Virtual Synchronous Control Research Articles

An improved virtual synchronous generator power control strategy considering time-varying characteristics of SOC

Virtual synchronous generator (VSG) is proposed to solve the lack of inertia of the power system. However, as the core of the VSG providing inertia, the dynamic characteristics of energy storages tend to be ignored in the VSG. In this paper, the dynamic characteristics of energy storages are first considered into the VSG control. Based on this concept, an improved VSG power control strategy considering time-varying characteristics of state of charge (SOC) is proposed in this paper. Firstly, the dynamic characteristics are analyzed in terms of external and output characteristics. Then, Radial Basis Function (RBF) neural network is used to learn data features and handle nonlinear relationships between port voltage and output power of energy storages. The dynamic response of the VSG is improved since the dynamic indexes of the power are optimized during transients. Finally, simulation and experimental results validate the efficacy of the proposed power control strategy.Index Terms—Energy storage, VSG, SOC, RBF neural network, Power control.

Large-Signal Stability Analysis and Control for Small-Scale AC Microgrids With Single Storage

Small-signal stability studies of ac microgrids have obtained abundant results, however, the large-signal stability is still a great challenge and this topic is little studied. In this article, the nonlinear reduced-order model of small-scale ac microgrids with single storage is established considering the structure changes of converters under large disturbances. Then, the large-signal stability is analyzed and the unstable mechanism is revealed. It is found that the structural changes caused by the limitation links have a great influence on the system stability. Based on the unstable mechanism, the virtual synchronous damping control is proposed to enhance the large-signal stability of ac microgrids. The effectiveness of the proposed analysis and control methods is validated through related case studies.

A New Perspective for Relating Virtual Inertia With Wideband Oscillation of Voltage in Low-Inertia DC Microgrid

Virtual synchronous generator (VSG) has been a grid-friendly integration control technique for the integration of grid-connected inverters. However, the emulated inertia and damping of VSG control technique can also be used in the field of dc systems. In this article, a virtual synchronous control is proposed to dampen the wideband oscillation of dc voltage in a dc microgrid. The proposed control strategy contributes to maintaining the synchronous operation of dc converter with the network. Besides, the relationships among damping, inertia, wideband oscillation, rate of change of voltage (RoCoV) as well as DC voltage nadir (DCVN) are studied. It is concluded that the RoCoV and DCVN are similar as the oscillation frequency and fluctuation ranges of poorly-damped oscillation, respectively. A unified concept is proposed by connecting the oscillation-related stability with the inertial transient response originated from the imbalanced powers/mismatched currents. Besides this, the inertia plays the same role as damping because the inertia contributes to maintaining the original state and damping to impeding further change. A new feedback analytical method is proposed to clarify the important role of RoCoV and DCVN on the motion of dc voltage. Finally, the theoretical results are compared with simulations and experiments.

Stability analysis of doubly-fed wind generation systems under weak power grid based on virtual synchronous control combined with adaptive robust control

Doubly-fed induction generator (DFIG) of wind generation system is generally located at the end of the distribution power-grid. Under the weak power grid, the grid connection of wind turbines is unstable, and the current quality is poor. Based on DFIG output impedance model, a stability analysis method combined with adaptive control method is proposed in this paper. The DFIG impedance model with power outer-loop is derived based on the virtual synchronous control of DFIG by using adaptive robust control method. Using generalized Nyquist criterion, the impacts of different power grid conditions and control parameters on the system stable operation under virtual synchronous control are analyzed and verified. The proposed control method in this paper is verified by experiments. The proposed control method improves the quality of grid-connected current.

