Virtual Synchronous Generator System Research Articles

Study on improved control strategy of virtual synchronous generator

Virtual synchronous generator (VSG) control technology for photovoltaic, energy storage, wind power, and other new energy to provide flexibility in the grid interface characteristics, is conducive to improving the stability of the power system, and has been widely considered by many scholars. Firstly, an improved VSG control method is proposed through simulation and analysis, which realizes the complete decoupling of the frequency response time constant and the inertia quantity of the active power control loop and reduces the complexity of the parameter design of the VSG system. Secondly, to avoid the frequent action of the VSG system caused by small-scale frequency change perturbation, this study proposes a VSG frequency optimization control method for VSG frequency control with rated angular velocity ωset feedforward composed of multivariate factors when considering a primary frequency regulation dead zone. Thirdly, the impact of VSG parameter design on the system is investigated through the system response characteristics of power scheduling, primary frequency regulation at the grid connection, and the small-signal dynamic characterization of the improved VSG. Finally, Simulation and experimental verification yielded an active power overshoot of 7% and a maximum frequency deviation of 0.17 Hz for the improved system. The improved control method resulted in an improvement of 0.1 s in the frequency response time and a reduction of 0.15 Hz in the oscillation amplitude. The response speed of the improved control method is much better, while the oscillation amplitude is reduced to meet the grid's regulation requirements. The simulation and experimental analysis verify the feasibility of the improved VSG control method.

An improved design for virtual synchronous generator control loop based on synergy theory

Virtual synchronous generator (VSG) technology has achieved some results in distributed generation networks and enhanced system stability. For problems that may occur in the system due to ignoring parameter linkages, this paper designs a synergistic controller that establishes the connection between the active frequency and the reactive voltage control loop based on synergy theory. As the output of the synergetic controller, the derived control law u is added to the power frequency control in the form of negative feedback. Two disturbance forms, a step disturbance, and a three-phase short-circuit fault, are set up. The conventional droop control and synergetic droop control results are compared using a MATLAB simulation system. Then, the system parameter variations were studied and analyzed, and the sensitivity of the control system with different values of parameters K1, K2, and K3 in the case of a three-phase short circuit was analyzed, which provides a reference guide for selecting parameters in later control optimization. Finally, the control effectiveness of a multi-machine VSG system was tested. The simulation results show that the proposed method has a pronounced effect on making the system stabilize quickly, which illustrates the effectiveness of this technique.

Improved VSG strategy of grid-forming inverters for supporting inertia and damping

A virtual synchronous generator (VSG) strategy can introduce the rotational inertia and damping characteristics of the synchronous generator to the static inverter, e.g., PV, wind generation, and ESS, which are used to enhance the system frequency support characteristics of the micro-grid. However, under various operations of the VSG, the inertia and damping support capabilities are different, and the conventional VSG strategy with a fixed control coefficient sacrifices a certain degree of dynamic regulation performance with less robustness. This research proposes an improved VSG strategy with adaptive inertia and damping coefficients to increase the flexibility of VSG. To this end, a mathematical model is first established to analyze the impact of various parameters on the characteristics of VSG, and then the root trajectory is used to explore the impacts of various rotational inertia and damping coefficients on the stability of the VSG system. Second, an improved control strategy with adaptive inertia and damping coefficients is proposed based on the characteristics of the second-order system and the system frequency deviations. Finally, a simulation system with the VSG is constructed based on MATLAB/Simulink. The effectiveness of the proposed control strategy is verified by comparing the simulation results to the conventional control strategy with the fixed control coefficients under over-frequency and under-frequency disturbances.

