Abstract: Disclosure is made of a rectifier-inverter system of a converter substation intended for the transmission of energy between a.c. networks having different frequencies, which system comprises one or more pairs of rectifier and inverter converters connected to a common d.c. circuit.According to the invention, the poles of at least one rectifier or inverted converter are connected to a protective grounding via resistor-capacitor elements.The absence of direct contact with the ground in the d.c. circuit rules out the possibility of a low-resistance closed circuit being formed as a result of a short-circuit earthing at any point of the d.c. circuit. This reduces the voltage across the insulation with respect to the ground at each pole of all the converters and diminishes the steady voltage component across the insulation with respect to the ground of transformer valve windings.