Design, optimization and analysis of grid connected hybrid energy system under unbalanced loads

The unbalanced currents are significant power quality issues that interact with each other and particularly affect low-voltage power plants. Unbalanced load groups and/or unbalanced voltages are common conditions that cause unbalanced currents in three phase systems. Electricity services and distribution network operators are obliged to provide symmetrical electrical characteristics at the common connection point between distribution network and consumer loads. For this purpose, balanced current flow in the three phase systems is the responsibility of both electricity suppliers and consumers. This thesis presents an improved grid-inverter control scheme tested in hybrid photovoltaic (PV)/fuel cell (FC) energy system with unbalanced sensitive loads. In designed system, unbalanced loads draw unbalanced currents at each phase and consume different power values from energy units. This situation causes the currents flowing into the grid to become imbalance. This problem in the grid cannot compensated by a conventional power flow controller used in electrical grid inverters. For this purpose, a phase balancing control method is proposed in order to improve the grid quality under unbalanced loading conditions. The proposed method calculates power differentiation values and it is based on multiple-frame αβ/dq transform instead of abc/dq transform in the conventional method. With the proposed method, it eliminates negative & zero sequence components in addition to power flow control. In this regard, it removes the deficiency of conventional method, which generates the symmetrical reference values under unbalanced conditions. In this thesis, the system with unbalanced loads is performed for different case studies, which supply different power from energy units to grids. The optimization of energy systems is also provided through a maximum power point tracking algorithm. The results show that the proposed method provides power balancing at grid-side and elimination of negative & zero sequence components at grid-side currents. Also, the performance results are compared to the conventional method in order to verify the validity of the proposed approach under the designed load groups.