Dynamic Coupling Mechanism Analysis Between Voltage and Frequency in Virtual Synchronous Generator System

Dynamic Coupling Mechanism Analysis Between Voltage and Frequency in Virtual Synchronous Generator System

Adaptive active and reactive power control strategy for virtual synchronous generator

SummaryWith the increase in the penetration level of renewables into the grid, the virtual synchronous generator (VSG) concept has emerged as an attractive solution for controlling grid‐tied inverters. The variation in reactive power also affects the real power of the system as well. To address the voltage variations and phase angle variations occurring because of the energy storage elements, the control of the reactive power loop (RPL) is important. Also, it is used to maintain voltage regulation and improve the power factor of the VSG system. This paper focuses on the damping of oscillations occurring because of load variation by proper adjustment of the rotational inertia, damping coefficient, droop coefficient, and regulatory coefficient of reactive power with a proposed adaptive control strategy. Small‐signal analysis of the active and reactive power and point of common coupling (PCC) have been derived, and the detailed dynamic performances are analyzed. The influences of parameters perturbation on PCC voltage and reactive power were investigated. Furthermore, adaptive control includes optimizing dynamic indexes such as rise time, delay time, and response settling time. Finally, the performance and improvement in the results of VSG with and without adaptive control are compared.

Optimal frequency fault‐tolerant control of virtual synchronous generator based on adaptive dynamic programming with fuzzy critic estimator

AbstractWith the popularity of renewable energy sources, fault‐tolerant frequency stability of distributed generation systems has become a major challenge. This paper proposes an optimal frequency fault‐tolerant control method in the distributed generation system, utilizing adaptive dynamic programming combined with fuzzy critic estimation. First, based on the operational state of the system, a non‐linear dynamic model for the virtual synchronous generator is established. Then, considering the occurrence of actuator faults, the fault‐tolerant control problem is formulated as an optimal control problem, where a fault observer is designed to estimate the fault signals. Furthermore, the proposed adaptive dynamic programming method based on fuzzy critic estimation is employed to effectively solve the highly coupled nonlinear Hamilton–Jacobi–Bellman equation and ensure the state convergence of the system with uniform ultimate boundedness. The optimal fault‐tolerant control law ensures a bounded frequency error for the virtual synchronous generator system in the event of faults. Finally, the effectiveness of the optimal control method is verified by two sets of simulations.

An improved VSG control strategy based on transient electromagnetic power compensation

Virtual synchronous generator (VSG) not only increases the inertia of grid-connected system, but also brings the problem of active power oscillation under grid disturbance. Therefore, VSG control strategy and system model order reduction method with transient electromagnetic power compensation are proposed. The closed-loop active power small signal model of the system is established, and the influence of transient electromagnetic power compensation on the power stability of VSG is analyzed based on root locus method. By removing the items which have little influence on the stability of the system in the small signal model, the order is reduced to obtain the equivalent second-order model of the system. According to the second-order model, the quantitative design criteria of the system parameters are given. The proposed transient electromagnetic power compensation strategy not only increases the transient equivalent damping of the system, but also does not affect the primary frequency modulation characteristics and will not cause large overshoot of the output active power. The experimental results are consistent with the theoretical analysis, which testify the effectiveness and correctness of the system control strategy and the model reduction method.

Fuzzy control based adaptive rotational inertia VSG grid integration research

Compared with the traditional inverter grid-connected control strategy, virtual synchronous generator (VSG) has received attention because it can effectively reduce the current shock to the grid caused by load fluctuation during grid-connected process by adding rotational inertia and damping parameters. A fuzzy control scheme is added to the rotor equation of motion of VSG to establish fuzzy rules by finding the relationship between frequency fluctuation and rotational inertia during grid connection. To improve the immunity of the frequency converter to interference during grid connection, and then improve the stability of the output power in the grid-connection process. The feasibility of the proposed method is verified by simulink model construction for simulation and comparison with the conventional virtual synchronous generator system.

Dynamic Interaction Analysis and Damping Control Strategy of Hybrid System With Grid-Forming and Grid-Following Control Modes

The dynamic behavior of converters with different synchronous control modes is significantly different. Therefore, the renewable energy grid-connected system cannot be equivalent as the same type of power electronics equipment. There are dynamic interactions among the multiple time scale controllers of each equipment, which may lead to the small signal instability of the hybrid system. In this paper, a hybrid grid-connected system with phase-locked loop (PLL) controlled doubly fed induction generator (DFIG) and virtual synchronous generator (VSG) controlled converter is studied. On the basis of the open-loop modal resonance theory, the mechanism of small signal instability of hybrid system is revealed from the perspective of mode interaction. Furthermore, a design method of additional damping controller is proposed based on normal matrix, which can achieve the minimum eigenvalue sensitivity of the hybrid system and avoid the interaction between different control loops. Finally, a hybrid system including a grid-forming VSG system and a grid-following DFIG wind farm is investigated in this paper. The correctness and effectiveness of interaction analysis and damping control strategy are confirmed by the results of modal analysis and simulation.

