An Islanding Microgrid Power Sharing Approach Using Enhanced Virtual Impedance Control Scheme

To address the load sharing problem in islanding microgrids, this paper proposes an improved approach which regulates the distributed generation (DG) unit interfacing virtual impedance at fundamental and selected harmonic frequencies. In contrast to the conventional virtual impedance control where only a line current feed-forward term is added to the DG voltage reference, the proposed virtual impedances at fundamental and harmonic frequencies are realized using DG line current and point of common coupling (PCC) voltage feed-forward terms, respectively. With this modification, the mismatched DG feeder impedances can be properly compensated, resulting in accurate reactive and harmonic power sharing at the same time. In addition, this paper shows that the microgrid PCC harmonic voltages can be mitigated by reducing the magnitude of DG unit equivalent harmonic impedance. Finally, an improved proportional plus resonant voltage controller is developed to ensure accurate power sharing and PCC harmonic voltage compensation without using any fundamental/harmonic component detections.

For a conventional islanded microgrid, each distributed generation (DG) unit usually equips with an LC filter and a closed-loop filter capacitor voltage control is applied to ensure an accurate tracking of DG voltage. In this article, a cost reduction microgrid and DG units configuration is developed via the following two steps. First, an L -type filter is adopted by each DG unit and a centralized shunt capacitor is placed at point of common coupling (PCC). Then, the inherent synchronization feature of droop control is further exploited as an inherent synchronizer to identify the carrier angle relationships between parallel inverters, but without any communications between them. With the carrier information obtained, a decentralized interleaving pulsewidth modulation (PWM) among parallel inverter can be easily achieved via a slow proportional integral (PI) regulation of each DG unit relative carrier. Accordingly, the high-frequency switching harmonics of PCC voltage are suppressed even with simplified and size reduced DG output chokes. Second, a virtual impedance control is applied to DG units to cancel the voltage drops on the DG feeder in a direct PWM manner. Accordingly, mitigating the low-order harmonic PCC voltages is obtained without using any closed-loop voltage tracking at DG unit local processor. Comparative experimental results have been provided to validate the correctness of the proposed method.

This paper presents the control strategies and output (virtual) impedance design approach for parallel operated distributed generation (DG) units in a low voltage (LV) microgrid system. To achieve communicationless control among DG interfacing converters, virtual impedances are placed between the interfacing converter outputs and the grid to improve system stability and mitigate coupling between real and reactive power. In order to properly design the virtual impedance, small-signal models of a microgrid in different operation modes are developed using complex number matrix, which is flexible and can be easily extended when more DG units are added. Based on the developed microgrid models, system damping and stability are evaluated. Together with the DG power capacity and power decoupling requirements, the desired DG output virtual impedance can be designed. To realize the virtual impedance, a robust implementation method that avoids using of any high/low pass filters is presented. Moreover, to further improve the power control performance during transients, a transient power decoupling algorithm and an adaptive transient impedance control scheme are proposed. Experimental results are provided to validate the impedance design approach, the virtual impedance implementation method and the proposed transient power control strategies.

This paper proposes an improved power control approach to address the load sharing problems in islanded microgrids. The proposed method first estimates distributed generation (DG) unit feeder impedances through measuring low order point of common coupling (PCC) harmonic voltages and DG unit line harmonic currents. With the knowledge of feeder impedances, reactive power sharing performance can be enhanced by the regulation of DG unit output virtual impedance. The proposed method realizes accurate real and reactive power sharing in proportion to DG unit rated power. Simulated and experimental results from a single-phase microgrid are provided to validate the feasibility of the proposed method.

The application of underground cables and shunt capacitor banks may introduce power distribution system resonances. In this paper, the impacts of voltage-controlled and current-controlled distributed generation (DG) units to microgrid resonance propagation are compared. It can be seen that a conventional voltage-controlled DG unit with an LC filter has a short-circuit feature at the selected harmonic frequencies, while a current-controlled DG unit presents an open-circuit characteristic. Due to different behaviors at harmonic frequencies, specific harmonic mitigation methods shall be developed for current-controlled and voltage-controlled DG units, respectively. This paper also focuses on developing a voltage-controlled DG unit-based active harmonic damping method for grid-connected and islanding microgrid systems. An improved virtual impedance control method with a virtual damping resistor and a nonlinear virtual capacitor is proposed. The nonlinear virtual capacitor is used to compensate the harmonic voltage drop on the grid-side inductor of a DG unit LCL filter. The virtual resistor is mainly responsible for microgrid resonance damping. The effectiveness of the proposed damping method is examined using both a single DG unit and multiple parallel DG units.

