Virtual Synchronous Generator Control Strategy Research Articles

Energy router with virtual inertia control schemes and concurrency operation

SummaryThe energy router is one unified interface solution for organizing distributed energy running in a microgrid operation mode. Because of its flexible and controllable features, this model is expected to become an important scheme for the electricity market liberalization process. Power electronic equipment could not provide inertia and damping like that of the traditional synchronous grid, which leads to poor grid‐tied integrity and reliability. This paper proposes an energy router microgrid‐operation model in which the AC‐interface and DC‐interface conventional control strategies are replaced by a virtual synchronous generator (motor) and virtual DC motor control strategy such that the energy router can mimic the inertia and damping characteristics of a rotating motor in order to improve the stability of the distributed power network. The simulation results prove the effectiveness of the energy router microgrid running mode and the control strategy.

A Virtual Synchronous Generator Control Strategy for VSC-MTDC Systems

With the rapid increase of renewable energy sources and power electronic converters in the power system, the inertia of the power system is declined and the stability problem is more serious than ever before. In this paper, a virtual synchronous generator (VSG) control strategy is proposed to enable the voltage-source converter-based multiterminal high-voltage direct current (VSC-MTDC) system to damp the low-frequency oscillation modes. Both rotational inertia part and governor part are modeled in the VSG controller. The $V^2-P-\omega$ characteristic is developed based on the $V^2-P$ droop control to emulate the action of the speed governor when the frequency of ac grid deviates from the nominal value. The virtual inertia part is used to emulate the inertia response of the synchronous generator. The communication between VSC stations is unnecessary with the autonomous power allocation ability of the $V^2-P-\omega$ characteristic. Modal analysis is utilized to test the system stability and optimize the controller parameters. Two test systems which include offshore wind farms, VSC-MTDC systems, and onshore power grids are built in the DIgSIENT/PowerFactory. The simulation results verify the effectiveness of the proposed VSG strategy.

A hybrid AC/DC microgrid control system based on a virtual synchronous generator for smooth transient performances

This paper presents a high-performance control strategy to support an optimised transient performance for a hybrid AC/DC microgrid system based on an improved virtual synchronous generator (VSG). The standard VSG is modified and an improved control strategy is developed. A pre-synchronization controller is embedded within the improved VSG controller for grid-connection use. In addition, this paper builds the small-signal model for the improved VSG controller in order to analyse the system’s stability. A controller for the battery energy storage system is developed in order to assist the power output of the hybrid microgrid. The microgrid system is designed in a MATLAB/SIMULINK simulation environment based on the under-construction hybrid AC/DC microgrid system at Griffith University, Australia. A comparative study of droop control, conventional VSG control, and the improved VSG control is carried out under different possible transient cases. The pre-synchronization method is also tested. The simulation results show that the improved VSG control strategy is evidently able to ensure smooth variations in frequency, voltage and active power during transient cases.

Research on an improved virtual synchronous generator control strategy for VSC‐HVDC supplying power to a passive network

The traditional double‐closed‐loop control system in a virtual synchronous generator controlled high‐voltage DC generator (VSC‐HVDC) system supplying power to a passive network cannot provide the necessary inertia and damping coefficient, whereas the traditional virtual synchronous generator (VSG) control can realize primary frequency regulation to provide frequency support of the system without the feature of error‐free adjustment. To mitigate the inadequacies of the two methods, an improved frequency synchronous control (IFSC) is proposed. The control simplifies the traditional VSG controller, and error‐free frequency regulation can be achieved without the detection of the active power. Meanwhile, the proposed method retains the two important parameters of VSG technology, namely the virtual inertia and damping coefficient, and inherits the excellent characteristics of the virtual synchronous machine. A simulation model is built on the MATLAB/Simulink platform, and two situations, i.e., normal working condition and the power change of load, are simulated and discussed. The simulation results show that, in the start‐up process, IFSC control has a damping effect of power and frequency. The overshoots of power and frequency decrease and reach their stable values more quickly. When the load power changes, the power and frequency have inertia effect in the dynamic process and the proposed IFSC method achieves the frequency regulation and realizes error‐free adjustment. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

On the Super-Lorenz Chaotic Model for the Virtual Synchronous Generator

Distinguished from the previous virtual synchronous generator (VSG) using the power equation of the three-phase voltage-source inverter (VSI) as the swing equation, this brief considers a new VSG that utilizes the voltage equation of the dc-link capacitor of the three-phase VSI as the swing equation, which reflects directly the dynamics of the rotor of the synchronous generator compared to the previous power one. A novel control strategy for the new VSG is derived by comparing the ac-side voltage equations of the three-phase VSI with the stator voltage ones of the permanent-magnet synchronous generator. The swing equation and the ac-side voltage equations of the controlled three-phase VSI together constitute the complete model of the new VSG, which is further transformed into a super-Lorenz chaotic model by applying an affine transformation and a time-scaling transformation. The condition for the appearance of the Lorenz-like chaotic attractor in the super-Lorenz model is discussed. With variation of the bifurcation parameter, the chaos phenomenon appears and it devastates the stability of the inverter and the grid.

