Chọn công suất máy biến áp theo phụ tải tính toán

How this is calculated#

A transformer is sized from the maximum demand it will carry, with room for the load to grow. Too small and it runs hot and drops too much voltage; too large and it costs more and wastes energy in its core every hour of the year. This tool finds the size and shows what it does at the load.

You enter the maximum demand, in kVA or in kW with a power factor, and a growth allowance. The design load is the demand plus that growth. The tool picks the smallest rating from a list of typical sizes that carries the design load within the loading limit you set. The list is common distribution and site ratings, not a standard's list.

At that size it works the loading today and with growth, then the voltage regulation: the drop through the transformer's own resistance and reactance at the design load and power factor. The impedance on the rating plate splits into the two by the X/R ratio. The losses are the core loss, which is there whenever the transformer is energised, plus the winding loss, which rises with the square of the loading. The full load and fault currents at the size come from the transformer tool.

The impedance, X/R ratio and losses are maker's data. The defaults are typical figures, and the result names them as assumptions.

Worked examples#

• A 220 kVA site with 20 % growth needs a 300 kVA transformer. It runs at 88 % with growth and loses 2.59 % of its voltage.
• A 180 kW load at 0.85 power factor with 25 % growth also needs 300 kVA, at 88.2 % with growth.
• A 420 kVA factory with 30 % growth, loaded to 80 % at most needs 750 kVA, at 72.8 % with growth.
• A 60 kVA single‑phase rural supply with no growth needs 75 kVA, at 80 %.
• A 900 kVA plant at 0.8 power factor, 6 % impedance and X/R of 7 needs 1,250 kVA. Its regulation is 3.57 % at 82.8 % with growth.

Limits#

The regulation is the drop through the transformer alone. The network upstream and the cables downstream add their own drop; the total voltage drop tool adds the cable parts. A tap changer or off-load taps can raise the secondary voltage to make up for it.

The tool does not cover transformers in parallel, harmonic loads, the temperature rise of the windings or the network company's own sizing rules for a supply transformer.

See also: Transformer, Maximum demand and Fault level at a board.