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A 40 MVA Auto transformer OF 115 KV/69 KV is equipped with automatic tap changer by having a series winding in the HV part of the winding. The problem is when the voltage increased in the 69 kv side there is a decrease in the 115 kv side. But in the two winding traditional transformer, the HV voltage is always the same, and only LV side will change. Why in auto transformer both voltages are changing?