An impedance injection unit, IIU, system configured to be connected to a high voltage HV transmission line, comprising: a plurality of IIUs configured to generate a plurality of rectangular impedance waveforms; and a controller, integrated with or coupled to the plurality of IIUs (400-A1, 400-A2, 400-B1, 400-B2, 400-C1, 400-C2, 400-D1, 400-D2, 400); wherein the controller is configured to: activate each of the plurality of IIUs in a sequence of activation in each of a plurality of successive time periods (T1, T2, T3, T4) such that the plurality of rectangular waveforms are injected to form a pseudo-sinusoidal impedance waveform onto a high voltage HV transmission line; and repeatedly change an ordering of IIUs (400-A1, 400-A2, 400-B1, 400-B2, 400-C1, 400-C2, 400-D1, 400-D2, 400) in the sequence of activation from one of the plurality of successive time periods (T1, T2, T3, T4) to a next one of the plurality of successive time periods (T1, T2, T3, T4), such that the electrical stress across the plurality of IIUs (400-A1, 400-A2, 400-B1, 400-B2, 400-C1, 400-C2, 400-D1, 400-D2, 400) over the plurality of successive time periods (T1, T2, T3, T4) is equalized; wherein at least a part of the plurality of IIUs (400-A1, 400-A2, 400-B1, 400-B2, 400-C1, 400-C2, 400-D1, 400-D2) are connected forming at least one sub-group of IIUs (500-1, 500-2, 500-3, 500-4), each sub-group of IIUs (500-1, 500-2, 500-3, 500-4) comprising at least two parallel connected IIUs (400-A1, 400-A2, 400-B1, 400-B2, 400-C1, 400-C2, 400-D1, 400-D2); and wherein the controller is further configured to cumulative count the sum of the injection durations (d1, d2, d3, d4) of each IIU and/or sub-groups of IIUs (500-1, 500-2, 500-3, 500-4) in the plurality of successive time periods (T1, T2, T3, T4), such that the step of repeatedly change an ordering of the plurality of IIUs (400-A1, 400-A2, 400-B1, 400-B2, 400-C1, 400-C2, 400-D1, 400-D2) in the sequence of activation from one of the plurality of successive time periods (T1, T2, T3, T4) to a next one of the plurality of successive time periods (T1, T2, T3, T4) is carried out by the controller so that the sum of the injection durations (d1, d2, d3, d4) of each IIU and/or sub-group of IIUs (500-1, 500-2, 500-3, 500-4) is equal over the plurality of successive time periods (T1, T2, T3, T4). a plurality of IIUs configured to generate a plurality of rectangular impedance waveforms; and a controller, integrated with or coupled to the plurality of IIUs (400-A1, 400-A2, 400-B1, 400-B2, 400-C1, 400-C2, 400-D1, 400-D2, 400); wherein the controller is configured to: activate each of the plurality of IIUs in a sequence of activation in each of a plurality of successive time periods (T1, T2, T3, T4) such that the plurality of rectangular waveforms are injected to form a pseudo-sinusoidal impedance waveform onto a high voltage HV transmission line; and repeatedly change an ordering of IIUs (400-A1, 400-A2, 400-B1, 400-B2, 400-C1, 400-C2, 400-D1, 400-D2, 400) in the sequence of activation from one of the plurality of successive time periods (T1, T2, T3, T4) to a next one of the plurality of successive time periods (T1, T2, T3, T4), such that the electrical stress across the plurality of IIUs (400-A1, 400-A2, 400-B1, 400-B2, 400-C1, 400-C2, 400-D1, 400-D2, 400) over the plurality of successive time periods (T1, T2, T3, T4) is equalized; activate each of the plurality of IIUs in a sequence of activation in each of a plurality of successive time periods (T1, T2, T3, T4) such that the plurality of rectangular waveforms are injected to form a pseudo-sinusoidal impedance waveform onto a high voltage HV transmission line; and repeatedly change an ordering of IIUs (400-A1, 400-A2, 400-B1, 400-B2, 400-C1, 400-C2, 400-D1, 400-D2, 400) in the sequence of activation from one of the plurality of successive time periods (T1, T2, T3, T4) to a next one of the plurality of successive time periods (T1, T2, T3, T4), such that the electrical stress across the plurality of IIUs (400-A1, 400-A2, 400-B1, 400-B2, 400-C1, 400-C2, 400-D1, 400-D2, 400) over the plurality of successive time periods (T1, T2, T3, T4) is equalized; wherein at least a part of the plurality of IIUs (400-A1, 400-A2, 400-B1, 400-B2, 400-C1, 400-C2, 400-D1, 400-D2) are connected forming at least one sub-group of IIUs (500-1, 500-2, 500-3, 500-4), each sub-group of IIUs (500-1, 500-2, 500-3, 500-4) comprising at least two parallel connected IIUs (400-A1, 400-A2, 400-B1, 400-B2, 400-C1, 400-C2, 400-D1, 400-D2); and wherein the controller is further configured to cumulative count the sum of the injection durations (d1, d2, d3, d4) of each IIU and/or sub-groups of IIUs (500-1, 500-2, 500-3, 500-4) in the plurality of successive time periods (T1, T2, T3, T4), such that the step of repeatedly change an ordering of the plurality of IIUs (400-A1, 400-A2, 400-B1, 400-B2, 400-C1, 400-C2, 400-D1, 400-D2) in the sequence of activation from one of the plurality of successive time periods (T1, T2, T3, T4) to a next one of the plurality of successive time periods (T1, T2, T3, T4) is carried out by the controller so that the sum of the injection durations (d1, d2, d3, d4) of each IIU and/or sub-group of IIUs (500-1, 500-2, 500-3, 500-4) is equal over the plurality of successive time periods (T1, T2, T3, T4).