What occurs to the low side voltage of a 345/115 kV auto-transformer if the high side tap is decreased by 5%?

In an auto-transformer, the voltage is divided between the high and low sides based on the tap configuration. When the high side tap is decreased by a certain percentage, it affects the turns ratio between the high voltage winding (in this case, the 345 kV side) and the low voltage winding (the 115 kV side).

Decreasing the high side tap effectively means reducing the number of turns in the high side winding, which alters the ratio of the voltages across the high and low sides. As the turns on the high side decrease, the voltage on the low side will increase as a consequence of the transformation ratio. This is due to the principle of an auto-transformer where the relationship between the input (high side voltage) and output (low side voltage) is mathematically linked through the turns ratio.

In this scenario, a 5% decrease in the high side tap increases the low side voltage proportionally, leading to a rise of 5% in the low side voltage. This understanding of transformer operation and the effects of changing tap settings is fundamental in power systems and transmission networks, especially in maintaining voltage stability and reliability.