Tap-changer for transformers with less harmonics

On-load tap changing is one of the requirements of an efficient transmission system. Static methods using switching devices have been suggested. In many of the suggested methods, the receiving-end voltage is a phase-shifted version of the Input voltage and unwanted harmonics are generated during the process of the addition of voltage to the sending-end voltage. These harmonics at the receiving-end may cause severe problems. To circumvent the disadvantages encountered with the generation of harmonics, a new method of control using PWM technique is suggested (Multi level power Inverter) in this paper. The additional advantage of the proposed scheme where series transformer is used for the addition of voltage to the sending-end voltage is the level of voltage that could be achieved using low-voltage and low dv/dt switches. Indeed, this principle is also applicable to the control of drives. The basic principles are brought-out and verified for the practical implementation.