FLI for Unbalanced and Harmonic Current Mitigation in Rooftop Solar Connected Distribution Network

This paper proposes a three-phase four-leg inverter (FLI) current control in a rooftop solar (RS) system connected to a distribution network (grid) via distribution transformers. A DC/AC converter, when the RS works in parallel with the distribution system, can also inject harmonics. The RS injection current's value is fluctuating and unbalanced in the three phases due to irradiation changes. This condition causes the current that the transformer must supply by each phase is different. The instantaneous fundamental power demand, both active and reactive on the transformer at the PCC point, is also not the same. Likewise, when there are harmonics in the system. FLI is needed to inject or draw current on the PCC to balance the transformer's current. The primary purpose of proposing this control system is to balance the current and eliminate the harmonic output current from the distribution transformer due to unbalanced RS injection capacity and unbalanced load. The proposed control strategy is to use single-phase PQ theory in controlling the FLI of each phase independently. The instantaneous power demand by the combined load and RS on the transformer is calculated and averaged. The result is evenly distributed across the transformers and FLI. Therefore, when viewed by the transformer, the load and RS become balanced even in unbalanced and fluctuating conditions. The simulation results with Matlab/Simulink, applying FLI with PQ Theory, improve the transformer's current unbalance from 35% to 0.16%. THDi on phases A, B, and C decreased from 22.98%, 18.08%, 12.08% to 8.22%, 11.60%, 9.21%, respectively.