A New Power-Frequency Oscillation Suppression and Power Allocation Control Method Based on MPC in Multi-VSGs Parallel System

The virtual synchronous generator (VSG) technology can provide frequency support for power systems that are dominated by renewable power sources. However, it also introduces the characteristic of active power oscillation, which can cause power-frequency oscillation between parallel VSGs when there are disturbances in the load power or command power. To address this problem, this paper proposes that the idea of equivalent comparison can be used to compare the power-frequency interaction of N-VSGs parallel systems with the current-voltage interaction of N-RLC circuits. This allows us to establish a mathematical model of the parallel VSGs system based on Kirchhoff’s equation of the circuit. Using this model, we designed a new power-frequency oscillation suppression and a power allocation control method based on the model predictive control (MPC) in multi-VSGs parallel systems. This method can effectively suppress power-frequency oscillation in each VSG branch after a disturbance occurs, and accurately allocate active power according to instructions. Finally, simulation results confirm the effectiveness of the controller.

Active Power Oscillation Suppression Based on Decentralized Transient Damping Control for Parallel Virtual Synchronous Generators

Similar to the traditional synchronous generator (SG), a drawback of the virtual synchronous generator (VSG) is the active power oscillation, especially in the system with parallel VSGs. In this paper, a decentralized transient damping (DTD) control strategy is proposed to suppress active power oscillation during load fluctuations in the parallel VSGs system, which is based on embedding the transient electromagnetic power compensation and transient damping power compensation (TDPCP) control into the original VSG active power controller, providing two new degrees of freedom for the flexible regulation of the system damping ratio without frequency measurement. In this way, the damping ratio of the parallel VSGs system can be adjusted more freely without affecting the steady-state characteristics of the system. Therefore, this flexible transient damping characteristic can facilitate the suppression of active power oscillation under load fluctuations. Moreover, the proposed method is immune to the parameter mismatch of the parallel VSGs. The design of key parameters and stability analysis are presented. Finally, the feasibility of the proposed control strategy is verified through the simulation and experimental results.

A New Virtual Inductance Control Method for Frequency Stabilization of Grid-Forming Virtual Synchronous Generators

Frequency stabilization is the premise of guaranteeing grid-friendly integration of virtual synchronous generator (VSG). Based on that premise, this article, focused on frequency stability, establishes the small signal model of grid-forming VSG system. At first, the mechanism of frequency oscillation occurring in system of active- and reactive-power coupling is analyzed by the defined feedback effect factor in this article. Besides, a new dynamic model is established for identifying dynamic interaction of voltage magnitudes and frequency by means of feedback effect. The analytical results of established models agree with that of eigenvalue analysis. Furthermore, a new virtual inductance control strategy is proposed to mitigate the unstable oscillation of frequency and powers, enhance damping performance, and improve stability margins. Unlike the conventional virtual inductance control, which is reliable on dual-loop control framework, the proposed virtual inductance control in this article is based on principle of energy conservation and can be applicable for the grid-forming inverter without inner dual-loop control structure. Finally, the proposed modeling as well as virtual inductance control method is experimentally verified.

Low-Frequency Oscillation Analysis of Grid-Connected VSG System Considering Multi-Parameter Coupling

With the increasing integration of new energy generation into the power system and the massive withdrawal of traditional fossil fuel generation, the power system is faced with a large number of stability problems. The phenomenon of low-frequency oscillation caused by lack of damping and moment of inertia is worth studying. In recent years, virtual synchronous generator (VSG) technique has been developed rapidly because it can provide considerable damping and moment of inertia. While improving the stability of the system, it also inevitably causes the problem of active power oscillation, especially the low mutual damping between the VSG and the power grid will make the oscillation more severe. The traditional time-domain state-space method cannot reflect the interaction among state variables and study the interaction between different nodes and branches of the power grid. In this paper, a frequency-domain method for analyzing low-frequency oscillations considering VSG parameter coupling is proposed. First, based on the rotor motion equation of the synchronous generator (SG), a second-order VSG model and linearized power-frequency control loop model are established. Then, the differences and connections between the coupling of key VSG parameters and low-frequency oscillation characteristics are studied through frequency domain analysis. The path and influence mechanism of a VSG during low-frequency power grid oscillations are illustrated. Finally, the correctness of the theoretical analysis model is verified by simulation.