Research on Combined Frequency Regulation Control Method of Wind Storage with Storage System Optimized Intervals Considered

To solve the insufficient frequency regulation capacity and inertia of the power system caused by the increase of grid-connected wind capacity, a combined wind-storage frequency regulation control strategy considering the optimized intervals of the energy storage system is proposed. This article selects the joint frequency regulation of wind turbine overspeed control and energy storage virtual synchronous control, and virtual synchronous control is used as the upper-level control of the entire wind-storage system. In the process of frequency modulation, the output power of wind turbine and energy storage is reasonably distributed through the upper control. On this basis, to ensure the long-term and stable operation of energy storage, the state of charge (SOC) partition principle of energy storage is set up, which is divided into different areas according to the size of the SOC, and the output of the energy storage device is adjusted according to the divided regions. The results of simulation analysis and verification show that the proposed strategy provides additional frequency regulation capacity and inertia during the frequency variation and greatly improves the frequency stability. Besides, the SOC zoning principle ensures the stable operation of the energy storage.

Current-Limiting Virtual Synchronous Control and Stability Analysis Considering DC-Link Dynamics Under Normal and Faulty Grid Conditions

An improved nonlinear virtual synchronous control for three-phase grid-connected inverters, that can maintain a reliable operation under both normal and faulty grid conditions, i.e. balanced grid voltage sags, is proposed. The proposed controller can ensure a desired RMS current limitation at all times, provide virtual inertia and damping via the DC-link voltage and AC system frequency coupling, and realize the desired real and reactive power regulation without requiring accurate knowledge of the system parameters. Opposed to the conventional methods that use saturated PI controllers with or without anti-windup techniques to limit the reference value of the inverter current, the proposed controller includes a nonlinear bounded integrator, which limits the actual value (instead of the reference) of the inverter RMS current and leads to a fast system recovery even after significant grid voltage sags. The closed-loop stability of entire system is rigorously proven using nonlinear singular perturbation theory. Moreover, analytic conditions for the controller parameter selection to guarantee the stability of entire inverter system with the DC-link dynamics are provided. To prove the effectiveness of proposed controller and its superior performance compared to the traditional approaches, extensive Matlab/Simulink-based simulations are performed, followed by Typhoon-HIL hardware-in-the loop implementation using a TI microcontroller.

Voltage Source Converter–Based Voltage Stiffness Compensator to Improve Grid Voltage Dynamics

This study proposes a novel voltage source converter (VSC)-based voltage dynamic regulatory device based on energy storage and virtual synchronous control, called as a rotating inertia and voltage stiffness compensator (RIVSC), to enhance grid voltage dynamics. In the proposed RIVSC, there is no inner cascaded current control, and the output voltage of the VSC, i.e., inner potential, is directly connected to the grid via a filter. Stiffness characteristic is featured in the inner potential of the RIVSC by the virtual synchronous control. On the basis of the stiffness characteristic, the RIVSC can naturally provide dynamic support for grid voltage. Compared to the traditional synchronous condenser, the proposed RIVSC is more flexible and contributes less short-circuit current. Compared to the existing power electronic-interfaced voltage regulatory device, dynamic support is natural without any time delay, and the proposed RIVSC can improve the dynamic performance of the grid voltage, which is helpful to alleviate the temporary overvoltage at the sending terminal. The basic principle, control methods, and application analysis are presented. Meanwhile, a 10 kW prototype is set up, and the experiment results validate the feasibility and effectiveness of the proposed RIVSC on improving grid voltage dynamics.

A survey on development and prospect of wind turbines virtual synchronous control technology

The ongoing energy revolution has led to the gradual replacement of traditional synchronous generators by renewable enecorresponding authorrgy. This transformation of power generation mode reshapes power system dynamics and poses many challenges to reliable and efficient grid operation. With the continuous improvement of the penetration level of renewable energy represented by wind power in the power system, the synchronous mode of wind turbine will gradually become one of the main modes of system synchronization. In the case of coexistence of multiple synchronization modes, the synchronization mechanism of the whole power system will be fundamentally changed; at the same time, due to the response characteristics of wind turbine to the frequency and voltage dynamics of power grid and the traditional power supply, the synchronous mode of wind turbine will gradually become one of the main modes of system synchronization The frequency and voltage stability of power system also faces new challenges. The virtual synchronous generation technology can make the grid connected inverter simulate the operation mechanism of step generator and provide an effective method to solve the above problems. In this paper, the main technical routes to realize virtual synchronous control of wind turbines are summarized. From the aspects of stability analysis, control system optimization, performance analysis and evaluation, the research and engineering status of virtual synchronization technology in wind turbines are introduced, and the problems to be solved and technical requirements are prospected.

