Multiple Virtual Synchronous Generator Research Articles

Current coordinated distribution and voltage optimization control for multi-VSGs system considering nonlinear and negative sequence loads

In a microgrid system where multiple virtual synchronous generators (VSGs) are interconnected through high impedance cables, the total output negative sequence and harmonic current of multiple VSGs cannot be coordinated based on the capacity of each power conversion system (PCS). In addition, there is significant distortion in the voltage at the point of common coupling (PCC). Because the impedance amplitude and impedance angle of the cable change with the harmonic frequency, the resistive virtual impedance reshaping method and fixed impedance angle virtual impedance reshaping method used in the current research cannot solve the harmonic current distribution problem. To solve the above problems, a multi-frequency points VSG impedance reshaping control strategy is proposed in this paper, which can adjust the resistive and inductive output impedance of each harmonic frequency independently. The strategy is based on the harmonic separation algorithm of LPF with Feedforward Compensation (FCLPF) and the voltage controller of Vector Proportional Integral (VPI) in fundamental dq rotation coordinate system, which effectively improve the flexibility and accuracy of harmonic impedance reshaping. Among them, FCLPF harmonic separation algorithm has the characteristics of target frequency bandpass and non-target frequency notch, and can eliminate the coupling interference of each harmonic virtual impedance when accumulating virtual impedance voltage references at various frequency points. The closed-loop transfer function of the multi-parallel VPI voltage controller has the characteristics of unit gain and zero phase shift at each harmonic frequency point, which means the static response error of virtual impedance voltage command can be almost completely eliminated. Under the proposed control strategy, the fundamental positive power can be allocated according to the droop coefficient, the fundamental negative current and harmonic current can be allocated according to the sum of the reshaped virtual impedance and the actual line impedance. Reshaping virtual impedance at each harmonic frequency to negative value can also compensate the harmonic voltage drop on the high impedance cable and improve the voltage quality at the PCC.

Active Damping for Dynamic Improvement of Multiple Grid-Tied Virtual Synchronous Generators

To eliminate low-frequency oscillations, this paper proposes an active-damping method for multiple grid-tied virtual synchronous generators (VSGs) in a power plant. Firstly, using the Lyapunov's indirect method, the damping ratio of multiple VSGs in parallel is analyzed. The average damping ratio reveals that this multi-VSG power plant can be poorly damped in a wide range of inertia and damping settings. Then, self- and mutual-damping controllers are developed to suppress self- and mutually induced low-frequency power oscillations, respectively. For the practical implementation, an adaptive tuning algorithm that enables automatic realization is proposed. Through a reassessment, a remarkable damping-ratio improvement is validated. Moreover, the inertial response improvement is validated by the frequency response analysis. Finally, simulations in Digsilent/PowerFactory and experiments are performed to demonstrate the accuracy of the analyses and the effectiveness of the proposed method.

Consensus-based secondary frequency control for parallel virtual synchronous generators

In off-grid operating modes, microgrids with Virtual Synchronous Generators (VSG) controlled inverters are limited to primary frequency control capabilities. When operating under the primary frequency control, if VSG's output frequency exceeds the safe operational range, it can adversely affect stable operation. Consequently, there is a need to implement a secondary frequency control strategy. This paper presents an analysis of the conventional secondary frequency regulation in VSGs. It reveals that when multiple VSGs in parallel to the Point of Common Coupling (PCC) with varied line impedances, the proportionate distribution of each VSG's active power output, post frequency control, does not align with predetermined ratios. In response to this challenge, the paper introduces a quasi-distributed secondary frequency control approach, tailored for systems with multiple feed-in points. This approach modifies the traditional method of compensating for frequency deviation through average ratio integral compensation, replacing it with a compensation based on a consensus algorithm. This negates the need for global communication and additional impedance measuring equipment. By facilitating information exchange between neighboring converters, it enables global control, effectively resolving the issue of line impedance parameters impacting the distribution of VSG's active power output. This decouples the previously strong correlation between line impedance and VSG's active power. The proposed method is characterized by its strong adaptability, scalability, and swift dynamic adjustments, significantly enhancing the fault tolerance of information exchange and the stability of microgrid operations. Finally, the correctness and effectiveness of the proposed improved control strategy are verified by simulation and HIL-based platform.

