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On-load tap-changers (OLTCs) are indispensable components in modern converter transformers for HVDC transmission lines. They compensate the system voltage variation, optimize the control angle of converter valves and realize the operation with reduced DC voltage. The non-sinusoidal current waveshape with the high rate of rise augments the switching duties and leads to an over sizing of the OLTC. The frequent tap-change operations result in high maintenance demand. The switching condition of OLTCs in converter transformers is compared to that of power transformers in this paper. The introduction of vacuum switching technology in OLTCs provides, besides the elimination of oil contamination, the unique advantage for heavy switching duties as that in converter transformers for HVDC applications. The rating of the OLTC can be adjusted optimally to the service duties. Since this new OLTC technique is maintenance-free up to 300,000 operations, the maintenance work can be reduced to a minimum. The reliability and the availability of converter transformers are therefore considerably improved. The sparing in volume and weight through the optimized rating of OLTCs makes a contribution to the future effort in ultra HVDC development.