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Aiming at the power coupling problem caused by the high impedance ratio between the distributed energy(DG)controlled by the virtual synchronous generator(VSG)and the power grid. In this paper,the power coupling mechanism is analyzed,and the coupling relationship between power and impedance ratio and power angle is established by relative static gain coefficient. A virtual power control technology based on virtual negative impedance is proposed. The virtual impedance is used to calculate the virtual voltage,and the obtained virtual voltage is fed back to the power calculation part to form the virtual power to realize power decoupling. Based on the constraint condition of coupling degree,the specific solution method of virtual resistance and inductance is given. Finally,the simulation model is built by using MATLAB/ Simulink simulation software. By comparing with the traditional virtual impedance technology,it is verified that the proposed virtual power control strategy has a significant effect on weakening the coupling between the active and reactive power loops. [ABSTRACT FROM AUTHOR]

A virtual synchronous generator (VSG) has a good adaptability to the weak grid but its grid-connected active power (GCAP) has the problem of a slow dynamic response under the active power command step. An optimization strategy of the GCAP dynamic response for the VSG based on the virtual negative impedance combined with the active power transient damping control algorithm is proposed in this paper. The optimization strategy first uses the virtual negative impedance control method to reduce the VSG equivalent output impedance and the GCAP dynamic response time of the VSG. Then, the transient damping as well as the inhibition ability of the GCAP dynamic oscillation for the VSG are enhanced by the active power transient damping control algorithm. The Matlab/Simulink simulation software is used to study the GCAP dynamic response performances of the VSG in the condition of the active power command step, and the experimental test platform of a VSG grid-connected system is established. The simulation and experimental results jointly verify the feasibility and superiority of the proposed strategy in improving the GCAP dynamic response characteristics of the VSG under a weak grid. [ABSTRACT FROM AUTHOR]

A virtual synchronous generator (VSG) has a good adaptability to the weak grid but its grid-connected active power (GCAP) has the problem of a slow dynamic response under the active power command step. An optimization strategy of the GCAP dynamic response for the VSG based on the virtual negative impedance combined with the active power transient damping control algorithm is proposed in this paper. The optimization strategy first uses the virtual negative impedance control method to reduce the VSG equivalent output impedance and the GCAP dynamic response time of the VSG. Then, the transient damping as well as the inhibition ability of the GCAP dynamic oscillation for the VSG are enhanced by the active power transient damping control algorithm. The Matlab/Simulink simulation software is used to study the GCAP dynamic response performances of the VSG in the condition of the active power command step, and the experimental test platform of a VSG grid-connected system is established. The simulation and experimental results jointly verify the feasibility and superiority of the proposed strategy in improving the GCAP dynamic response characteristics of the VSG under a weak grid.

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Due to the existence of line impedances and low-bandwidth communication, the traditional peer-to-peer control method based on droop control has difficult meeting the requirements of current sharing and voltage stability in islanded DC microgrids at the same time. In this paper, a novel current-sharing control strategy based on injected small ac voltage with low frequency and low amplitude is proposed for multiple paralleled DC−DC converters. The small ac voltage is superimposed onto the output voltage of each converter. Then, the reactive circulating power is generated and used to regulate the output DC voltage of each converter. Under the droop characteristic between the injected frequency and output DC current, a feedback mechanism is generated to realize the accurate current sharing. On this basis, a reactive power-voltage limiter link and virtual negative impedance are added. Under the interaction of the two links, the bus voltage drop caused by line impedances can be almost completely eliminated. This method does not need any communication or to change the hardware structure. The controller design process is presented in detail along with a system stability analysis. Finally, the feasibility and effectiveness of the proposed control strategy are validated by the results obtained from simulations and experiments.

Multi-mode operation and transient stability are two problems that significantly affect flexible microgrid (MG). This paper proposes a multi-mode operation control strategy for flexible MG based on a three-layer hierarchical structure. The proposed structure is composed of autonomous, cooperative, and scheduling controllers. Autonomous controller is utilized to control the performance of the single micro-source inverter. An adaptive sliding-mode direct voltage loop and an improved droop power loop based on virtual negative impedance are presented respectively to enhance the system disturbance-rejection performance and the power sharing accuracy. Cooperative controller, which is composed of secondary voltage/frequency control and phase synchronization control, is designed to eliminate the voltage/frequency deviations produced by the autonomous controller and prepare for grid connection. Scheduling controller manages the power flow between the MG and the grid. The MG with the improved hierarchical control scheme can achieve seamless transitions from islanded to grid-connected mode and have a good transient performance. In addition the presented work can also optimize the power quality issues and improve the load power sharing accuracy between parallel VSIs. Finally, the transient performance and effectiveness of the proposed control scheme are evaluated by theoretical analysis and simulation results. [ABSTRACT FROM AUTHOR]

• A current sharing method based on virtual frequency is proposed. • The virtual voltage does not introduce actual reactive power and harmonics. • This method improves the current sharing accuracy without causing a voltage drop. • This method improves the communication fault tolerance of current sharing control. • This method improves the plug-play performance of the converters. In the DC microgrid with traditional droop control, the inconsistency of circuit parameters and line impedance results in the diverse droop characteristics of DC-DC converters, which reduces the current sharing accuracy of converters. Meanwhile, droop control inevitably leads to DC-bus voltage deviation and the secondary control has the problem of poor communication fault tolerance. To improve the control performance of DC microgrid, a droop control method based on virtual frequency is proposed by simulating the frequency droop control mechanism in AC microgrid. A virtual voltage with a frequency proportional to the output current is constructed and shared among the converters through low bandwidth communication (LBC). The phase difference among virtual voltages generates virtual reactive power, which is calculated by Fourier transform to coordinate the output current of each converter. The algorithm can keep high performance under some communication faults and can guarantee that the converter with serious communication faults can automatically exit the current sharing control without affecting other converters. Moreover, the actual reactive power and harmonics wouldn't be introduced, and the power quality of the microgrid can be ensured. The stability of the algorithm is demonstrated by small-signal analysis and mathematical calculation. Finally, simulation and experiment show that the algorithm has high current sharing accuracy, excellent communication fault tolerance and plug-play performance. [ABSTRACT FROM AUTHOR]

Due to the existence of line impedances and low-bandwidth communication, the traditional peer-to-peer control method based on droop control has difficult meeting the requirements of current sharing and voltage stability in islanded DC microgrids at the same time. In this paper, a novel current-sharing control strategy based on injected small ac voltage with low frequency and low amplitude is proposed for multiple paralleled DC–DC converters. The small ac voltage is superimposed onto the output voltage of each converter. Then, the reactive circulating power is generated and used to regulate the output DC voltage of each converter. Under the droop characteristic between the injected frequency and output DC current, a feedback mechanism is generated to realize the accurate current sharing. On this basis, a reactive power-voltage limiter link and virtual negative impedance are added. Under the interaction of the two links, the bus voltage drop caused by line impedances can be almost completely eliminated. This method does not need any communication or to change the hardware structure. The controller design process is presented in detail along with a system stability analysis. Finally, the feasibility and effectiveness of the proposed control strategy are validated by the results obtained from simulations and experiments. [ABSTRACT FROM AUTHOR]