Power system (PS) requires new flexible AC transmission systems, new solutions for voltage instability problems with simple design and fast response and its equipment, should allow reactive control, short-circuit capacity, dynamic behavior, flexibility and more efficiency control for the grid associated to reliability and security. This research develops a magnetically controlled reactor (MCR) design for a weak system. A methodology for design is proposed with a primary winding connected to the PS and the secondary winding connected to voltage control, the magnetic flux is modified by the direct current, therefore, the equivalent impedance is modified. This proposal is validated in a case study for Peruvian grid with low short circuit and reactive power. Our findings are: (i) A dynamic model in failure condition regarding the error signal. (ii) The MCR can assume reactive compensation in the unsaturated and saturated regions, (iii) it can be scalable into several stages with banks.