Abstract
For islanded microgrids, droop-based control concepts have been developed both in single and three-phase variants. The three-phase controllers often assume a balanced network; hence, unbalance sharing and/or mitigation remains a challenging issue. Therefore, in this paper, unbalance is considered in a three-phase islanded microgrid in which the distributed generation (DG) units are operated by the voltage-based droop (VBD) control. For this purpose, the VBD control, which has been developed for single-phase systems, is extended for a three-phase application and an additional control loop is added for unbalance mitigation and sharing. The method is based on an unbalance mitigation scheme by DG units in grid-connected systems, which is altered for usage in grid-forming DG units with droop control. The reaction of the DG units to unbalance is determined by the main parameter of the additional control loop, viz., the distortion damping resistance, Rd. The effect of Rd on the unbalance mitigation is studied in this paper, i.e., dependent on Rd, the DG units can be resistive for unbalance (RU) or they can contribute in the weakest phase (CW). The paper shows that the RU method decreases the line losses in the system and achieves better power equalization between the DG unit’s phases. However, it leads to a larger voltage unbalance near the loads. The CW method leads to a more uneven power between the DG unit’s phases and larger line losses, but a better voltage quality near the load. However, it can negatively affect the stability of the system. In microgrids with multiple DG units, the distortion damping resistance is set such that the unbalanced load can be shared between multiple DG units in an actively controlled manner rather than being determined by the microgrid configuration solely. The unit with the lowest distortion damping resistance provides relatively more of the unbalanced currents.
Highlights
Microgrids can become incubators of smart grid technologies
The resistive for unbalance (RU) method shows a resistive behavior towards unbalanced currents; it generates some unbalance in the terminal voltage of the distributed generation (DG) units (VUF(vg ) > 0) to counteract the unbalance in the line currents current unbalance factor (CUF)(ig )
A negative Rd in the CW method better mitigates the voltage unbalance of the loads, at the expense of larger line losses and the output power of the DG unit being more unevenly spread between the three phases, which was expected in Section 4.1; see Table 1
Summary
Microgrids can become incubators of smart grid technologies. These new technologies can be integrated faster and at a lower cost in microgrids than in the overall electric power system [1], and they can be selected to meet the specific needs of the local microgrid users. The VBD controller is extended for three-phase application, keeping its intrinsic advantages, such as maximizing the renewable energy capture and taking into account the specific nature of the low-voltage islanded microgrids, e.g., the low inertia and the predominantly resistive line parameters. In [17], a communication-based controller is added to the droop controller for improving the voltage quality of a critical load, i.e., compensation of the voltage unbalance and the harmonics This compensation effort is shared among the DG units according to their rated powers. The VBD control is extended for three-phase application, and an additional control loop, characterized by the distortion damping resistance, Rd , is included for taking into account the unbalance in the system.
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