ORC system for generation of energy by Organic Rankine Cycle having two or more pressure levels, comprising: at least one heat exchanger to exchange heat between a high temperature source and an organic working fluid, to heat, evaporate and superheat said organic working fluid; at least one turbo-expander presenting an inlet in use fed with the vaporized organic working fluid coming out from the heat exchanger, to make a conversion of the thermal energy present in the organic working fluid into mechanical energy according to a Rankine cycle; at least one condenser where in use the vaporized organic working fluid coming out from an outlet of said at least one turbo-expander is condensed; at least one pump to feed the organic working fluid coming out from the condenser to said at least one heat exchanger; at least an auxiliary circuit for injection of the organic working fluid into said turbo-expander; characterized in that said at least one turbo-expander is of radial centrifugal type and comprises: a single rotor disc carrying a plurality of rotor blades (111 a, 111 b, 111 c); at least an auxiliary opening interposed between the inlet and the outlet and in fluid communication with said auxiliary circuit, to inject the organic working fluid at an intermediate pressure between an inlet pressure and an outlet pressure; wherein the turbo-expander comprises a plurality of stages (111 a, 113a, 111 b, 113b, 111 c, 113c) radially disposed one after the other and wherein the auxiliary opening opens between two of said stages; wherein the radial distance (Rd) between the two stages between which the auxiliary opening opens is greater than the radial distance between the other stages to delimit an injection chamber of the organic working fluid. at least one heat exchanger to exchange heat between a high temperature source and an organic working fluid, to heat, evaporate and superheat said organic working fluid; at least one turbo-expander presenting an inlet in use fed with the vaporized organic working fluid coming out from the heat exchanger, to make a conversion of the thermal energy present in the organic working fluid into mechanical energy according to a Rankine cycle; at least one condenser where in use the vaporized organic working fluid coming out from an outlet of said at least one turbo-expander is condensed; at least one pump to feed the organic working fluid coming out from the condenser to said at least one heat exchanger; at least an auxiliary circuit for injection of the organic working fluid into said turbo-expander; a single rotor disc carrying a plurality of rotor blades (111 a, 111 b, 111 c); at least an auxiliary opening interposed between the inlet and the outlet and in fluid communication with said auxiliary circuit, to inject the organic working fluid at an intermediate pressure between an inlet pressure and an outlet pressure; wherein the turbo-expander comprises a plurality of stages (111 a, 113a, 111 b, 113b, 111 c, 113c) radially disposed one after the other and wherein the auxiliary opening opens between two of said stages; wherein the radial distance (Rd) between the two stages between which the auxiliary opening opens is greater than the radial distance between the other stages to delimit an injection chamber of the organic working fluid.