In power stations, generator step up transformers and unit auxiliary transformers are most common. Generator step up transformers are large three phase power transformers that raise generator voltage to high transmission levels such as 132 kV, 220 kV or higher. They are designed for high efficiency, strong short circuit withstand and often include on load tap changers. Unit auxiliary or station service transformers step down generator or transmission voltage to medium and low voltages for plant loads such as pumps, fans, controls and auxiliary equipment.