Technical Analysis of Paralleling 2x25 MVA Power Transformers in CEPALCO’s PDO Substation

The increasing power demand in Cagayan de Oro City, Philippines, requires the expansion of Pueblo de Oro substation, owned by the electric distribution utility Cagayan Electric Power & Light Co., Inc. (CEPALCO). This expansion involves adding a new $\mathbf{6 9 k V} / \mathbf{3 4. 5 k V}$ 25MVA power transformer and operating it in parallel with the existing $69 \mathrm{kV} / 34.5 \mathrm{kV}$ 25MVA transformer. This study evaluated the technical feasibility and impact of this parallel operation on CEPALCO’s distribution system. This study also examines the allowable differences in impedance and X/R ratio between the two transformers and their optimal tap position settings, and compares the impacts of their independent and parallel operations through simulations. The results revealed that the parallel operation of the two transformer is feasible with identical or slightly different tap position settings, ranging from 0.925 to 1.000 for both transformers. Specific tap position settings of 0.938 or 0.950 may be necessary to maintain a 1 pu voltage at the secondary bus. The additional transformer’s percent impedance (% Z) should closely match the existing transformer’s 10.440%, and its X/R ratio should be higher than 22.373. In conclusion, paralleling the existing and additional transformers in the Pueblo de Oro substation is technically feasible and offers several advantages over independent operation, specifically improved system reliability and efficiency.