Extensive assessment of virtual synchronous generators in intentional island mode

Virtual Synchronous Generators (VSGs) are considered one of the most effective cutting-edge technologies for seamlessly integrating Renewable Energy Systems (RESs) into power grids. While various VSG control schemes have been proposed and analysed for different technical aspects, not many experimental demonstrations have been published about VSGs operating in Intentional Island Mode (IIM). Inversely, the IIM has become increasingly attractive, nowadays, to energize those remote areas of the world where entire districts are served by a small number of RESs operating off-grid. In this scenario, the main objective of this paper is to present an extensive performance assessment of the VSG operating in IIM. Different working conditions are considered, with special focus on: black start capability, grid-connected-to-IIM transition, grid reconnection, feeding of non-linear and asymmetric local loads, load sharing between multiple VSGs operating in parallel. In order to achieve this objective, an improved structure of VSG is implemented on 8kVA three-phase converters and significant experimental results are presented. Algorithms for black start capability and grid synchronization are illustrated in detail as well. Meaningful information is provided for engineers involved in activity of designing and testing islanded VSGs.

Systematic Methods to Eliminate the Transient Circulating Powers in the Multi-VSGs System

When multiple virtual synchronous generators (VSG)s operate in parallel in an islanded microgrid, the sudden load power change could lead to significant transient circulating (and oscillating) power. In this paper, the transient circulating power is first analyzed with the two-VSGs small signal model. On this basis, the paper defines parameter design principles that theoretically eliminate all transient circulating power. Due to the line impedances uncertainty in practice, the ideal parameter design is not always available. Then, to suppress the remaining transient circulating power, an enhanced VSG (EVSG) control strategy is further proposed, by deriving and revising a two-VSGs transfer function from load disturbance to power angles difference. The proposed parameter design principles and the EVSG can be applied sequentially, so that the transient circulating power can be systematically suppressed without the side-effects as in existing methods. An experimental platform with 3 VSGs is established. The parameter design principles and the EVSG are thoroughly validated by experimental results. This is also the first time the 3-VSGs experimental results are reported, while all the existing literature has 2-VSGs data. It confirms that the two-VSGs analytical results can be extended to systems with more VSGs.

Analysis of Offline Transient Power Oscillation and Its Suppression Method in the Microgrid with Multiple Virtual Synchronous Generators

When multiple Virtual Synchronous Generators (VSGs) operate in parallel in an islanded grid, power and frequency oscillations will occur when one VSG goes offline. However, the existing literature does not cover the related analysis and transient suppression schemes for this scenario. To analyze these complex high-order system dynamics, this paper first establishes an intuitive equivalent circuit model for multiple VSGs, and the frequency domain expressions of the multi-machine VSG system during the VSG offline transient simulation are then derived. Based on the multi-VSG model and its transient oscillations analysis, this paper further proposes a configuration scheme for the equivalent circuit parameters. Equivalently, the virtual inertia, damping coefficient and virtual impedance of the VSGs can be configured. With the proposed parameter configuration scheme, the power oscillation during the VSG offline transient can be eliminated, as verified by experiments with a microgrid lab platform using three VSGs. Compared with the existing multi-VSG studies, the proposed scheme is not only the first attempt to study the transient suppression when a VSG goes offline, but also is more intuitive in analysis and less complicated in the controller parameter tuning.

A Distributed Dynamic Inertia-Droop Control Strategy Based on Multi-Agent Deep Reinforcement Learning for Multiple Paralleled VSGs

The virtual synchronous generator (VSG) control method for energy storage is a promising way to improve the frequency stability of the power system with large-scale renewable energy. However, due to the mismatch parameters, power oscillations may occur when multiple VSGs are connected in parallel to the grid. In this paper, the relationship between oscillation and the inertia-droop parameters distribution of multiple paralleled VSGs is derived by the simplified frequency response model. Furthermore, to coordinate the inertia-droop parameters of paralleled VSGs for oscillation damping, this paper formulates the parameter tuning problem as a Markov game with an unknown transition function and proposes a distributed dynamic inertia-droop control strategy based on multi-agent deep reinforcement learning (MADRL). Based on the local and adjacent VSG information, each agent learns the optimal inertia-droop control parameters independently by interacting with the environment. The soft-actor-critic (SAC) framework is introduced to train each agent. The well-trained agent can modify the parameters of VSG dynamically to suppress the power oscillation under different operating conditions. Finally, several time-domain results demonstrate the effectiveness and robustness of the proposed approach.

