A Control Strategy for Islanded Microgrids With DC-Link Voltage Control

Abstract

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.

Development of a smart transformer to control the power exchange of a microgrid

“…If the microgrid elements are equipped with the voltage-based droop (VBD) control strategy that is presented in [5], [6], these elements automatically change their input/output power dependent on the grid voltage.…”

“…3, has originally been presented for ensuring a stable operation of lowvoltage islanded microgrids [5]. For the active power control of [4] the DG units, this VBD controller consists of a combination of a V g /V dc droop controller and a P/V g droop controller, with V dc the dc-link voltage, where the power of the dc link is provided by the available renewable energy, and V g the terminal voltage of the DG unit.…”

Test field for LV distribution systems

“…A lot of simulation has been executed, although practical validation of these simulations should be executed. [9] [10] 3. The massive implementation of DG, possible storage equipment, DSM etc.…”

Soft curtailment for voltage limiting in low-voltage networks through reactive or active power droops

“…6, with their operation dependent on the rms voltage [8]. Key in this control strategy is that it operates without communication and is based on the intrinsic characteristics of the considered microgrid such as the resistive lines, lack of rotating inertia, and the high share of renewables.…”

“…Therefore, the so-called reversed droops, P /V droops, have been developed [7]. A variant of this strategy, the voltage-based droop (VBD) control presented in [8], combines P /V droop control with dc-link voltage droops. With this VBD control strategy, the power changes of renewable energy sources can easily be delayed (to more extreme voltage conditions) compared to those of the dispatchable DG units.…”