Can I approximate impedance Z% to X% neglecting resistance in > 25MVA transformers?
If you assume Z = X and R =0, what would the actual value of Z if X/R = 5? Say X = 2%. For X/R = 5, R would be 0.4%. The aortal value of Z is the square root of the sum of the squares of X and Z or Z = 2.04%. So you have a small error. In other words, if X/R is 5 or larger, R is negligable.