The article outlines the basic theory, operation principle, peculiarities of electromagnetic processes, and circuit and design solutions of a fundamentally new three-phase magnetic-valve controllable reactor, which combines increased response speed with high-precision stabilization of the current reactive power value. The advisability of a monoblock design with all power elements of the device placed in one transformer-type tank is substantiated. An example of the design of a magnetic-valve controllable transformer-type reactor for a capacity of 25 Mvar, and rated voltage of 35 kV is given along with an analysis of the effectiveness of its three-year operation as part of the Petrovsk-Zabaikalskaya 220/110/35 kV digital substation. It is shown that a reactive power source based on a new magnetic-valve controllable reactor is able---in addition to its main function of optimizing reactive power flows between power supply centers and load nodes---to normalize the voltage quality in a three-phase network with a nonlinear asymmetrical load in terms of such indicators as compensation of slow deviations of the three-phase voltage, symmetry of line-to-line voltages, and elimination of their waveform distortion.