Conventional Virtual Synchronous Generator Research Articles

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.

Adaptive Supplementary Control of VSG Based on Virtual Impedance for Current Limiting in Grid-Connected and Islanded Microgrids

By applying virtual synchronous generator (VSG) method in high penetration level microgrids, current limiting issue under fault condition has become an important challenge. The main objective of this paper is to propose a novel current limiting strategy which has the capability of voltage tracking error reduction to avoid voltage controller saturation. This purpose is obtained by adaptive virtual impedance-based reference voltage correction during fault condition. Furthermore, positive and negative peak detection-based current limiting method implemented in double loop control system prepares soft transition ability. Moreover, this current limiting method generates the adaptive term of virtual impedance part. In contrast to conventional VSG methods, the proposed approach provides high quality voltage and current for microgrid in both grid-connected and islanded modes during fault conditions. The offline and hardware in the loop (HIL) simulation results confirm that the peak current of the inverter is restricted to the threshold value under symmetrical and asymmetrical faults for two operating modes of microgrid.

Adaptive optimization of virtual synchronous generator based on fuzzy logic control and differential evolution

Maintaining frequency stability in low inertia microgrids with high renewable energy source (RES) penetration is a major problem. To resolve this issue, the use of the virtual synchronous generator (VSG) was proposed in the literature. VSGs can provide damping and inertial response mimicking the operation of synchronous generators, hence the name. However, the damping and inertia coefficients of VSGs are typically fixed which limits the capabilities of VSG in stabilizing the frequency. Higher frequency oscillations may develop due to the fixed damping factor and virtual inertia constants. This paper proposes an optimization method that sets the VSG damping and inertia coefficients in an adaptive manner based on the fuzzy logic controller (FLC). Further, the differential evolution (DE) algorithm is used to solve the formulated multi-objective function to optimize the FLC gains. In this way, the damping and virtual coefficients are continuously and adaptively adjusted to provide optimal frequency stabilization. To verify the efficiency of the proposed optimization and control method, a comprehensive nonlinear simulation analysis was carried out using MATLAB/Simulink to compare the proposed method with the conventional VSG. The simulation was performed under multiple renewable penetration levels. The proposed method showed remarkable performance during disturbances and at varied RES penetration levels with 7.3% less frequency deviation and 46% less rate of change of frequency.

Adaptive VSG control strategy considering energy storage SOC constraints

The virtual synchronous generator (VSG) control strategy is proposed to mitigate the low inertia problem in the power system brought about by the high percentage of distributed generation connected to the grid and the application of power electronic devices. In order to maximize the effectiveness of the advantages of the flexible and adjustable parameters of VSG control, an adaptive VSG control strategy considering SOC constraint of the energy storage unit is proposed in this paper. Considering the significant loss of service life by operating the energy storage unit at its limit state, based on the rate and degree of change in system frequency, the adaptive control strategy realizes the online adaptive adjustment of the inertia factor and damping factor under different perturbation conditions by adaptively adjusting the control parameters when the system frequency oscillates. Finally, the effects of this adaptive VSG control method and conventional VSG control method are compared by simulation in PLECS. Hardware-in-the-loop (HIL) experimental platforms and semi-physical simulation experiments are constructed on RTBOX, and the feasibility and validity of this adaptive VSG control strategy are verified.

An Adaptive Inertia and Damping Control Strategy Based on Enhanced Virtual Synchronous Generator Model

In modern converter-dominated power systems, total inertia is very variable and depends on the share of power generated by renewable-based converter-interfaced generation (CIG) at each specific moment. As a result, the limits required by the grid codes on the rate of change of frequency and its nadir or zenith during disturbances become challenging to achieve with conventional control approaches. Therefore, the transition to a novel control strategy of CIG with a grid-forming power converter is relevant. For this purpose, a control algorithm based on a virtual synchronous generator (VSG) is used, which simulates the properties and capabilities of a conventional synchronous generation. However, due to continuously changing operating conditions in converter-dominated power systems, the virtual inertia formed by VSG must be adaptive. At the same time, the efficiency of adaptive algorithms strongly depends on the used VSG structure. In this connection, this paper proposes an enhanced VSG structure for which the transfer function of the active power control loop was formed. With the help of it, the advantages over the conventional VSG structure were proven, which are necessary for the effective adaptive control of the VSG parameters. Then, the analysis of the impact of the VSG parameters on the dynamic response using the transient characteristics in the time domain was performed. Based on the results obtained, adaptive algorithms for independent control of the virtual inertia and the parameters of the VSG damper winding were developed. The performed mathematical modeling confirmed the reliable and effective operation of the developed adaptive control algorithms and the enhanced VSG structure. The theoretical and experimental results obtained in this paper indicate the need for simultaneous development and improvement of both adaptive control algorithms and VSG structures used for this purpose.