New Framework of RoCoF-FD for Wideband Stability Evaluation in Renewable Energy Generators With Virtual Impedance Control

This research proposes a new perspective for identification of wideband stability in virtual synchronous generator (VSG) with virtual impedance control strategy. At first, the drawback of magnitude-phase-motion-equation (MPME) of grid-forming VSG system is briefly discussed. Then, based on the derived MPME model, a RoCoF-FD framework, which includes two most important indices of frequency performance, e.g., rate of change of frequency (RoCoF) and frequency nadir/deviation (FN/FD), is established to illustrate the interaction between RoCoF and FD based on feedback effect. Moreover, the vector analysis can quantitatively sketch the dynamics of RoCoF impacted by the FD via the loop of feedback effect. In addition, the secure operation regions are defined for the VSG system. It is found that system has a higher risk of oscillation instability when ratio of grid resistance divided by grid inductance gets smaller. However, greater ratio will lessen grid strength/stiffness, which will trigger aperiodic loss of synchronization. To cope with such issues, an adjustable-parameter virtual impedance control strategy (APVICS), based on principle of energy conservation, is proposed to dampen oscillation as well as avoid loss of synchronization of the system, thereby enhance damping behaviors and grid stiffness. Finally, theoretical analysis is validated experimentally.

Frequency Stability Assessment of Grid-Forming VSG in Framework of MPME With Feedforward Decoupling Control Strategy

Virtual synchronous generator (VSG), recognized as a grid-friendly control technique, contributes to providing inertia and damping support from the grid-tied power electronic systems. However, similar to the conventional synchronous generator, the VSG is vulnerable to wideband oscillations and can even become unstable when the system is subjected to a variety of disturbances. To tackle this issue, this article proposes a magnitude-phase motion equation (MPME) for grid-forming VSG attached to a weak grid. Besides, a feedback analytical method is introduced to identify the frequency dynamic stability, judged by the sign of ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dω</i> / <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dt</i> )/ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">δω</i> . Besides, the rate of change of frequency (RoCoF) and frequency nadir (FN) are evaluated by the two indices, i.e., <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dω</i> / <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dt</i> and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">δω</i> . In addition, the relationship between the low-inertia-RoCoF-FN-related stability and the weak-damping-oscillation-related stability is illustrated by the feedback analysis. Moreover, in the framework of MPME, a reactive-power feedforward (RPFF) decoupling control strategy is proposed for the enhancement of stability and robustness performance of the VSG system. Finally, the proposed modeling and the RPFF decoupling control strategy are demonstrated with the experiments.

Characteristics of Parallel Inverters Applying Virtual Synchronous Generator Control

The virtual synchronous generator (VSG) is a promising way to deal with the lack of physical inertia and damping introduced by the power-electronic-based power supply system. However, the inertia and damping characteristics may be complicated in a paralleled VSG system due to the lack of a stiff power grid and the interactions between the control system of the inverters. In this paper, the dynamics of parallel inverters with VSGs is studied. The equivalent inertia and damping characteristics are derived and analyzed in detail considering both changing of set-points and load disturbance, where the transient load sharing is also investigated. Furthermore, a better dynamics is achieved by an inertia switching strategy. Experimental results verify the effectiveness of the analysis and the proposed strategy.

