The voltage regulation operation with on-load tap-changing device is carried out by the operating personnel according to the voltage curve determined by the supervisory dispatching department. The number of regulation times per day is generally no more than 20 times for 35kV main transformers and no more than 10 times for 110 - 220kV main transformers (each regulation of a tap is counted as one time). The reverse regulation method is adopted to control the power supply voltage as close as possible to the optimal level.