A comprehensive review of virtual synchronous generator

Although the low-frequency/harmonic oscillation of grid-tied inverters under weak grids has been widely researched, the classical impedance-based approach focuses more on the identification of stable operating points and the stability margin of return-ratio matrix. Furthermore, it cannot provide stability regions of system parameters. Therefore, this paper proposes a line impedance cooperative stability region identification method. Firstly, the output impedance matrix and the input admittance matrix are respectively built in $d-q$ axis. Secondly, a novel stability forbidden region is proposed. It is less conservative than the forbidden region criteria in existing literatures. Based on the stability forbidden region, the stability operation region is established via mirror, translation and rotation mappings. It is less conservative compared with the norm-based stability criteria. The solving of stability operation region is first transformed into the eigenvalue identification problem in this paper. Furthermore, the detailed line impedance cooperative stability region is solved by guardian map theory. It can provide guidance for system planning and stabilization method researches with simplified computational process. Finally, the simulation and experimental results show that the proposed line impedance cooperative stability region identification method is more effective and less conservative.

Line Impedance Cooperative Stability Region Identification Method for Grid-Tied Inverters Under Weak Grids

The droop control is an advantageous approach for stand-alone supply systems consisting of multiple batteries, allowing among various inverters without intercommunication. The droop coefficients of batteries always vary with their state-of-charge (SoC) and charge/discharge mode, resulting in small-signal instability. Nevertheless, the existing impedance-based approaches can only assess the droop coefficients stability point, but not the stability region. Therefore, this article proposes a droop coefficients stability region analysis approach. First, the charge/discharge SoC-based droop controlled battery, the P&Q controlled distributed generator and the constant power load are separately discussed. Meanwhile, the state matrix and return-ratio matrix are established, respectively. Furthermore, the novel forbidden region criterion based on the return-ratio matrix is constructed, which reduces conservatism compared with norm-based impedance criteria and partial forbidden region criteria. Such a forbidden region criterion is first switched to the Hurwitz identification problem regarding the equivalent return-ratio matrix. Combined the state matrix and the equivalent return-ratio matrix, the generalized incidence matrix is constructed to simultaneously identify subsystem stability and interactive stability. Based on the generalized incidence matrix, an adaptive step search strategy is proposed to obtain the droop coefficients coordinated stability region. Finally, the simulation and experimental results illustrate the validity of the proposed method.

SoC-Based Droop Coefficients Stability Region Analysis of the Battery for Stand-Alone Supply Systems With Constant Power Loads

Active power oscillation is observed when two or more parallel synchronverters undergo load fluctuations, potentially impacting the safe operation of the synchronverters. This article provides a fundamental analysis for the power oscillation mechanisms and proposes a new control strategy for suppressing power oscillation especially the oscillation excitation is proposed. It is found that power oscillation is inherently caused by weak damping and large inertia of the synchronverters under the condition of undesirable instantaneous active power sharing. Then, the condition for reducing the oscillation excitation of synchronverters, which is caused by undesirable instantaneous active power sharing, is deduced. The new control strategy effectively suppresses the power oscillation by adding a virtual damping element for enhancing the system damping and a virtual reactance for reducing the oscillation excitation to the conventional control scheme without changing the basic dynamic and steady characteristics of the synchronverters. The theoretical analysis and the proposed oscillation suppression control techniques are validated with an extensive hardware-in-loop simulation study.

Active Power Oscillation and Suppression Techniques Between Two Parallel Synchronverters During Load Fluctuations

This paper proposes a dual control of energy hub considering security operation and economic efficiency which is constituted by the control of energy hub outputs and inner devices. As security operation is a significant requirement of multi-carrier energy system (MCES) with energy hubs, it is necessary for MCES to regulate the outputs of hubs which highly influence the performance. The control approach of energy hub outputs can proportionally allocate the electricity and heat outputs of energy hubs. Meanwhile, the decentralized control of energy hubs can be realized to eliminate the impact of communication on security operation by implementing the proposed method. Furthermore, in view of the characteristics of energy hub, the elements in hubs also need to be regulated to reduce energy consumption. The control of inner devices based on the improved equal incremental consumption principle can reach to the minimal loss of the energy. Numerical simulations demonstrate the effectiveness of the proposed method.

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The Dual Control With Consideration of Security Operation and Economic Efficiency for Energy Hub

The premise that the virtual synchronous generator (VSG) can actively support the weak grid is that the VSG can operate stably. In this paper, the small-signal sequence impedance models of the voltage-controlled VSG and the current-controlled VSG are built and the sequence impedance characteristics of these two types of VSGs are compared and analyzed. The sequence impedance of the voltage-controlled VSG, being consistent with the grid impedance characteristics, is generally inductive. By contrast, the sequence impedance of the current-controlled VSG is mainly negative impedance in the middle- and high-frequency area and the impedance amplitude is quite low. Based on the built sequence impedance models and the impedance stability criterion, the influence of the weakness of the grid on the stability of these two types of VSG grid-connected system is analyzed. The results of stability analysis show that the current-controlled VSG is prone to instability while the voltage-controlled VSG remains stable. Therefore, the voltage-controlled VSG is more suitable than the current-controlled VSG for grid-connected renewable energy generation in an ultraweak grid from the point of view of system stability. Finally, experiment results validate the stability analysis.

Mingmin Zhang

Papers

Sequence Impedance Modeling and Stability Comparative Analysis of Voltage-Controlled VSGs and Current-Controlled VSGs