Power Oscillation Suppression in Multi-VSG Grid by Adaptive Virtual Impedance Control

When multiple virtual synchronous generators (VSGs) operate in parallel, severe power oscillation may be suffered. To deal with this issue, this article offers an adaptive virtual impedance method applied to a multi-VSG grid. The core idea is to change the output active power by adaptive virtual impedance. When the angular frequency of the local controller is greater than the angular frequency of the neighbors’ controllers, the output active power of local VSG falls, and thus <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dω_i/dt</i> rises. Then, as the angular frequency is lower than neighbors’, the output active power rises, and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dω_i/dt</i> falls. As a result of this, the method facilitates the consistency of response curves of all angular frequencies. Besides, the stability of the offered algorithm is demonstrated. A final proof of the efficacy of the proposed control method is shown by simulation and hardware-in-loop results.

A Two-Stage Simultaneous Control Scheme for the Transient Angle Stability of VSG Considering Current Limitation and Voltage Support

Since the wide application of virtual synchronous generators (VSGs), the power grid faces great challenges in the safe and stable operation due to their limited thermal capacity and weak anti-disturbance ability. During transient period, for example, a fault occurs in the transmission line, the VSG may lose the transient angle stability and provoke the current hard limit. Even if the fault is cleared by tripping of line, it still faces the problem of instability and voltage dips. To address this problem, in this paper, the post-fault large-signal model of VSG is derived first via the travelling waves based fault information acquisition. Subsequently, with the effect of both active and reactive power loops taken into account, a two-stage simultaneous control scheme is proposed for improving the transient stability of VSG, while considering the current limitation during fault state and voltage support after fault clearance. This method is fulfilled by mode switching and an additional feedback control based on the fault signal. Finally, the effectiveness of the proposed method under both symmetrical and asymmetrical faults is verified. Moreover, the application of the proposed method in a multiple VSGs system is also verified. Besides, the robustness to parameter mismatch and the feasible operating region for the method are discussed.

Power‐frequency oscillation suppression algorithm for AC microgrid with multiple virtual synchronous generators based on fuzzy inference system

In the microgrid, virtual synchronous generator technology can significantly enhance the anti-interference characteristics of the system frequency and bus voltage, as well as solve the problems of insufficient damping and low inertia. However, the system frequency and active power oscillation caused by power fluctuations and grid faults threaten the stable operation of the grid seriously. Therefore, for an alternating current (AC) microgrid multi-virtual synchronous generator (VSG) parallel system, an improved virtual synchronous generator control algorithm based on a fuzzy inference system is proposed, which adjusts the values of virtual inertia and damping coefficient dynamically through fuzzy logic rules to realize the coordinated control of the two. The enhanced VSG algorithm described in this research has a substantial influence on power-frequency oscillation suppression, decreases active power and frequency overshoot, shortens the adjustment time, and improves system frequency stability active power, according to simulation and experimental findings.

Frequency Stability Assessment of Multiple Virtual Synchronous Generators for Interconnected Power System

Virtual inertia (VI) control is a promising concept to ensure the frequency stability of systems dominated by high renewable energy. Supply of VI is established by an inverter-based energy storage system, which can emulate a necessary behavior of synchronous generation in terms of inertia and damping properties, enhancing the entire system stability. This work investigates the dynamic behavior of multiple-VI in an interconnected system. A frequency control model of a three-area interconnected system with multiple-VI control is presented. Then, contrastive test scenarios are conducted to evaluate the necessity of the VI control in both islanded and grid-connected modes. The obtained results show that the genuine dynamic behavior of a grid-connected mode can lead to critical frequency issues with damping insufficiency. The instability in a grid-connected mode can be resolved by the development of multiple-VI control. The promising results will be useful and essential for further research on an interconnected system in respect to a detailed model-based frequency and inertia power response.

