The power of a transformer is determined by the load, i.e. P2=U2II2I+U2III2II++ U2nI2In(VA)  P1=P2/η(VA)

In the formula: Calculation value of secondary power of P2 transformer. Low level power calculation value of P1 transformer.

U2I and U2II The voltage (V) of each secondary winding of the transformer is determined by the load.

I2I and I2II The current (A) of each secondary winding of the transformer is determined by the load.

For transformers with a capacity of less than 1KVA, where the capacity is small and the efficiency is low, η can generally be taken as 0.8 to 0.9. For transformers with a capacity of less than 100VA, the smaller value of η should be selected; Choose the larger value for transformer capacity between 100VA and 1000VA For poor quality silicon steel sheets, an optional value of 0.7 can be chosen.

I1=P1/U1 (1.1 to 1.2) (A), where U1 is the low-level voltage (V)

1.1 to 1.2 are empirical coefficients considering the no-load excitation current.

The capacity of a power transformer is the apparent power S=1.732 × U × I, where s is the apparent power, U is the line voltage, and I is the line current. Due to the presence of inductive and capacitive loads, the power factor of the line cannot be 1, so P=S × 0.8, where p is the active power and 0.8 is the assumed power factor. 4500 ÷ 0.8=5625, so a transformer of 5625KW is needed. However, the manufacturing of transformers is divided by capacity, and it is impossible to manufacture them separately based on your calculation results. Therefore, only the capacity level of 6300KVA can be selected. KVA is the unit of apparent power, and KW is the unit of active power.

Here is also an example for everyone:

The efficiency of transformer capacity is generally around 0.8. When the load power factor of 500 × 0.8=400 (KW) is 0.8, a 500KVA transformer can normally load 400KW power. As the power factor increases, the load power of the transformer also increases accordingly. The power factor has been increased from 0.7 to 0.95, and for every KW of power, a compensation capacitance of 0.691 kilovolts is required. Therefore, 500 × 0.691 ≈ 346 kilovolts (Kvar)