New opportunities for optimally integrating the increasing number of distributed-generation (DG) units in the power system rise with the introduction of the microgrid. Most DG units are connected to the microgrid via a power-electronic inverter with dc link. Therefore, new control methods for these inverters need to be developed in order to exploit the DG units as effectively as possible in case of an islanded microgrid. In the literature, most control strategies are based on the conventional transmission grid control or depend on a communication infrastructure. In this paper, on the other hand, an alternative control strategy is proposed based on the specific characteristics of islanded low-voltage microgrids. The microgrid power is balanced by using a control strategy that modifies the set value of the rms microgrid voltage at the inverter ac side as a function of the dc-link voltage. In case a certain voltage, which is determined by a constant-power band, is surpassed, this control strategy is combined with -droop control. This droop controller changes the output power of the DG unit and its possible storage devices as a function of the grid voltage. In this way, voltage-limit violation is avoided. The constant-power band depends on the characteristics of the generator to avoid frequent changes of the power of certain DG units. In this paper, it is concluded that the new control method shows good results in power sharing, transient issues, and stability. This is achieved without interunit communication, which is beneficial concerning reliability issues, and an optimized integration of the renewable energy sources in the microgrid is obtained.

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a b s t r a c tThe 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.

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