An improved fault control strategy for virtual synchronous generator with the coordination of STATCOM during unbalanced fault

AbstractThe virtual synchronous generator (VSG) is a good solution for stabilizing the power system with high penetration of renewable energy. However, in case of serious unbalanced voltage disturbance/fault, the conventional VSG may lose voltage, inertia, and damping support characteristics to the grid and even can cause disconnection of renewable energy. This paper proposes an improved fault control strategy for VSG with the coordination of STATCOM. The proposed method can provide sufficient voltage support while keeping continuous system inertia and damping support under severe unbalanced fault. In the paper, an improved VSG and STATCOM control topology based on positive and negative sequence current control are first proposed so as to keep the damping and inertia support to grid during the grid fault. Secondly, the voltage support control method for the VSG with improved topology during unbalanced fault is introduced, which can achieve multiple control objectives, in terms of voltage support, current limitation, and active power output simultaneously. Then a coordination control scheme of improved VSG and STATCOM is developed so as to optimize the maximum control objectives in all possible scenarios, especially in the case of severe unbalanced fault. Finally, the effectiveness of the method is verified by using the MATLAB/SIMULINK simulation platform.

A Deep Reinforcement Learning Design for Virtual Synchronous Generators Accommodating Modular Multilevel Converters

The deep reinforcement learning (DRL) technique has gained attention for its potential in designing “virtual network” controllers. This skill utilizes a novel solution that can avoid the specific parameters and system model required in classical dynamic programming algorithms. However, addressing the issue of system uncertainties and performance deterioration remains a challenge. To overcome this challenge, the authors propose a new control prototype using a twin delayed deep deterministic policy gradient (TD3)-based adaptive controller, which replaces the conventional virtual synchronous generator (VSG) module in the modular multilevel converter (MMC) control. In this approach, an adaptive programming module is developed using a critic fuzzy network point of view to determine the optimal control policy. The modification presented in this framework is able to improve the system stability and resist disruptions while retaining the merits of the conventional VSG control model. The proposed approach is implemented and tested using the DRL toolbox in MATLAB/Simulink.

Online Grid Impedance Estimation-Based Adaptive Control of Virtual Synchronous Generators Considering Strong and Weak Grid Conditions

The conventional virtual synchronous generator (VSG) is typically designed to meet certain operational and control requirements in the islanded mode. However, once the VSG is switched to grid-connected mode (GCM), the robust operation cannot be guaranteed under different grid conditions. It can lead to poor dynamic performance, especially in strong grids, such as significant oscillation, long settling time, and large overshoot. To improve the VSG performance in the GCM, this article first analyzes in depth the inherent coupling between the active and reactive power and its dependence on grid conditions, such as the short circuit ratio and the grid impedance ratio. Subsequently, an adaptive VSG (AVSG) control strategy based on online grid impedance estimation is proposed to ensure robust operation of the VSG considering both strong and weak grid conditions. This technique allows the operator to specify the desired settling time of the output power and damping ratio. To estimate the grid impedance in real time without additional hardware and reduce the associated impacts on power quality, an online event-based grid impedance estimation algorithm is embedded in the control loop of the AVSG. The simulation and experimental results indicate that, compared with the conventional fixed-parameters-based controller design method, the AVSG exhibits desired performance such as no oscillation, specified time duration for the settling time, and minimal overshoot regardless of the grid conditions.

