Thyristor assisted on-load tap changers for transformers

Mechanically operated on-load tap changers are widely used on power transformers to vary the output voltage by tap selection. During the process of tap changing it is essential that there is no interruption of the current flow. A method commonly used is shown where transition resistors are connected across the transformer tappings during the changeover period. The double compartment type tap change gear to which the authors refer comprises two major components, a tap selector and a diverter switch each of which are located in transformer oil. The tap selector selects the tapping; its electrical contacts are designed to carry the transformer rated current but not to make or break such current. The diverter switch is designed to carry, make and break the load current in circuits previously selected by the tap selector. The authors describe a new circuit arrangement whereby the diverter switch contacts are relieved of their duties to make and break current resulting in reduced contact erosion and oil contamination