Coordinated Control Strategy of Virtual Synchronous Generator Based on Adaptive Moment of Inertia and Virtual Impedance

The application of virtual synchronous generators in the power system eases the pressure on the grid caused by penetration of lots of power electronic devices, but worsens the dynamic frequency stability of the grid. Meanwhile, in order to solve the problem of power coupling in low-voltage micro-grid systems, most measures introduced virtual impedance technology to adjust the equivalent output impedance of the system. Therefore, most studies have adopted the adaptive control strategy of the moment of inertia, to improve the dynamic frequency adjustment of the grid. However, it will cause the frequency response speed reduction problem. In order to deal with the contradiction between the moment of inertia and frequency response speed, the voltage phasor relationship of the micro-source - grid system under signal disturbance in the introduction of virtual impedance is analyzed. Then an adaptive virtual impedance control strategy is proposed to accelerate the active power adjustment process of the virtual synchronous generator system, which helps to complete the first frequency modulation and improve the inertia adjustment ability. Finally, combining these two control strategies, a coordinated adaptive moment of inertia and virtual impedance control strategy is proposed. From the perspective of the moment of inertia and virtual impedance, the proposed method fully exploits the control advantages of the virtual synchronous generator system and achieves the purpose of adjusting the inertia of the virtual synchronous generator system while considering the acceleration of the frequency response speed. The comprehensive simulation results verify the feasibility and effectiveness of the proposed method.

Artificial Intelligence-Based Control Design for Reliable Virtual Synchronous Generators

Virtual synchronous generator (VSG) is a promising solution for inertia support of the future electricity grid to deal with the frequency stability issues caused by the high penetration of renewable generations. However, the power variation in power electronic interface converters caused by VSG emulation increases the stress on power semiconductor devices and hence has a negative impact on their reliability. Unlike existing works that only consider stability for VSG control design, this article proposes a double-artificial neural network (ANN)-based method for designing VSG inertia parameter considering simultaneously the reliability and stability. First, a representative frequency profile is generated to extract various VSG power injection profiles under different inertia values through detailed simulations. Next, a functional relationship between inertia parameter (H) and lifetime consumption (LC) of VSG is established by the proposed double-ANN reliability model: ANN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> provides fast and accurate modeling of thermal stress in the semiconductor devices from a given operating profile; with the aid of ANN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> , ANN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</sub> is built for fast and accurate estimation of LC for different inertia parameters in the next step. The proposed approach not only provides a guideline for parameter design given a certain LC requirement, but can also be used for optimal design of VSG parameter considering reliability and other factors (e.g., inertia support in this article). The proposed technique is applied to a grid-connected VSG system as a demonstration example.

Renewable Energy System on Frequency Stability Control Strategy Using Virtual Synchronous Generator

This study constructs a novel virtual synchronous generator system based on a transfer function, and optimizes the parameters of the model by using the improved whale algorithm to improve the frequency control ability of virtual synchronous generator. Virtual synchronous generator technology helps to solve the problem that the integration of large-scale renewable energy generation into the power system leads to the deterioration of system frequency stability. It can maintain the symmetry of grid-connected scale and system stability. The virtual synchronous generator technology makes the inverter to have the inertia and damping characteristics of a synchronous generator. The inverter has the inertia characteristics and damps to reduce the frequency instability of high penetration renewable energy power system. The improved whale algorithm is efficient to find the best combination of control parameters and the effectiveness of the algorithm is verified by microgrid and power system. The results show that the proposed frequency coordination control scheme suppresses the frequency deviation of power system and keep the system frequency in a reasonable range.

Power Coupling Mechanism Analysis and Improved Decoupling Control for Virtual Synchronous Generator

A virtual synchronous generator (VSG) control-based grid-connected converter (GCC) is an attractive solution to improve the stability of a more renewable-energy-integrated power system. Unfortunately, the inherent power coupling (i.e., the interaction between the active power loop and the reactive power loop) defect of VSG control severely restricts the power delivery capacity and the grid support capability of the GCC. The virtual inductor is commonly used to reduce coupling, but its decoupling capability is very limited. In addition, the power coupling mechanism and its limiting factors are not clear. For this issue, the nature of power coupling in the VSG system is investigated first. The decoupling capability of the virtual inductor is studied, and the reason for decoupling effectiveness is revealed. It indicates that the effectiveness of decoupling results from the proper voltage compensation, but this kind of positive effect is limited by the d -axis voltage drop across the virtual inductor. Then, a q -axis voltage-drop-based power decoupling control (QVPDC) is proposed to further reduce the power coupling, which does not consider the d -axis voltage drop when applying the virtual inductor. Compared with the virtual-inductor-based decoupling method, the decoupling performance of QVPDC is better, and the computation burden is reduced by half. Finally, the analysis and the proposed method are validated by simulation and experiment.