Microgrids consist of multiple parallel-connected distributed generation (DG) units with coordinated control strategies, which are able to operate in both grid-connected and islanded mode. Microgrids are attracting more and more attention since they can alleviate the stress of main transmission systems, reduce feeder losses, and improve system power quality. When the islanded microgrids are concerned, it is important to maintain system stability and achieve load power sharing among the multiple parallel-connected DG units. However, the poor active and reactive power sharing problems due to the influence of impedance mismatch of the DG feeders and the different ratings of the DG units are inevitable when the conventional droop control scheme is adopted. Therefore, the adaptive/improved droop control, network-based control methods and cost-based droop schemes are compared and summarized in this paper for active power sharing. Moreover, nonlinear and unbalanced loads could further affect the reactive power sharing when regulating the active power, and it is difficult to share the reactive power accurately only by using the enhanced virtual impedance method. Therefore, the hierarchical control strategies are utilized as supplements of the conventional droop controls and virtual impedance methods. The improved hierarchical control approaches such as the algorithms based on graph theory, multi-agent system, the gain scheduling method and predictive control have been proposed to achieve proper reactive power sharing for islanded microgrids and eliminate the effect of the communication delays on hierarchical control. Finally, the future research trends on islanded microgrids are also discussed in this paper.

In this paper, an enhanced hierarchical control structure with multiple current loop damping schemes for voltage unbalance and harmonics compensation in ac islanded microgrid is proposed to address unequal power sharing problems. The distributed generation (DG) is properly controlled to autonomously compensate voltage unbalance and harmonics while sharing the compensation effort for the real power, reactive power, unbalance and harmonic powers. The proposed control system of the microgrid mainly consists of the positive sequence real and reactive power droop controllers, voltage and current controllers, the selective virtual impedance loop, the unbalance and harmonics compensators, the secondary control for voltage amplitude and frequency restoration, and the auxiliary control to achieve a high voltage quality at the point of common coupling (PCC). By using the proposed unbalance and harmonics compensation (UHC), the auxiliary control, and the virtual positive/negative-sequence impedance (VPI/VNI) loops at fundamental frequency, and the virtual variable harmonic impedance (VVHI) loop at harmonic frequencies, an accurate power sharing is achieved. Moreover, the low bandwidth communication (LBC) technique is adopted to send the compensation command of the secondary control and auxiliary control from the microgrid control center (MGCC) to the local controllers of DG unit. Finally, the hardware-in-the-loop (HIL) results using dSPACE 1006 platform are presented to demonstrate the effectiveness of the proposed approach.

In this paper, the modeling, controller design, and stability analysis of the islanded microgrid (MG) using enhanced hierarchical control structure with multiple current loop damping schemes is proposed. The islanded MG is consisted of the parallel-connected voltage source inverters using LCL output filters, and the proposed control structure includes: the primary control with additional phase-shift loop, the secondary control for voltage amplitude and frequency restoration, the virtual impedance loops which contains virtual positive-and negative-sequence impedance loops at fundamental frequency, and virtual variable harmonic impedance loop at harmonic frequencies, and the inner voltage and current loop controllers. A small-signal model for the primary and secondary controls with additional phase-shift loop is presented, which shows an over-damped feature from eigenvalue analysis of the state matrix. The moving average filter-based sequence decomposition method is proposed to extract the fundamental positive and negative sequences, and harmonic components. The multiple inner current loop damping scheme is presented, including the virtual positive, virtual negative and variable harmonic sequence impedance loops for reactive and harmonic power sharing purposes and the proposed active damping scheme using capacitor current feedback loop of the LCL-filter, which shows enhanced damping characteristics and improved inner-loop stability. Finally, the experimental results are provided to validate the feasibility of the proposed approach.

