Three-phase inverter-connected DG-units and voltage unbalance

The increasing presence of single-phase distributed generators and unbalanced loads in the electric power system may lead to unbalance of the three phase voltages, resulting in increased losses and heating. The distribution network operators (DNOs) are increasingly being challenged to maintain the required power quality. To reduce voltage unbalance DNOs are seeking to connect larger DG units to the three phases instead of a single-phase connection. The three-phase connection can be realised by three single-phase inverters or by a three-phase inverter. Each inverter topology can be implemented with different control strategies. The control can be equiped with active power filtering functions which can improve the power quality. In this paper, the effect of connecting DG units by means of a three-phase connection instead of a single-phase connection on voltage unbalance is studied. Besides two commonly used control strategies, two other control strategies that combine DG and active power filtering functions are implemented and their effect on voltage unbalance is studied. The last two control strategies lead to the reduction of voltage unbalance such that the voltage requirements are maintained.