To address the unbalanced and harmonic power sharing issue among parallel inverters caused by feeder impedance mismatch in islanded microgrids, an adaptive virtual impedance control method is proposed based on the injection of an extra small AC signal (SACS) in the output voltage of each inverter. Similar to the principle of active power-frequency droop, the frequency of the injected signal droops with the output unbalanced and harmonic power, while the active power produced by the injected SACS is detected to adjust the virtual impedance at the fundamental negative sequence and selected harmonic frequencies, which will tune the distribution of unbalanced and harmonic power in the system. When the injected SACSs of each inverter synchronize with each other and reach a common frequency in steady state, the virtual impedance of each inverter will be matched to each other for evenly sharing the unbalanced and harmonic power. This proposed method requires neither communication links among parallel inverters nor feeder impedance information. Furthermore, the control parameter design method based on modeling and stability analysis of the proposed control structure is discussed in detail. Finally, simulation and experimental results are provided to validate the effectiveness of the proposed scheme.

In an islanded microgrid composed of droop-controlled parallel inverters, the system frequency endures deviations as the load changes. To compensate for frequency deviation without involving communication infrastructures among distributed generators (DGs), the proportional-integral regulator based secondary frequency control (PI-SFC) method has been proposed and implemented. However, PI-SFC may incur real power-sharing errors because the integrator accumulates disturbances and noise in each DG, leading to different compensation values of nominal real power. To achieve frequency restoration while maintaining equal real power sharing among DGs, this paper proposes a small-AC-signal injection-based secondary frequency control method (SACS-SFC), which is implemented by injecting an additional AC signal into the output voltage of each DG. Furthermore, a droop relation between the frequency of the injected SACS and the compensation value of nominal real power is innovatively established to trim the output real power of each DG to be equal. Frequency deviations caused by primary droop control are thus eliminated, and even real power sharing can be maintained among DGs. Moreover, the control parameters of the proposed SACS-SFC are comprehensively designed via modeling and stability analysis. Simulation and experimental results demonstrate the effectiveness of the proposed method.

In an autonomous ac microgrid system, a peer-to-peer decentralized solution would be a preferred choice for the coordinative control of distributed generation units. However, due to the dispersed sources and loads, and mismatched line impedances, it is hard to achieve satisfactory power sharing performance and power quality without communication links. Therefore, a comprehensive decentralized solution based on dual-frequency-droop is proposed in this article. With only local information, appropriate virtual impedances could be obtained to compensate the mismatched complex line impedances in the required frequency range, and hence active, reactive, unbalanced, and harmonic powers could be shared accurately among parallel inverters under various load conditions. Then, the frequency deviation caused by the droop control is effectively attenuated without affecting power sharing through a dynamic droop-slope regulator. Moreover, the causes of steady-state ripples on virtual impedance waveforms are carefully analyzed, and the design criterions to eliminate these ripples are provided. Finally, case studies are provided in simulations and experiments to validate the effectiveness of this method.

As a key parameter in microgrids, feeder impedance is closely related to power distribution, power quality, and system stability. This letter proposes a novel noninvasive feeder impedance estimation method (FIEM), which uses the microgrid central controller (MGCC) and inherent harmonic signals introduced by a nonlinear or unbalanced load. First, the sequence impedance model for a droop-controlled inverter is built to demonstrate that the inverter output impedance is negligible at low-order harmonic frequencies. Then, the bus voltage and output current at a specific dominant harmonic frequency are extracted in MGCC, based on which feeder impedance can be obtained. Finally, the estimated feeder impedance is used to generate proper virtual impedance to enhance power sharing and voltage quality as an example, based on a hierarchical control structure. Experimental results are provided to validate the proposed method.

Grid-forming inverters, which are represented by droop control and virtual synchronous generator control, have been widely studied and applied because of their excellent grid-supporting ability and smooth off-grid switching. When a grid-forming inverter is connected to a microgrid or utility grid, the control loops of active power and reactive power will be coupled because of the voltage phase difference, which will affect the power control performance. This paper first derives the small-signal linearized model of the system, based on which a frequency feedforward control and an amplitude feedforward control are proposed to decouple the active power and reactive power control loops, respectively. The proposed decoupling strategy directly modifies the reference values through feedforward with an easily implementable principle that is applicable to various control coordinate systems, control coordinate systems, and control structures. By comparing system models with and without the proposed decoupling strategy, its effectiveness can be theoretically proven. Time-domain simulations and hardware experiments are presented to further validate its effectiveness.

Aiming at the problem that the bus voltage in a low-inertia DC microgrid is prone to be affected by internal power fluctuations, an adaptive virtual inertia control strategy for a grid-connected converter of a DC microgrid based on an improved model prediction is proposed. Firstly, the adaptive analog virtual synchronous generator (AVSG) is introduced into the voltage outer loop by combining the inertial parameters with the voltage change rate, and the flexible adjustment of the inertial parameters is realized. Secondly, the improved model predictive control is introduced into the current inner loop to realize the fast-tracking of the given current value and improve the dynamic characteristics of the control system. Finally, a system model is established based on Matlab/Simulink for simulation. The results show that compared with the traditional virtual inertia control strategy, the proposed control strategy has smaller bus voltage fluctuation amplitude and better dynamic performance; when a 10 kW load mutation occurs, the magnitude of bus voltage drop is reduced by 60%, and the voltage recovery time is shortened by 30%. The proposed control strategy can effectively improve the stability of DC bus voltage and the operation ability of the system under asymmetric conditions.