498 research outputs found
Reactive power sharing and voltage harmonic distortion compensation of droop controlled single phase islanded microgrids
When paralleling multiple inverters that are capable
of operating as an island, the inverters typically employ the
droop control scheme. Traditional droop control enables the
decentralized regulation of the local voltage and frequency of
the microgrid by the inverters. The droop method also enables
the inverters to share the real and reactive power required by the
loads. This paper focuses on some of the limitations of parallel
islanded single phase inverters using droop control. Algorithms
with the aim to address the following limitations in islanded
operation were proposed: reactive power sharing and reduction
of the voltage harmonic distortion at the point of common
coupling (PCC). Experimental results were then presented to
show the suitability of the proposed algorithms in achieving
reactive power sharing and in improving the voltage harmonic
distortion at the PCC.peer-reviewe
Three-phase primary control for unbalance sharing between distributed generation units in a microgrid
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 where 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 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 resistance provides relatively more of the unbalanced currents
Selective virtual capacitive impedance loop for harmonic voltage compensation in islanded microgrids
Parallel inverters having LCL output filters cause
voltage distortions at the point of common coupling (PCC)
in islanded microgrids when non-linear loads are present. A
capacitive virtual impedance loop could be used to provide
selective harmonic compensation in islanded microgrids, instead
of introducing additional active or passive filters into the system
that could compromise the stability of the microgrid. However,
the performance of these compensation loops becomes degraded
when a virtual resistance is introduced with the aim to improve
the overall stability of the parallel inverters. With the capacitive
virtual impedance, there is effectively a compromise between
the additional stability provided by the virtual resistance and
the harmonic compensation due to the virtual capacitance. This
paper focuses on overcoming this limitation of the capacitive
virtual impedance with additional virtual resistance for selective
harmonic compensation in islanded microgrids. Simulation results
were given to show the suitability of the proposed algorithms
in reducing the voltage harmonics at the PCC.peer-reviewe
A Hierarchical Control Scheme for Reactive Power and Harmonic Current Sharing in Islanded Microgrids
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