Virtual Synchronous Generator Research Articles

Coordination of synthetic inertia from wind turbines and battery energy storage systems to mitigate the impact of the synthetic inertia speed-recovery period

It is well-known that wind power plants can provide short-term frequency control responses; however, when wind turbines are operated to extract maximum power from the wind, any additional energy supplied to the grid in response to the frequency event must be returned to the wind turbine to accelerate it back to optimum speed. The wind turbine must temporarily lower its power output to facilitate acceleration back to optimum speed. In power systems that have many wind turbines providing short-term frequency control services, the reduction in power generation associated with optimum speed recovery could be significant enough to cause a secondary frequency dip shortly after the initial dip, the nadir of which could be lower than the initial dip. This paper proposes 1) an improved speed-recovery scheme for wind turbines that provide synthetic inertia, and 2) a coordinated control scheme for wind turbines and BESSs to mitigate the secondary frequency dip that can occur during the synthetic inertia speed-recovery phase. The control schemes presented can be easily integrated into existing wind turbine and BESS active power controllers as implementation can be achieved using only the active power and rotational speed measurements that are available in the wind turbine's active power controller.

High frequency resonance characteristics analysis of voltage‐source virtual synchronous generator and suppression strategy

AbstractThe voltage‐source virtual synchronous generator is a grid‐forming type power electronic control technology and is able to support converters that have the ability of inertial, primary frequency and voltage regulation. Firstly, the authors build the small‐signal model of the voltage‐source virtual synchronous generator and conventional converters, compare and analyse the characteristics of the high‐frequency resonance modes, the results show that the voltage‐source virtual synchronous generator has almost the same high‐frequency resonance characteristics as the conventional converter. Secondly, the different control parameters and power grid strength that affect the high‐frequency resonance characteristic are analysed with eigenvalue, and the mechanism of voltage‐source VSG that caused high‐frequency resonance is derived. The leading factor that causes high‐frequency resonance is power grid strengths, and the authors proposed introducing virtual impedance into the front‐end channel of the current inner loop control strategy to suppress resonance for the voltage‐source virtual synchronous generator based on the theoretical analysis with the small‐signal model and eigenvalue analysis. In addition, the electromagnetic transient simulation model and RT‐LAB controller‐in‐loop platform are built, and the simulation and experiment results indicate the effectiveness of the virtual impedance control strategy.

A Feedforward Control-Based Power Decoupling Strategy for Grid-Forming Grid-Connected Inverters

Grid-forming inverters, which are represented by droop control and virtual synchronous generator control, have been widely studied and applied because of their excellent grid-supporting ability and smooth off-grid switching. When a grid-forming inverter is connected to a microgrid or utility grid, the control loops of active power and reactive power will be coupled because of the voltage phase difference, which will affect the power control performance. This paper first derives the small-signal linearized model of the system, based on which a frequency feedforward control and an amplitude feedforward control are proposed to decouple the active power and reactive power control loops, respectively. The proposed decoupling strategy directly modifies the reference values through feedforward with an easily implementable principle that is applicable to various control coordinate systems, control coordinate systems, and control structures. By comparing system models with and without the proposed decoupling strategy, its effectiveness can be theoretically proven. Time-domain simulations and hardware experiments are presented to further validate its effectiveness.

Real-time validation of RoCoF – Enhancing advanced coordinated control strategy for voltage unbalance mitigation implementing demand side management

The absence of rotational masses in converter-interfaced microgrids has led to a reduction in system inertia. As a consequence, the stability of modern microgrids is at risk, especially in the case of unbalanced systems, resulting in significant issues such as an increase in the rate of change of frequency (RoCoF), disturbances in the nominal frequency, and unfavourable voltage unbalances. This research article introduces an advanced concept centered around a virtual synchronous generator designed to enhance RoCoF performance. This is achieved through the implementation of a positive and a negative (PN) - sequence controller scheme in coordination with demand side management to ensure that RoCoF remains within the limit of 2 Hz/s according to IEEE Standard 1547–2018 and the voltage unbalance factor (VUF) adheres to the established standard of 2 % according to IEC 61,000–3–13. The behaviour of the proposed control strategy during transient conditions has also been demonstrated. The framework of the virtual synchronous generator is utilized in this paper to demonstrate the influence of damping coefficient and moment of inertia on the low inertia system's frequency and transient response where it has been established that greater inertia and damping are essential for their enhancement. The study also conducts a small signal stability analysis to assess the effects of various parameters on system stability. Furthermore, the real-time efficacy of the coordinated control methodology based on virtual inertia has been experimentally verified using OPAL-RT simulator. The simulation and experimental results, both provide evidence of the successful performance of the proposed approach in realizing efficient RoCoF enhancement and VUF mitigation.