Line impedance mismatch and unregulated harmonic currents cause serious problems for an islanded microgrid, such as inaccurate reactive power sharing and voltage distortion at the point of common coupling (PCC). To overcome these issues, a coordinated virtual resistance and capacitance are introduced together with its control scheme for parallel distributed generation (DG) in an islanded microgrid. The proposed coordinated virtual impedance control scheme directly modifies the DG output impedance at the fundamental frequency to realise accurate current sharing among DG units. In addition, a variable harmonic impedance control loop is proposed for each DG to absorb the non‐linear load harmonic current. Thanks to the proposed control scheme, the PCC harmonic voltage distortion is successfully compensated and power‐sharing among DG units is accurately achieved. The stability of the microgrid system was analysed in detail to verify the feasibility of the coordinated virtual impedance control scheme. Experiments on a laboratory prototype microgrid are performed to validate the performance of the proposed control scheme.

Among parallel inverters in islanded microgrid, circulating current originated by output impedance mismatches and output voltage differences cannot be neglected. Conventional droop control and fixed virtual impedance control are sensitive when the load changes. In this paper, the concept of equivalent feeder is proposed to cover the mismatch of external feeder impedance. The differences of external inductor, local load, and transmission line among parallel inverters are considered. Based on equivalent feeder impedance calculation, an aggregated control strategy of improved droop control and adaptive virtual impedance control is introduced. To improve power sharing accuracy, the equivalent feeder voltage drop of each inverter is calculated and compensated to the reference voltage of droop control. Conventional virtual impedance control is defective to cope with sudden load perturbations, especially when local loads exist. In this regard, the adaptive virtual complex impedance control can adjust to the equivalent feeder impedance, which provides accurate and quick response to load changes and line impedance mismatches. Simulation results show that the proposed control strategy can reduce circulating current effectively as well as improve power sharing accuracy.

Distributed virtual negative-sequence impedance control for accurate imbalance power sharing in islanded microgrids

A robust decentralized control strategy for the islanded operation of a microgrid system is proposed in this paper. The microgrid consists of parallel connection of two electronically-coupled distributed generation (DG) units. The DG units are connected to a point of common coupling (PCC) where a passive load is also connected. In the islanded microgrid, a master DG fulfills voltage and frequency control of the load, and a slave DG utilizes the conventional dq-current control strategy to regulate its power components. It is shown that the parallel connection of two DG units in the islanded mode constitutes an interconnected composite system consisting of two subsystems. Moreover, it is shown that the overall islanded microgrid can be controlled by local controllers, i.e., there is a solution to the robust decentralized servomechanism problem for the composite system. Performance of the proposed control strategy is verified using MATLAB/SimPowerSystems Toolbox.

Summary If sensitive loads are connected to the point of common coupling (PCC), compensation should be carried out in order to reduce PCC voltage harmonics. In this case, compensation is performed by the interface converters of distributed generation (DG) units. In this paper, a novel method is proposed for voltage harmonics compensation in islanded microgrids. The proposed method is capable of adjusting compensation percentage in proportion to the existing level of distortion at PCC. Also, this controller provides a proper sharing of compensation workload, considering the nominal capacity of DGs, which will improve the sharing of nonlinear load. Also, to decrease the asymmetry among phase impedances of microgrid, a novel structure is proposed to generate virtual impedance. At fundamental frequency, the proposed structure for the virtual impedance improves the control of the fundamental component of the power, and at harmonic frequencies, it acts to adaptively improve nonlinear load sharing among DG units. In the structures of the proposed harmonics compensator and the proposed virtual impedance, a self-tuning filter is used for separating the fundamental component from the harmonic components. This self-tuning filter decreases the number of the phase-locked loops. The simulation results in MATLAB/SIMULINK environment show the efficiency of the proposed approach in improving load sharing and decreasing voltage harmonics. Copyright © 2016 John Wiley & Sons, Ltd.

