Determine How To Load Two Paralleled Transformers

Two three phase transformers with ratings shown below are paralleled on their secondary side.

Transformer No. 1 – 1000kVA, 4160V/480V, Z = 5.0%

Transformer No. 2 – 1500kVA, 4160V/480V, Z = 8.0%

The maximum load in kVA the transformers can supply (combined) without overloading any one , is determined by the transformers impedance. They load up inversely proportional to their impedance.

Therefore,

Before calculating the load, we need to get the transformers on the same base impedance. This can be done using the following formula.

Z_{p.u._{new}}=Z_{p.u._{old}}(\frac{S_{base_{new}}}{S_{base_{old}}})(\frac{V_{base_{old}}}{V_{base_{new}}})^2

Changing transformer T1 base impedance to T2 base impedance

X_{t1}= j0.05(\frac{1500}{1000})(\frac{4160}{4160})^2 = j0.075 pu

When transformer T1 is completely loaded to 1000kVA

T2 can be loaded to

Load_{T_{2}}= (1000)(\frac{j0.075}{j0.08}) = 937.5kVA

The total load the transformers can supply combined is

1000kVA + 937.5kVA = 1937.5kVA

Aleen Mohammed

Registered Professional Engineer | MSEE
Skillset: Substation Design | Power System Studies | Arc Flash Study
DISCLAIMER: ALL VIEWS EXPRESSED IN THIS ARTICLE ARE MINE ALONE.