Power transformer rating
Design apparent power from the installed load, the demand factor and cos φ, and the transformer rating from the standard series for the allowed loading and the number of transformers in the substation.
S_p = K_d · P_inst / cos φS_n ≥ S_p / (N · β)β_f = S_p / (N · S_n)The rating is the next larger one of the GOST 9680-77 series (… 25, 40, 63, 100, 160, 250, 400, 630, 1000, 1600, 2500 kVA …). With two transformers and β = 0.7, the one left in service carries 140 % — the emergency overload an oil-immersed transformer allows during load peaks (GOST 14209 and the manufacturer's instructions). For large motors, also check the voltage dip at starting.
Source: Rating series per GOST 9680-77; demand and loading factors from industrial power supply design practice
Inputs
You can change a field's unit: the value is converted to the formula's units automatically.
Sum of the ratings of the connected motors and other loads, without standby units.
Share of the installed load drawn at the same time: pumps, compressors, fans 0.7–0.8.
Without correction 0.75–0.85, with correction 0.92–0.95.
Two-transformer substations with category I loads 0.65–0.7, category II 0.7–0.8; single-transformer (category III) 0.9–0.95.
Unit converter for this formulaPower · Apparent power · Fraction and percent
- kW1
- W1,000
- MW0.001
- PS (metric hp)1.35962
- kcal/h859.845
- hp1.34102
- BTU/h3,412.14
PS is the metric horsepower (735.5 W), hp the mechanical horsepower (745.7 W).
- S_p — Design load–
- β_f — Actual loading–
- β_1 — Loading of one transformer carrying the whole load (with two, after the other trips)–
More in Motors and transformers
Reactive power of a capacitor bank that raises the power factor from cos φ_1 to cos φ_2, and the resulting reduction of the supply current and line losses.
Synchronous speed from the supply frequency and the number of poles, slip from the rated speed, and the speed on a variable frequency drive — for example, of an ESP motor or a pumping unit drive.
Estimate of the voltage dip on the substation busbars during direct-on-line starting of an induction motor, and of its reduced starting torque — to check whether a large pump or compressor can be started from a given transformer.
Results are engineering estimates from standard formulas; for design decisions check them against the codes, project documents and specialists' calculations. The formulas carried over from the original set are unchanged, and their errors are described in the notes.