For an islanded microgrid, load power should be properly shared among multiple distributed generation (DG) units, not only for the fundamental power but also for the harmonic power. In this paper, the operation of microgrids under imbalanced and nonlinear load conditions is studied, and a dynamic consensus based power controller is proposed for DG units to realize enhanced power sharing performance. Firstly, the fundamental and low order harmonic powers of a DG unit are detected by using second-order generalized integrator (SOGI) modules. Then, the obtained local power information is transmitted to the neighbor DG units to help tuning their virtual impedances at fundamental and selected harmonic frequencies. After this, accurate fundamental and harmonic power sharing performance will be achieved. In the proposed control strategy, DG units only communicate with their neighbor units, thus the “plug and play” function will be reserved. Compared with those centralized control methods, the proposed strategy with distributed consensus protocol can simplify the maintenance and expansion of the system, making the microgrid more flexible. Moreover, as the knowledge of the detailed network is not required, it is convenient to be applied in practice. Simulation results are presented to demonstrate the proposed method.

With high penetration of power electronics-based distributed generation (DG) units, the decentralized control strategy based on droop control with virtual impedance has been widely studied in recent years. But the conventional virtual impedance cannot properly deal with the massively increasing of nonlinear loads. Therefore, an improved virtual impedance method based on second-order general-integrator (SOGI) is presented. Besides, a further value correction of reactive power in droop control is also proposed in this paper. The proposed approaches have been applied and verified in a parallel system formed by two DG units connected with nonlinear loads. The form of SOGI and derivative formula of corresponding virtual impedance in the system has been analyzed in detail. The obtained experimental results show the proposed approaches can achieve less output voltage and current distortion of DG units with nonlinear loads, and also realize good performance of power sharing.

As a new emerging approach, virtual synchronous generator (VSG) control for interfacing inverters in renewable distributed generation (DG)-based microgrids has attracted significant research interest. However, VSG control cannot ensure proportional reactive power sharing among DG units when unequal transmission line impedances are involved, although active power sharing remains proportional among DGs. In this article, an adaptive virtual impedance-based VSG control approach for grid-connected and islanded microgrids is proposed to alleviate impedance difference at the inverter output and improve proportional reactive power sharing among DGs. The virtual impedance consists of an adaptive virtual resistance and a fixed virtual inductance, where the virtual resistance is designed adaptively based on operation points of the microgrid. In this article, the proposed adaptive virtual impedance control is implemented on a modified VSG control, known as fuzzy secondary controller-based VSG control, offering excellent voltage and frequency regulation at the PCC. Its performance is demonstrated through case and sensitivity studies using MATLAB/Simulink simulation; and further validated by comparing with an existing method.

Inverter-based distributed generation (DG) units in microgrid (MG) call for robust and precise control methods that attain high performance under both normal and unbalanced conditions. Efficient control scheme needs to be investigated for the proper operation of a microgrid in both grid-tied and islanding condition. Using the excellent performance of slide mode controller(SMC) for nonlinear loads and the ability of proportional resonant(PR) controller to precisely track reference sinusoidal signal, a cascaded SMC and PR controller has been proposed for stable operation of microgrid. The phenomenon of chattering a drawback in SMC is eliminated by using fixed frequency switching of the inverter circuit using smooth control law in a narrow boundary layer. For effective sharing of the loads among different DER’s during isolation mode of operation a droop controller is proposed. The proposed controller showed excellent results for linear and nonlinear loading. To analyze the performance of droop controller, proposed for parallel operation of two DG units, simulations are performed under linear and distorted load conditions. Results obtained shows that proposed cascaded controller and droop controller improves the output by the reduction of the total harmonic distortion (THD), high degree of robustness, fast response time, and efficient power sharing among DG units.

Microgrid (MG) is experiencing a rapid growth and better integration of renewable distributed generation (DG) units with utility grid and plays a key role in grid infrastructure upgrade. It is required from these DG units to stay connected during and after any voltage disturbance to support the voltage, further ensuring the power system stability. In this context, an adapted inverter control scheme for fault ride through (FRT) is proposed to enhance the MG stability after fault provoked islanding. Hence, a maximum available support of inverter DG unit regarding the reactive power enhancement is achieved. In addition, the influence of using the droop control strategy on the MG stability is analyzed with and without the proposed FRT scheme. Superior feature for the FRT based on the overloading capability of the inverter DG unit is proposed to enhance MG performance. The MG under study includes a synchronous diesel DG unit, an inverter DG unite, and a combined load (static, and dynamic induction motor (IM) load). The structure of MG and the proposed inverter control strategy and scheme are employing using Matlab/Simulink package. A wide perspective on the simulation results verify the feasibility of the proposed FRT to enhance MG stability at the fault incidents.