The main operating transformer connected by a adscititious tandem on-load-tap-changing one is the important mode.About t通过基本定义和有关名牌值进行了计算和分析，认为在正负极限档，外加串联调压变的阻抗值相对额定档的偏差很小，所以可不必在意这种变压器整体阻抗的偏差值。