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.

Presynchronization control for ship microgrid of merchant marine inverters based on VSG algorithm with MFAC

The paper studies a presynchronization control of grid connection for large merchant marine microgrid inverters. We present a virtual synchronous generator (VSG) algorithm with model-free adaptive control (MFAC) to optimize the stable grid connection of ship microgrid and shore-to-ship power. To solve poor precision of presynchronization control under nonideal ship microgrid condition, an MFAC controller and its presynchronization method are developed for grid connection of ship-distributed generation inverters. The proposed presynchronization control method effectively avoids a high transient overcurrent and achieves a seamless grid connection to different types of shore power. The simulation results verify the effectiveness of the proposed control method.

Frequency control of voltage sourced converter-based multi-terminal direct current interconnected system based on virtual synchronous generator

Frequency control of voltage sourced converter-based multi-terminal direct current interconnected system based on virtual synchronous generator

Virtual synchronous generator frequency response study of energy computing and storage devices

Virtual synchronous generator frequency response study of energy computing and storage devices

Train based on virtual synchronous generator technology uninterrupted phase-separation passing study

Train based on virtual synchronous generator technology uninterrupted phase-separation passing study

Coordinated frequency control study of variable speed variable pitch of DFIG load shedding unit based on VSG

The involvement of wind turbines operating at load shedding levels in primary frequency regulation serves as a crucial technical solution to provide high levels of inertial response capability and frequency regulation capability in power systems with significant proportions of renewable energy. Load-shedding units' frequency regulation capacity is constrained by the optimal operating speed, which prevents the utilization of the rotor's full kinetic energy and consequently limits the system's ability to provide strong inertia support. Additionally, the units are restricted by the maximum speed in the medium wind speed range, resulting in a reduced reserve capacity. The control mechanism of virtual synchronous generator (VSG) can ensure voltage stability at the machine end, and enable real-time tracking of the grid frequency, thereby imparting to wind turbines the on-grid operation characteristics of synchronous generator sets. To achieve this goal, this paper proposes a coordinated frequency control method for variable-speed variable-pitch doubly fed induction generator units based on VSG. Differentiated control strategies are employed for varying wind speed ranges, while an additional frequency control module is utilized to realize frequency regulation functionality. In conjunction with VSG, this approach enhances the inertial response of the units and strengthens their ability to support grid frequency. Simulation results indicate that the proposed control method mitigates the rate of decline in grid frequency and reduces frequency deviation, ultimately promoting grid stability. Additionally, a simulation system was designed based on the conventional four-machine two-area model to verify the effectiveness of this approach.

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.

Synergistic adaptive control of virtual inertia and damping coefficient in virtual synchronous generators for standalone microgrid applications

The current control methods for virtual synchronous generators (VSG) in regulating inverter frequency in standalone microgrids at border posts and remote mountainous regions remain suboptimal. This study introduces a small-signal VSG model to elucidate the intrinsic dynamics of the virtual inertia and damping coefficient, along with their coupled interrelationship. A novel VSG control approach is proposed, featuring synergistic adaptive regulation of both virtual inertia and damping coefficient. This approach is designed to optimize the interaction of the virtual inertia and damping coefficient with the frequency difference and rate of frequency variation, within a predefined operational range. Additionally, it adaptively modulates these parameters to mitigate further frequency reductions, taking into account the frequency difference and active power when deviations occur outside the predefined range. The experiments demonstrate that this approach effectively moderates the rate of frequency change, diminishes frequency departure velocity for approximately 4 times the original during disturbances, expedites frequency stabilization post-disturbance, the stabilization time is reduced by at least half of the original and prevents excessive frequency deviations. The implementation of this method significantly enhances the response speed and accuracy of frequency control in standalone microgrids, contributing to improved overall system stability.

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.