Fuzzy-Secondary-Controller-Based Virtual Synchronous Generator Control Scheme for Interfacing Inverters of Renewable Distributed Generation in Microgrids

In this paper, a fuzzy-secondary-controller-based virtual synchronous generator (VSG) control scheme for interfacing inverters of distributed generation units is proposed for voltage and frequency regulation in microgrids. The proposed control scheme shows significantly improved dynamic performance when compared with the conventional droop control and conventional VSG control through several case studies. Its effectiveness and robustness is further evaluated by sensitivity studies.

Comprehensive control strategy of virtual synchronous generator under unbalanced voltage conditions

The virtual synchronous generator (VSG) is emerging as an effective approach for controlling converter to mimic the traditional synchronous generator (SG). VSG can provide virtual inertia, handle both reactive and active power, which is grid-friendly for integrating distributed generations. Generally, when grid voltage is unbalanced, the negative sequence voltage components will appear. Due to traditional VSG control strategy does not consider the negative sequence components and cannot eliminate them, thus it will lead to current unbalance and power oscillations. To address these problems, a comprehensive control strategy of VSG under unbalanced voltage conditions is proposed. Herein, the basic principle of VSG and inherent reasons for VSG current unbalance and power fluctuation are demonstrated with quantitative analysis. The cascaded control framework and design process of the comprehensive control strategy are presented, integrating the traditional VSG control algorithm, novel current reference generator and typical current regulator. Additionally, related critical issues such as sequence components extraction and control parameter design are discussed. The proposed control strategy can flexibly meet different operation demands, including current balancing and suppression of active power or reactive power oscillations. The validity and effectiveness of the proposed control strategy are verified with simulation and experimental results.

Research on the virtual synchronous generator control strategy of grid-connected permanent-magnet direct-driven wind power system

Renewable energy, distributed generation, and micro-grid technology have been widely concerned for a long time. The traditional grid-connected inverter control strategy does not take into account the problem of inertia which is short and fast to cause the frequency change. The virtual synchronous generator control strategy is adopted to simulate the synchronous generator characteristics, which enhanced the inertia and damp of the system. For the micro-grid of wind power grid-connected, the storage battery is arranged on the AC side of the permanent magnet direct-drive wind turbine, and the model of the virtual synchronous generator is established. Thus the grid-connected performance of large-scale wind farm is improved. Here, the effect of moment of inertia in the virtual synchronous generator and the grid-connected regulation of virtual synchronous generator are verified by using PSCAD/EMTDC. The simulation results show that the grid-connected inverter controlled by the virtual synchronous generator is approximately equivalent to the synchronous generator in external characteristic. The grid-connected inverter based virtual synchronous generator control has a beneficial to adjust frequency and voltage, and can enhance the standby inertia of new energy and grid power generation

Enhanced Virtual Synchronous Generator Control for Parallel Inverters in Microgrids

Virtual synchronous generator (VSG) control is a promising communication-less control method in a microgrid for its inertia support feature. However, active power oscillation and improper transient active power sharing are observed when basic VSG control is applied. Moreover, the problem of reactive power sharing error, inherited from conventional droop control, should also be addressed to obtain desirable stable state performance. In this paper, an enhanced VSG control is proposed, with which oscillation damping and proper transient active power sharing are achieved by adjusting the virtual stator reactance based on state-space analyses. Furthermore, communication-less accurate reactive power sharing is achieved based on inversed voltage droop control feature ( V–Q droop control) and common ac bus voltage estimation. Simulation and experimental results verify the improvement introduced by the proposed enhanced VSG control strategy.

Research on the control strategy of distributed energy resources inverter based on improved virtual synchronous generator

This paper focus on the power fluctuations of the virtual synchronous generator(VSG) during the transition process. An improved virtual synchronous generator(IVSG) control strategy based on feed-forward compensation is proposed. Adjustable parameter of the compensation section can be modified to achieve the goal of reducing the order of the system. It can effectively suppress the power fluctuations of the VSG in transient process. To verify the effectiveness of the proposed control strategy for distributed energy resources inverter, the simulation model is set up in MATLAB/SIMULINK platform and physical experiment platform is established. Simulation and experiment results demonstrate the effectiveness of the proposed IVSG control strategy.

