How to calculate the transformer DC resistance imbalance of the transformer?
Dividing the maximum number of three-phase subtracted by the three-phase average is the unbalance rate.
Such as OA=5.761, OB=5.772, OC=5.758
DC resistance unbalance rate = (OB-OC) / ((OA + OA + OC) / 3) = (5.772-5.758) / ((5.761 + 5.772 + 5.758) / 3) = 0.014 / 5.7636667 = 0.002429 * 100% =0.2429
Unbalance: It means that the three-phase current (or voltage) amplitude is inconsistent in the power system, and the amplitude difference exceeds the specified range.
According to the power transformer with the specified capacity of 1. 6MVA or above, the phase resistance unbalance rate should not exceed 2%, the winding without neutral point, the line resistance unbalance rate should not exceed 1%; the capacity of 1.6MVA and below Transformer, the phase resistance unbalance rate is generally not more than 4%, and the line resistance unbalance rate is generally not more than 2%.
From the engineering practical point of view, it can be considered that the line resistance imbalance is half of the phase resistance unbalance rate. Therefore, it is recommended that for all three-phase transformers, regardless of their capacity, only the unbalance rate limit of the DC line resistance can be specified as 2%.