A series injection device comprising a power splitter coupled to two or more lines (100, 102, 104) of an AC power system, wherein: the power splitter comprises a coupling transformer (300; 900, 902, 904) for each phase of a single phase or polyphase AC circuit that includes the two or more lines; each of the coupling transformers (300; 900, 902, 904) couples one of the phases of the two or more lines; the power splitter is configured to inject a first voltage of a first polarity into one or more of the two or more lines (102, 104) and inject a second voltage of a second polarity opposite the first polarity into at least one of the two or more lines (102, 104) via the same coupling transformers (300, 900, 902, 904) used to inject the first voltage; the first and the second voltages are controllable; the two or more lines (102, 104) are connected at a junction; each of the coupling transformers (300; 900, 902, 904) connects one of the lines (102, 104) to another one of the lines (102, 104) at the junction and is configured to inject a voltage at the junction so as to balance current flowing through the lines; a first one of the coupling transformers comprises a first winding connecting a first one of the lines (100, 102, 104) to a second one of the lines at the junction and a second winding connecting the first line (100a, 102) to a third one (100b, 104) of the lines at the junction; the series injection device further comprises a converter, wherein the first coupling transformer further comprises a third winding, wherein the converter is connected to the third winding, and wherein the converter is configured to inject a voltage to the third winding which is applied as a positive voltage to one of the second and the third lines and as a negative voltage to the other one of the second and the third lines (102, 104); characterized in that the series injection device further comprises a fuse connected in series between the first line and each winding (302, 304) of the first coupling transformer; a bypass impedance connected in parallel with each fuse, wherein the impedance of each bypass impedance is at least 10 times greater than a leakage impedance of the first coupling transformer. the power splitter comprises a coupling transformer (300; 900, 902, 904) for each phase of a single phase or polyphase AC circuit that includes the two or more lines; each of the coupling transformers (300; 900, 902, 904) couples one of the phases of the two or more lines; the power splitter is configured to inject a first voltage of a first polarity into one or more of the two or more lines (102, 104) and inject a second voltage of a second polarity opposite the first polarity into at least one of the two or more lines (102, 104) via the same coupling transformers (300, 900, 902, 904) used to inject the first voltage; the first and the second voltages are controllable; the two or more lines (102, 104) are connected at a junction; each of the coupling transformers (300; 900, 902, 904) connects one of the lines (102, 104) to another one of the lines (102, 104) at the junction and is configured to inject a voltage at the junction so as to balance current flowing through the lines; a first one of the coupling transformers comprises a first winding connecting a first one of the lines (100, 102, 104) to a second one of the lines at the junction and a second winding connecting the first line (100a, 102) to a third one (100b, 104) of the lines at the junction; the series injection device further comprises a converter, wherein the first coupling transformer further comprises a third winding, wherein the converter is connected to the third winding, and wherein the converter is configured to inject a voltage to the third winding which is applied as a positive voltage to one of the second and the third lines and as a negative voltage to the other one of the second and the third lines (102, 104); characterized in that the series injection device further comprises a fuse connected in series between the first line and each winding (302, 304) of the first coupling transformer; a bypass impedance connected in parallel with each fuse, wherein the impedance of each bypass impedance is at least 10 times greater than a leakage impedance of the first coupling transformer.