Abstract: Addressing the problem as staying with the imported medium voltage frequency converter for the main drive system of the Φ720 mm rotary expansion mill, i.e., poor protection and stability, a solution is proposed involving the replacement of the said imported converter with the home-made counterpart. Briefed here in the article are the structure and the technical characteristics of the home-made variable frequency speed regulation system, focusing on lot of innovative and optimal design items of the high-protection converter as applicable to the seamless steel tube-making field. Thanks to the localization-purposed modification, the stability of the rolling mill system is enhanced, and the cost input for maintenance of the system is reduced, resulting in excellent economic and social benefits, and being worth using as a technical guide for upgrade modification of similar imported medium voltage frequency converter for the main drive system.