Power oscillation suppression in multi-VSG grid with adaptive virtual inertia

Power oscillation is observed when a disturbance occurs in multiple virtual synchronous generators (VSGs) grid, potentially impacting the safe operation of the systems. This article proposes a distributed adaptive virtual inertia control method for suppressing the power oscillation and improving dynamic frequency response in the multiple VSGs grid. The basic principle of the method is that when the local frequency of a VSG deviates from the average frequency of its neighbors, the adaptive inertia control will exhibit a large inertia and when the local frequency starts to return, a small inertia is shaped to accelerate the convergence to the average frequency. The stability of the proposed algorithm is proved by the Lyapunov stability theory, and the guidelines on the key control parameters are provided. Finally, the simulation results and hardware-in-loop results demonstrate the effectiveness of the proposed control method.

A decentralized control for Non-error frequency regulation in an islanded microgrid containing multiple VSGs

Secondary control in the hierarchical control of microgrids can achieve frequency restoration and accurate proportional power sharing among distributed generators (DGs) but depends on complicated communication systems. The flexibility and reliability of microgrids can be significantly improved by reducing the dependence on the communication system. Therefore, in this paper, a new decentralized control strategy for non-error frequency regulation with no intercommunication is proposed in an islanded microgrid containing multiple virtual synchronous generators (VSGs). In the proposed strategy, the frequency recovery link of the VSG, i.e., Proportional Integral (PI) controller, is split into an integral link and a proportional link that are connected in series. The integral link rapidly regulates the microgrid frequency to its rated value, while the proportional link achieves accurate proportional power sharing among VSGs. The whole process can be completed only by local measured signals by each VSG and through local frequency feedback instead of relying on the microgrid central controller (MGCC) and the information interaction between the adjacent VSG. The proposed control strategy enables multiple VSGs to collectively achieve frequency restoration, thus avoiding the problem of frequency regulation overload of a single VSG. The impact of VSG control parameters over the control system is also analyzed by the root locus. Finally, the simulation and experimental results verify the effectiveness of the proposed control strategy in this paper. The results show that the proposed control strategy can achieve frequency restoration and accurate proportional power sharing among VSGs simultaneously with no intercommunication.

Small-Disturbance Stability of a Wind Farm With Virtual Synchronous Generators Under the Condition of Weak Grid Connection

Case-by-case study in the literature has found from the results of numerical computation and simulation that a grid-connected virtual synchronous generator (VSG) can maintain the stability under the condition of extremely weak grid connection. This paper presents the theoretical proof to the finding and thus, generally concludes that the grid-connected VSG is immune to the instability risk under the condition of weak grid connection. In addition, the paper extends the theoretical proof to the case of a grid-connected PMSG wind farm with multiple VSGs. An example grid-connected PMSG wind farm with twenty VSGs is presented to demonstrate and evaluate the theoretical analysis and conclusion made in the paper.

Coordinated Damping Control Design for Power System With Multiple Virtual Synchronous Generators Based on Prony Method

With more renewables integrated into power grids, the systems are being transformed into low inertia power electronic dominated systems. In this situation, the virtual synchronous generator (VSG) control strategy was proposed to deal with insufficient inertia challenge caused by the reduction of synchronous generation. However, as the VSG control method emulates the dynamic behavior of traditional synchronous machines, the interaction between multiple VSG controllers and synchronous generators (SGs) may cause low-frequency oscillation similar to that caused by the interaction between multiple SGs. This paper reveals that the system low-frequency oscillatory modes are affected by multiple VSGs. Then Prony analysis is utilized to extract the system mode information which will be subsequently used for VSG controller design, and a decentralized sequential coordinated method is proposed to design the supplementary damping controller (SDC) for multiple VSGs. The system low-frequency oscillation is first analyzed based on a modified two-area system with a linearized state-space model, and a further case study based on a revised New England 10-machine 39-bus system is used to demonstrate the effectiveness of the proposed coordinated method for multiple VSGs.