Flexible unbalanced control with peak current limitation for virtual synchronous generator under voltage sags

Virtual synchronous generator (VSG) is grid-friendly for integrating distributed generations (DGs) since it can emulate the operation mechanism of traditional synchronous generator (SG). However, the traditional VSG control strategy, which is mainly suitable for balanced voltage conditions, may lead to power oscillations, current unbalance and even overcurrent under unbalanced voltage sags. To overcome this difficulty, a flexible unbalanced control with peak current limitation for VSG under unbalanced operating conditions is proposed. Based on the basic VSG control algorithm, the control strategy integrates two novel control modules, which are current reference generator (CRG) and power reference generator (PRG). The proposed control strategy can flexibly meet different operation demands, which includes current balancing, constant active or reactive power. And the injected currents are kept within safety values for a better utilization of the VSG power capacity. Furthermore, the experimental platform is built. Experimental results demonstrate the validness and effectiveness of the proposed control strategy.

Enhanced Virtual Synchronous Generator Control with Fuzzy Logic Controller for Parallel Inverters in Microgrids

Objective: To reduce the power oscillations and also to provide accurate active power and reactive power sharing. Methods: In this paper enhanced Virtual Synchronous Generator (VSG) control strategy along with Fuzzy Logic Controller (FLC) is employed for parallel inverters in microgrids. It deals with design of the system and simulation results shows the improvement obtained through it. Findings: By using enhanced VSG control, the oscillation damping and proper transient active power sharing are achieved by adjusting the virtual stator reactance and also accurate reactive power sharing is achieved based on inversed voltage droop control. Furthermore, FLC is used as suitable controllable mechanism (which was PI in basic VSG) for controlling the parallel inverters in microgrids with which power oscillations are further damped and accurate active and reactive power sharing are achieved. The simulation results prove that the proposed method is better than the basic VSG for parallel inverters in microgrids. Application: This scheme is used for control of inverters in distributed source environments. This is essential for the operation of large AC systems, where distances between inverters make communication impractical. Use of multilevel inverters or an adaptive neuro-fuzzy inference system (ANFIS) can be considered as improvement for this paper. Keywords: Distributed Power Generation, Droop Control, Fuzzy logic controller, Microgrids; Reactive Power Control, Virtual Synchronous Generator Control1.

A Power Sharing Method for Parallel Inverters with Virtual Synchronous Generator Control Strategy

<p>A new power sharing method of a virtual sychronous generator control based inverters is introduced in this paper. Since virtual synchronous generator has virtual inertia and damping properties, it significantly enhances the grid stability. However, its output power considerably affects by the line impedance. Thus, in this paper, the relation between the droop control and the line impedance is analyzed at first. Then, by appling an improved droop control strategy to an inverter based on the virtual sychronous generator control, achieving proportional active and reactive power sharing unaffected by the line impedance is realized. The result shows that a smooth response is achieved. As well as, the voltage drop caused by the line impedance is totally compensated. As a result, the system stability is furtherly improved. At last, the effectiveness of the proposed method is verified through MATLAB/Simulink.</p>

Power Sharing for Inverters Based on Virtual Synchronous Generator Control

<p>Power sharing is the most important challenging today, especially for parallel operation inverters. Recently to fixed this problem the virtual synchronous generator control strategy has been used. This paper introduced the parallel operation of three phase multilevel inverters with different capacities to share load power by used a newly strategy named VSG with droop method controller. The control strategy is used to make the inverters to emulate the transient , dynamic features of conventional synchronous generator and to gives accurate load sharing among the inverters in proportional to their ratings. To verify the performance of the proposed technique MATLAB/Simulink package for simulation experiment is established.</p>

Control of Uninterrupted Switching Using a Virtual Synchronous Generator Between Stand‐Alone and Grid‐Connected Operation of a Distributed Generation System for Houses

SUMMARYThe home applications of distributed generations facilities such as photovoltaic and gas engine cogeneration systems are growing and many distributed generations are connected to a grid by a DC/AC inverter. The DC/AC inverter is controlled by a phase‐locked loop (PLL) in order to be synchronized with the power system frequency. However, the control scheme of stand‐alone operation uses voltage control, and the PLL is unnecessary. Therefore, it is hard to realize uninterrupted change between stand‐alone and grid‐connected operation. In this paper, we propose a virtual synchronous generator control scheme in order to use the same control scheme in stand‐alone and grid‐connected operation. We carried out experiments to demonstrate the control characteristics.