# 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,

$\frac{Load_{T_{1}}}{Load_{T_{2}}} = \frac{Z_{T_{2}}}{Z_{T_{1}}}$

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

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Just my opinion, did review the calculation the admin did it right @1937.5 thr transformer will contribute the load inversely proportional to the load. Convert impedance in pu to the second transformer use the relationship S1/S2=ZT2/ZT1. Hope this will help…[email protected]

Parag

The very basic condition to load and operate any two transformers in parallel is that both should have equal Impedence value without which as a law of least resistance path one will be more loaded than another one.

Siri

When I solve the above problem by transfering the T2 Z to T1 base values, I get below values.
When T2 is completely loaded to 1500 KVA, T1 supplies 1599 KVA (=1500*.0533/.05). Then the total load that can be connected = 2599 KVA, which is not equal to 1937.5 KVA (obtained above). I was expecting both values to be equal irrespective of whether we take first transformer base or second transformer base. Am I missing anything here? Could you please clarify?