Cooperative Control Strategy of Multiple VSGs in Microgrid Based on Adjacent Information

As an interface between distributed power source and large power grid, microgrid has attracted more and more attention. Due to the lack of inertial support for the power system, the virtual synchronous generator (VSG) technology that can simulate the external characteristics of synchronous generators came into being. Aiming at the problem of power and frequency oscillation in microgrid after the introduction of VSG technology, this paper conducts small-signal modeling of multiple VSGs in parallel, analyzes their operating characteristics, and proposes a multiple VSGs cooperative control strategy based on adjacent information. Depend on the adjacent information, the strategy optimizes the output control of each VSG, which makes each VSG tend to the same output frequency at any time. Therefore, the proposed strategy can suppress the power and frequency oscillation, and improve the dynamic characteristics of the system. In this paper, the stability of the proposed control strategy is studied by Lyapunov stability theory, and the effectiveness of the proposed control strategy is verified by simulation.

Research on multi-VSG fast pre-synchronization grid connection with dual power loop

This paper researched on the pre-synchronization grid connection method of a virtual synchronous generator (VSG), and proposed a fast pre-synchronization method based on the virtual power loop. According to the self-synchronization characteristics of the VSG power loop, the double power loop design was adopted in this paper. The two power loops had the same structure, which the power loop 1 was used to implement the basic VSG function, and the power loop 2 was used to achieve rapid pre-synchronous control. Therefore, the mathematical model of the VSG was similar to the synchronous generator in the pre-synchronization control. This method did not require a phase-locked loop and VSG has a good inertia and Damping performance. Moreover, this method was suitable for the networking of multiple VSGs.

An Autonomous Power-Frequency Control Strategy Based on Load Virtual Synchronous Generator

With the increasing penetration of the hybrid AC/DC microgrid in power systems, an inertia decrease of the microgrid is caused. Many scholars have put forward the concept of a virtual synchronous generator, which enables the converters of the microgrid to possess the characteristics of a synchronous generator, thus providing inertia support for the microgrid. Nevertheless, the problems of active power oscillation and unbalance would be serious when multiple virtual synchronous generators (VSGs) operate in the microgrid. To conquer these problems, a VSG-based autonomous power-frequency control strategy is proposed, which not only independently allocates the power grid capacity according to the load capacity, but also effectively suppresses the active power oscillation. In addition, by establishing a dynamic small-signal model of the microgrid, the dynamic stability of the proposed control strategy in the microgrid is verified, and further reveals the leading role of the VSG and filter in the dynamic stability of microgrids. Finally, the feasibility and effectiveness of the proposed control strategy are validated by the simulation results.

Self-Adaptive Secondary Frequency Regulation Strategy of Micro-Grid With Multiple Virtual Synchronous Generators

With the penetration rate of distributed generator and distributed energy storage growing, the frequency stability of microgrid (MG) is severely affected. In this paper, a self-adaptive secondary frequency regulation (FR) strategy based on virtual synchronous generator (VSG) for a microgrid containing wind turbine, photovoltaic array, and electric vehicle (EV) cluster is proposed. First, modified VSG control is applied to renewable energy units and EV cluster, where nonerror frequency regulation is realized by introducing proportional-integral control into power-frequency controller of VSGs. Secondly, the FR reserve capacity of wind turbine, photovoltaic array, and EV are predicted in real time through their respective characteristics. Then, a self-adaptive adjustment strategy for secondary FR parameters of VSGs is designed to control the FR power of VSGs according to real-time predicted FR capacity. Finally, the coordinated FR strategy of VSGs and conventional unit is proposed considering the situation of insufficient FR capacity of VSGs. Through the analysis of simulation results, the reasonability and validity of proposed self-adaptive FR strategy for multiple VSGs have been verified.

Frequency Restoration and Oscillation Damping of Distributed VSGs in Microgrid With Low Bandwidth Communication

The virtual synchronous generator (VSG) provides a promising way to increase the inertia of inverter-based microgrids. However, when multiple VSGs are parallel in the grid, severe oscillations may be incurred. To address this problem, this article proposes a secondary frequency control for distributed VSGs with low bandwidth communication. The control can damp the oscillations and restore the frequency to the rated value without changing the virtual inertia provided by the VSGs. In addition, the proportional active power sharing is achieved by event-triggered communication mechanism, thereby reducing the communication burden considerably. The stability and Zeno-free behavior of the proposed control strategy are demonstrated by the ultimately uniformly bounded (UUB) theory. Simulations are conducted to verify the effectiveness of the proposed control strategy on the PSCAD/EMTDC platform.