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Application of the third harmonic current injection in three-phase full-bridge thyristor rectifiers is analyzed. Spectra of the rectifier output terminal voltages are derived. Results of the optimal current injection for diode bridge rectifiers are generalized for the thyristor rectifiers, and the waveform of the optimal injected current is derived. It is shown that for the optimal third harmonic current injection, the current injection network takes 8.571% of the rectifier input power from the thyristor bridge output terminals, regardless the thyristor firing angle. A current injection network with a passive resistance emulator that provides recovery of the power taken from the thyristor bridge output terminals is proposed. Volt-ampere ratings of the magnetic components are derived. An algorithm to control the switches in the resistance emulator is presented. Analytically obtained results are experimentally verified.