Improved virtual synchronous generator control strategy for the flexible interconnection system in distribution transformer areas

• An improved virtual synchronous generator control strategy is proposed for the flexible interconnection system. • The DC side impedance small signal model is built. • The stability of the flexible interconnection system is analyzed mathematically. • The controller hardware-in-the-loop experiment is designed for verification. The flexible interconnection system (FIS) in the distribution transformer areas containing high-penetration distributed photovoltaics plays a significant role in balancing the load and improving new energy consumption. This paper proposes an improved virtual synchronous generator (VSG) control strategy for the FIS. Firstly, the impedance small signal model of the FIS is developed for analyzing the influence of the conventional VSG control strategy on the stability margin of the FIS in the case of bidirectional power flow. Then, the improved VSG control strategy is proposed based on the analysis, which can improve the stability of the FIS in the negative power direction. Finally, the controller hardware-in-loop experiments of the FIS verify the effectiveness of the proposed improved VSG control strategy.

Freely Customized virtual generator model for grid-forming converter with hydrogen energy storage

Currently, there is a large-scale penetration of renewable energy sources (RES) in power systems around the world, most of which are based on the power converter technology. The result of such penetration is the emergence of frequency and voltage regulation problems in power systems with RESs. An effective way to solve the mentioned problems is the use of energy storage systems (ESS), among which hydrogen ESSs are promising. However, the quality of such solution mainly depends on the approach to the power converters control. One of the most effective is a grid-forming power converter control strategy. Within this strategy, many scientific groups have proposed the concept of a virtual synchronous generator (VSG). However, the existing VSG models use a series structure with a stiff direction of signals, which completely mimics conventional synchronous generators. In this regard, the fundamental problems inherent in such a structure arise, e.g., a coupling between active and reactive power control. This paper describes a freely configurable VSG model (FC-VSG), in which the structural control blocks can be transferred from one level to another, and the levels themselves can be installed not only in series, but also in parallel. The case studies carried out made it possible to identify the configuration of the FC-VSG, which excludes the fundamental problems inherent in the conventional VSG structures. At the same time, the most efficient and reliable frequency and voltage regulation is provided, which has been proved by qualitative and quantitative comparison, including with the conventional VSG structure.

Multiobjective Laguerre Functions[formula omitted]Based Discrete[formula omitted]Time Model Predictive Control: A Fast Inner[formula omitted]Loop Controller for Grid[formula omitted]Forming Converters

In islanded ac microgrids, conventional primary control uses inner−loop cascaded linear controller and outer−loop droop to realize local voltage regulation and power sharing. However, it has a poor dynamic performance and fast rate of frequency change following disturbances. This paper addresses these issues by proposing a modified virtual synchronous generator (VSG) control. A Laguerre functions−based discrete−time model predictive control (LF−DMPC) with a multiobjective cost function is incorporated as the inner loop; yielding large prediction horizon, improved dynamic response, and inherent overcurrent protection in the case of faults. The swing−based and the reactive power droop controllers also form the outer loop aimed at inertia emulation and power sharing. The merits of proposed approach are verified by comparisons with conventional droop and VSG controls. Detailed model simulations are conducted on a 2−converter ac microgrid in MATLAB/Simulink to show the efficacy of the proposed controller.

Coordinated VSG Control of Photovoltaic/Battery System for Maximum Power Output and Grid Supporting

In a two-stage photovoltaic (PV) system, batteries are generally connected to the DC-link via a converter for buffering the power imbalance induced by the grid supportive services of grid-side inverter and the maximum power point tracking (MPPT) of PV source. Considering the limited battery capacity, the MPPT operation is easily compromised to avoid over-energizing the batteries in the conventional virtual synchronous generator (VSG) control, degrading the energy efficiency and extending the payback period. To concurrently achieve grid supporting and maximum PV power harvesting without increasing batteries, a coordinated VSG control for the photovoltaic/battery (PV/Bat) system is proposed in this paper. In the proposed strategy, the DC-link voltage level is segmented to differentiate the operations of converters. When the DC-link voltage is within the safety margin, the PV source operates at MPPT mode and the inverter output frequency is programmed proportionally to the DC-link voltage deviation. Meanwhile, droop control is implemented to associate the battery power with the DC-link voltage. When the DC-link voltage goes beyond the safety margin, PV de-loading and inverter rectification will be adaptively activated to restore DC-link voltage within the predefined range. The effectiveness and advantages of the proposed strategy are eventually verified by simulations and experiments.

