Zero-sequence current injection based power flow control strategy for grid inverter interfaced renewable energy systems

This paper deals with an energy management issue for the grid-connected renewable energy sources connected with unbalanced loads. In the grid-connected energy systems, conventional energy management methods are used to manage balanced active power supplied from energy units to grids/loads. However, unbalanced load groups, like single-phase loads, parallel rectifiers, and three-winding transformers tied to the electric networks, cause zero-sequence currents and impair the power stability at the grid-side. For this purpose, in this study, an improved power flow controller method with zero-sequence current injection is proposed in order to compensate zero-sequence currents and ensure phase equilibrium at grid-side. Therefore, it is tested in a photovoltaic/fuel cell-based hybrid energy system with unbalanced loads, including zero-sequence. Instead of the conventional abc/dq frame used in energy management control, the proposed method makes use of triple alpha beta transforms and implemented in each phase to be controlled separately under designed unbalanced load groups. Hence, it abolishes the weakness of the conventional abc/dq frame method that generates the undesirable mean reference for zero-sequence situations. In addition, the designed system cannot only mitigate the grid-side zero-sequence currents but also supply active powers to three-phase loads/grids. The optimization of energy systems is also provided through a maximum power point tracking algorithm. In the performance testing, the energy generation units are performed under three states, which produce different supplied currents at the output. In the performance section, it is obvious that zero-sequence components at grid-side currents are significantly reduced via the proposed power flow controller-based system. Also, the results are compared to the conventional method in order to verify the validity of the proposed approach under the designed load groups.