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In inverter operation of a converter for h.v.d.c. transmission, commutation failure is inevitable in such a case that a.c. voltage suddenly drops due to occurrence of a fault in the a.c. system. In this paper, the region of inevitable commutation failure is first discussed and then the constant power factor control system is proposed to squeeze the region. By using this control system, the extinction angle is kept larger in partial load than in full load. Also, the proposed control system makes it possible to rapidly increase d.c. transmission power from the minimum to full loads without controlling transformer taps.