Evaluation of 3 Inter Bus Transformer 500/150/71 kV 500 MVA in parallel operation

摘要

Field data showed that several paralleled transformers only fulfill some requirements stated in IEC 60076-8. Unequal voltage ratio and short circuit impedance difference between multiple Inter Bus Transformers (IBT) in parallel operation will generate serious problem. Therefore, an evaluation should be conducted before performing parallel operation. This study proposed a brief evaluation of transformer parallel operation using circulating current and load division calculation which was validated by simulation of 3 non identical IBT. Based on the simulation, circulating current flows from IBT with higher secondary voltage to the IBT with lower secondary voltage during load zero crossing and creates phase shift between each IBT's load current toward total loading current. The presence of circulating current will influence IBT's loading current by vector relationship. Unequal short circuit impedance leads to unbalanced load sharing which exceed the tolerance stated in standard. The short circuit current of 3 IBT in parallel operation is very close to the short circuit rating of the installed equipment. Therefore, equal voltage ratio, appropriate short circuit impedance and the short circuit rating of the equipments should be confirmed first before operating 3 IBT in parallel.