Distributed Low-Frequency Oscillation Damping in Low-Voltage Islanded Multi-Bus Microgrids With Virtual Synchronous Generators

Distributed Low-Frequency Oscillation Damping in Low-Voltage Islanded Multi-Bus Microgrids With Virtual Synchronous Generators

Study on frequency stability control strategies for microgrid based on hybrid renewable energy

The dynamic nature of renewable energy sources, such as wind and photovoltaic power generation, significantly impacts the frequency stability of microgrid systems due to their pronounced intermittency, inherent randomness, and limited output support. Despite ongoing research, a comprehensive understanding of control measures to enhance microgrid frequency stability remains lacking. This paper addresses this gap by summarizing domestic and global advancements in control strategies for microgrid frequency stability. Specifically, it examines the operating states of microgrids and associated frequency stability issues and expounds various methods for maintaining frequency stability. The paper proposes innovative control measures to enhance frequency stability, including improvements in master-slave control, droop control, phase-locked loop, and virtual synchronous generator (VSG) techniques, particularly during transitions between islanded and grid-connected modes. The findings demonstrate the effectiveness of these enhanced control strategies in maintaining frequency stability, and the paper concludes by suggesting future research directions in this field.

An Improved Grid-connected Pre-Synchronization Method based on Virtual Synchronous Generator Control in Power Conversion System

Under the background of vigorously promoting the construction of new power system，the power conversion system(PCS) plays an important role to transfer high reliable electrical power quality to grid. However, the system is sensitive to the voltage deviation on both side of point of common coupling(PCC) when switching from off-grid mode to grid-connected mode directly, it leads to surge currents or even threaten system stability.In this paper, an improved grid-connected pre-synchronization method for smooth grid connection is proposed. The system adds a pre-synchronization module on the basis of virtual synchronous generator(VSG) control to synchronize the voltage between the PCS and the grid. In addition, a slope control unit is introduced to further avoid the potential surge current problem at the moment of grid connection. Finally, the experimental results of simulation is verified the effectiveness and feasibility of the proposed scheme.

Control approach for photovoltaic inverters enhancing the primary grid using the virtual synchronous generator concept

This paper presents a control scheme for virtual synchronous generators (VSGs) in PV inverters, designed to enhance grid frequency and voltage. Through the skillful management of active and reactive power, this control scheme enables PV inverters to interact seamlessly with the main grid in response to grid events, including voltage and frequency fluctuations. The VSG controller is implemented using Matlab Simulink, and its effectiveness is rigorously assessed under various scenarios in a case study involves a 50 kVA rated PV inverter, a 50 kW rated PV system, and a 220 V grid phase voltage. In conditions of low power generation from the PV system (solar radiation of 200 W/m²) and high load power (120 kW and 37.5 kVAr), the load voltage drops to 202.4 V. The VSG controller successfully raises the grid voltage by 17.6 V, stabilizing it at 220 V. Conversely, in scenarios of high PV power generation (solar radiation of 1000 W/m²) and low load power (20 kW and 7.5 kVAr), the grid voltage surges by 10 V, reaching 230 V. The proposed control strategy adeptly fine-tunes the voltage, ultimately stabilizing at 220 V. Additionally, when the frequency deviates within ±0.4 Hz from the nominal frequency of 50 Hz, the proposed control effectively restores the frequency to its nominal value.

P-f Stability of 2030 Nordic Power System due to Non-Synchronous Generation

The Nordic Power System is undergoing significant transformation and development in the coming years. A driving factor for this is the increased dependency on renewable energy sources and high voltage direct current transmission (HVDC). Various stability challenges due to the dynamics of reactive power - voltage (Q-V) balancing and active power - frequency (P-f) balancing seep into the system with such rise of non-synchronous generation (NSG). This paper investigates the projected impact of NSGs on the P-f dynamic performance of Nordic power system in 2030. As NSGs are systems with low inertia, power discrepancies lead to larger oscillations, higher Rate of Change of Frequency (RoCoF) and bigger maximum frequency deviation (MFD) values. To address these issues, Emulated Inertia (EI) control for wind turbines and Synthetic Inertia (SI) control for VSC-HVDC links are proposed in this paper. The simulation is carried out using DIgSILENT PowerFactory 2022 SP1 simulation software. The results obtained confirm the Effectiveness of the proposed EI and SI methods to enhance frequency response and mitigate deviations.

Adaptive Control for Improved Virtual Synchronous Generator Under Imbalanced Grid Voltage

Adaptive Control for Improved Virtual Synchronous Generator Under Imbalanced Grid Voltage

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