Grid-connected inverters (GCIs) with LCL output filter have the ability of attenuating high-frequency (HF) switching ripples. However, by using only grid-current control, the system is prone to resonances if it is not properly damped, and the current distortion would be amplified significantly under highly distorted grid conditions. In this paper, a synchronous reference frame equivalent proportional-integral (SRF-EPI) controller in αβ stationary frame using the parallel virtual resistance-based active damping (PVR -AD) strategy for grid-interfaced distributed generation (DG) systems to suppress the LCL resonance is proposed. Although both proportional-resonant (PR) controller in αβ stationary frame and PI controller in dq synchronous frame achieve zero steady-state error, the amplitude-and phase-frequency characteristics differ greatly from each other except for the reference tracking at fundamental frequency. Therefore, an accurate SRF-EPI controller in αβ stationary frame is established to achieve precise tracking accuracy. Moreover, the robustness, harmonic rejection capabilities, and influence of control delay are investigated by the Nyquist stability criterion when the PVR-based AD method is adopted. Furthermore, the grid voltage feed-forward and multiple PR controllers are integrated in the current loop to mitigate the current distortion introduced by the grid background distortion. Besides, the parameters design guidelines are presented to show the feasibility and effectiveness of the proposed strategy. Finally, simulation and experimental results are provided to validate the feasibility of the proposed control approach.

This paper presents the dynamic modeling and the stator-voltage-aligned control (SVAC) strategies of the doubly fed induction generator (DFIG)-based wind energy generation system (WEGS). The state-space dynamic model of the DFIG is derived in the synchronous d-q reference frame. The control strategies of the DFIG-based WEGS includes three parts, namely, the network-side converter (NSC) control, the rotor-side converter (RSC) control and the maximum power-point tracking (MPPT) algorithm. The NSC and RSC control algorithms are implemented based on the internal model control principle. The SVAC algorithm is devised for the RSC, where the positions of the stator and rotor voltage-vectors are referenced to the stator-frame to simplify the controller. The hill-climbing method is adopted for the MPPT algorithm, and the iteration procedure for searching the optimal operation point is discussed. The simulation results obtained from the electromagnetic transient program (EMTP-ATP) are provided, which validates the effectiveness of the control algorithms. The theoretical analysis and EMTP-based digital simulation can be widely applied for the grid-connected converters in renewable generation and smart grid applications.

The model predictive torque control (MPTC) is an effective strategy for high performance motor systems. The strategy obtains the optimal voltage vector more quickly and accurately compared with the traditional direct torque control. However, some problems of the strategy are needed to be solved, such as few active vectors, difficult cost function design, and hard duty cycle regulation and so on. An improved MPTC is put forward for these problems in this study. The number of the vector is increased by constructing virtual vectors in the improved method. The finite control set under the rotating coordinate system based on the stator flux orientation is established to select the vector and reduce the computation load. The evaluation mechanism of the selected vector is set by combined with the duty cycle method, so the weight factor of cost function in the traditional method is eliminated. And the duty cycle can play a full role in the adjustment. The prototype experiment system is built for verifying the proposed method. The results demonstrate that the proposed method has better torque and flux control performances and effectively improve the above problems in the traditional method.

-This paper presents the small-signal state-space modeling and a new multifunctional multi-loop control strategy for three-phase inverter-based islanded DG systems under unbalanced and/or nonlinear load conditions. The proposed control methodology utilizes the parallel virtual resistance (PVR)-based active damping (AD) method for the islanded DG system using an extra feedback capacitor current shaping loop in the stationary reference frame to provide an AD to suppress the resonance-peak of the LC filter and improve both steady-state and transient performances, the inner voltage and current controllers are based on an enhanced proportional resonant (PR) structure to achieve zero steady-state error, and multi-resonant harmonic compensator (MRHC) plus PR controller to prevent low-order load current harmonics to distort the output voltage. The proposed small-signal model of the islanded DG system with multi-loop control strategy in the stationary reference frame is presented. Moreover, an enhanced delay compensation (EDC) scheme based on two integrators of the discrete PR controller is presented to improve stability margins with a higher accuracy compared with the existing methods. Then, a detailed systematic controller parameters design procedure is presented. Finally, simulation and experimental results are provided to validate the effectiveness of the proposed strategy.

No abstract