Virtual synchronous generator based pole control in high-voltage DC transmission systems

The use of renewable energy to realize decarbonization is rapidly increasing, and considerable research is being performed to achieve highly efficient high-voltage direct current (HVDC) transmission for offshore wind power generation over long distances. However, in the case of onshore AC power transmission and distribution systems, with the mass introduction of distributed energy resources (DERs), voltage and frequency fluctuations often occur. It is thus necessary to implement the control function of smart inverters, which can facilitate the stabilization of the AC system, in the grid-connected inverters of DERs. Virtual synchronous generator (VSG) control is a smart inverter control strategy that can contribute to the stability of the AC system. In this study, conventional power control and VSG control were applied to the control strategy of modular multi-level converter-based poles of the HVDC transmission system, and the AC system fault tolerances of the entire system were examined.

Grid-Following Mode Operation of Small-Scale Distributed Battery Energy Storages for Fast Frequency Regulation in a Mixed-Source Microgrid

As the share of power converter-based renewable energy sources (RESs) is high, a microgrid, in islanded mode, is more vulnerable to frequency instability due to (1) sudden power imbalance and (2) low inertia. One of the most common approaches to address this issue is to provide virtual inertia to the system by appropriately controlling the grid-side converter of the RESs. However, the primary frequency controller (PFC) presented in this paper focuses on the fast compensation of power imbalance without adding inertia to the system. The proposed method is based on estimating the real-time power imbalance caused by a disturbance and compensating it using multiple small-scale distributed battery energy storage systems (BESSs). The power imbalance is estimated by observing the initial rate of change of frequency (RoCoF) following a disturbance. Based on the estimated power imbalance and the rating of the BESSs, the reference power for the BESSs is determined. The BESSs are controlled in grid-following mode to compensate for the power imbalance. The performance of the proposed PFC is verified using a Typhoon real-time simulator for various scenarios and is compared with the conventional virtual synchronous generator (VSG) controller.

An oscillation damping method for frequency-detector-less virtual synchronous generators

This paper presents an active power oscillation damping method for frequency-detector-less virtual synchronous generator (VSG) to enhance its transient stability and to avoid the ill effect of frequency detector. In the paper, first, the mechanism and the low-frequency oscillation model of a conventional VSG is derived and reduced to a second-order model. Then, a new way to obtain the damping power without using frequency detector is proposed in order to simplify the conventional one, which includes frequency detector. The new approach presents an equivalent third-order model, and based on small-signal state-space analysis, the dominant poles are assigned to the same place as the original one, so that we can fairly compare their dynamic performances. Finally, experimental results are performed to compare the proposed approach with some other VSG control methods. The experimental results show that the proposed approach does better in transient stability and power-sharing ability than other conventional VSG control methods.

A Self-Regulated Virtual Impedance control of VSG in a microgrid

In this paper, a self-regulated virtual impedance (SRVI) control of virtual synchronous generator (VSG) is proposed. In the modern power system, large penetration of inverter-interfaced distributed generator (DG) decreases the equivalent inertia of the grid. The system, therefore, becomes more prone to frequency instability, and the fault current limits are reduced due to inverter power ratings. The VSG control of the inverter is used to maintain and enhance the transient stability of the system by introducing virtual inertia in it. In the proposed control, the SRVI control block is added in VSG control to further improve the damping and to limit the fault current during overloads and/or faults. The virtual impedance in the SRVI block is regulated as a function of the rate of change of root mean square (RMS) value of DG current during the transient case. The design of virtual impedance for the proposed control is also discussed in this paper. To see the effectiveness of the proposed technique, electromagnetic transient (EMT) simulation is performed using a real-time digital simulator (RTDS) for disturbances that result in large frequency deviations. The simulation results show that the proposed scheme outperforms the conventional VSG control.

