaded. In this mode, the factory can only work at about 45% of its maximum power capacity.

## Comparing Voltage Drop Calculation Results

Voltage drop in high to medium voltage transformers as well as voltage level at section 1 and section 2 are provided in Table 6. This analysis was carried out only for maximum load operation mode (both transformers are operating).  Results for case 1 and case 4 were reached by not considering the reserve power. However, it was considered for case 2 and case 3 calculations. In Table 6 all calculations were done when the tap changer position was not adjusted.

The transformers have automatic tap changers, that can automatically compensate for voltage drop by changing their positions. Consequently, during power flow calculations, the transformer tap changer’s position is automatically selected according to voltage level in the medium voltage network (6.3 kV voltage is maintained). Calculation results for when the tap position is changed to maintain the nominal voltage level in the network are displayed in Table 7.

The table above shows that it is possible to compensate voltage drop in transformers with the help of a tap changer in all cases.

## Motor Starting Currents

Motor data used in calculating motor starting currents and voltage levels in the network are provided in Table 8.

Motor starting current study was calculated only for maximum load operation mode (both transformers are operating). Results for case 1 and case 4 were reached by not considering the reserve power. However, it was considered for case 2 and case 3 calculations. Results can be seen in Table 9. As it can be seen in results, case 2 and case 3 show significantly lower voltage drops and smaller currents than in case 1. In case 4 many motors start dropped voltage levels lower than 10%. This could have some negative effects on motor starting and the entire network overall.

## Short Circuit Currents Calculations

Three-phase (K3), phase-to-phase (K2), phase-to-earth (K1), and two-phase-to-earth (K11) short circuit calculation results are provided in Table 10.

In the current situation, the K3 minimum short circuit current can reach up to 21.2 kA. With a 40 MVA transformer, the short circuit current would increase by 43% to 30.4 kA. In the case of 31 MVA transformers, the short circuit current would increase by 17%. In the case of 20 MVA transformers, the short circuit current would reduce by 23%. These are quite significant changes, therefore equipment and protection relay settings will have to be upgraded.

## Conclusion

After performing calculations for different size transformers (case 2 – 40MVA, case 3 – 31.5MVA, case 4 – 20MVA), results show:

• that reducing the existing transformer size to 20MVA would cause problems for main motors starting and might limit plants operation,
• that increasing existing transformer size would result in smoother motor staring, factory’s ability to work at higher capacity at emergency mode, potential to fully utilize reserve power or expand further.
• However, upgrading transformers would cause short circuit currents to be larger. Short circuit current value increase for 40MVA transformers is significant and should be considered, while short circuit current size increase when installing 31.5MVA transformers is not as noteworthy. This value increase could require some equipment to be upgraded to withstand higher short circuit currents.
• It should be noted, that a single 40MVA transformer could operate at maximum factory capacity in emergency mode.