Adaptive critic design-based reinforcement learning approach in controlling virtual inertia-based grid-connected inverters

In this paper, an adaptive critic design (ACD) approach is proposed to control the phase and voltage of a grid-connected virtual synchronous generator (VSG). The penetration of fast responding inertia-less power converters significantly affect the stability of the power system, especially weak systems such as micro grids. The concept of virtual inertia addresses this concern by virtually emulating the behavior of a synchronous generator. However, the conventional VSG is designed based on two conditions: (i) fixed operating point and (ii) inductive grid connections. The performance of VSGs in low-voltage semi-resistive microgrids is far from optimal. To overcome the aforementioned concerns, a heuristic dynamic programing (HDP) approach is proposed to optimally control grid-connected VSGs. The neural-network-based inherence of the HDP enables the proposed technique to adapt to any impedance angle. The HDP controller includes two subnetworks: (i) the action network that controls the system optimally and (ii) the critic network, which evaluates the effectiveness of the action network. The simulation and experimental results are provided to evaluate the effectiveness of the proposed technique. As shown, the HDP-based approach illustrates a better performance in comparison with the conventional PI-based VSG in various operating conditions.

Model Predictive Virtual Synchronous Control of Permanent Magnet Synchronous Generator-Based Wind Power System

As much wind power is integrated into the power grid through power electronic equipment, the use of wind power is increased rapidly. Wind power system makes the power grid lack inertia and damping, thereby reducing power grid stability; in severe cases, it may even be disconnected. virtual synchronous generator (VSG) has been put forward to enhance the anti-disturbance performance of power grid. However, conventional VSG adopts an outer power loop and inner-current loop control. The inner-current loop control needs a pulse width modulation (PWM) module and proportion integration (PI) parameter settings. In order to reduce the parameter settings and simplify control structures, in this study, model predictive control (MPC) is used instead of inner-current loop control. At the same time—for the overall stability and control flexibility of the back-to-back system—we further propose to use outer-voltage loop control (OVLC) and MPC to stabilize direct current (DC) voltage on the machine-side and to employ model predictive virtual synchronous controls to provide inertia and damping for the power grid. The proposed control method was simulated in Matlab/Simulink (MathWorks, Massachusetts, MA, 2016) and verified by experimental results.

Modified virtual synchronous generator based‐primary frequency regulation for renewable generation integrated into power system

To guarantee the frequency stability of renewable energy sources (RESs) generation integrated into the power system, primary frequency regulation (PFR) for RES generation controlled by virtual synchronous generator (VSG) has been concerned extensively. This study mainly focuses on the dynamic and steady-state characteristics of VSG in the PFR dead-band. Based on the typical structure of VSG, small-signal model of the conventional VSG is established to analyse the issue of the sharp change of angular frequency. Consequently, a novel modified VSG (M-VSG) controller is proposed, which can ensure a stable angular frequency and synchronise with the grid without the phase-locked-loop. The inertia and damping properties of M-VSG are studied theoretically by eigenvalue analysis, and the design procedure of control parameters is presented. To enhance the practicability, a flexible PFR strategy of M-VSG is added considering inertia mode, bidirectional mode, and unidirectional mode according to the operation state of RES generation. Moreover, after PFR is added, the proposed controller is analysed with eigenvalue loci to guarantee the stability and dynamic characteristics outside the dead-band. Finally, hardware-in-the-loop experiment results based on RT-LAB demonstrate the validity of the theoretical analysis and the superiority of M-VSG in participating in flexible PFR.

Virtual Synchronous Generator Control of VSC-HVDC System Based on MMC of Hybrid Topology

For a VSC-HVDC transmission system based on a hybrid topology converter of full-bridge and half-bridge, a kind of virtual synchronous generator (VSG) control strategy which can be applied to modular multilevel converter (MMC) grid-connected structure was researched and proposed. First, based on the conventional VSG control strategy, the energy stored in the equivalent capacitor of MMC power module was used to imitate the rotor inertia of synchronous generator. The characteristics of generator can be simulated during transient frequency fluctuations and it can help relieve the power fluctuations. Secondly, with respect to the structural characteristics of the direct grid connection through the reactor on the AC side of the MMC, which is unlike the microgrid inverter, there are no additional filter capacitors. So, the existing commonly used VSG control strategy of the microgrid inverter and double-closed-loop structure composed of filter capacitor voltage and AC current cannot be directly applied. For this, a method where the given values of inner current loop are calculated by grid impedance matrix was used. So, a double-closed-loop control structure composed of a power outer loop based on VSG control and a current inner loop is obtained. It can effectively improve the current control capability during the transient process. Finally, a hybrid MMC simulation model was built based on PSCAD to verify the correctness and effectiveness of the proposed methods.