Stability analysis of sub‐synchronous oscillation in power system connected with virtual synchronous DFIGs

Concerning sub-synchronous oscillation in power system connected with virtual synchronous DFIGs, an oscillatory stability analysis method based on the dissipation rate of dynamic energy is proposed. First, the dynamic energy function that reflects the sub-synchronous oscillation characteristics of virtual synchronous DFIG is built, and the expression of dynamic energy considering virtual synchronous control parameters is analysed. Then, according to Lyapunov's stability law, the variation rate of dynamic energy function is defined as the energy dissipation rate. The energy dissipation rate reflects the variation trend of system dynamic energy during oscillation, thus it is used as the stability criterion of sub-synchronous oscillation. On this basis, the effects of different virtual synchronous control parameters on sub-synchronous oscillation are analysed for a comprehensive consideration of system stability. Finally, simulation tests are conducted on RT-LAB platform. The results verify that, when the energy dissipation rate is positive, system oscillation diverges; when the energy dissipation rate is negative, system oscillation converges. Meanwhile, the larger the virtual resistance is, the worse the stability of system is; the larger PI control parameters of reactive power control loop are, the worse the stability of system is.

Research on Control Technology of Distributed Power Generation Virtual Synchronous Generator

Virtual synchronous generator control technology, as a method that enables distributed inverter power supplies to have similar characteristics to synchronous generators, has an important supporting role in improving the stability of the power system. Based on the rotation equation and electrical equation of the synchronous generator, this paper firstly constructed the virtual synchronous generator topology and mathematical model; then designed the corresponding virtual synchronous generator active-frequency droop control and reactive power-voltage droop control strategies, and through the frequency domain method, this paper analyzes the influence of key control parameters. Finally, the related control strategy is verified in the Matlab/Simulink simulation model. The results show that the proposed control strategy can seamlessly switch between off-grid and grid-connected modes, and has good frequency and voltage support capabilities.

Cooperative Control Strategy of Virtual Synchronous Generator Based on Optimal Damping Ratio

Wind power, photovoltaics and other new energy sources are connected to the grid on a large scale. The power electronic interface cannot provide inertia and damping support for the microgrid. The virtual synchronous generator technology is introduced into the inverter control system. Unlike the synchronous generator, its parameters are flexible and adjustable, which can enhance the robustness of the microgrid. This paper firstly analyzes the influence of the moment of inertia and damping on the system, it is based on the equivalent relationship between the three-phase inverter and the synchronous machine, combined with the VSG rotor motion equation, and conducts stability analysis based on the small signal model and root locus. Based on the existing control strategy, a virtual synchronous generator cooperative control strategy based on the optimal damping ratio is proposed. Finally, the VSG collaborative control model is built in MATLAB/Simlink, the experimental results show that the proposed control strategy effectively improves the transient performance of the VSG, the system active power overshoot is reduced from 5% to 0, and the frequency deviation peak is lowered from 0.174 Hz to 0.115 Hz, which confirms the effectiveness of the proposed control strategy.

Virtual synchronous motor based-control of Vienna rectifier

Vienna rectifiers are widely used because of their advantages of high power-factor, low voltage stress on the power semiconductor switches, high power density, stable output voltage, low current harmonic content, and good economics. And gradually applied to the front stage AC/DC part of the electric vehicle charger. Traditional Vienna rectifier control methods include double closed-loop control, hysteresis current control, and Sliding mode variable structure control strategy, but all strategies usually regulate DC-Voltage and keep the unity power factor. However, if it is used for the interface part of the electric vehicle charger with frequent load switching, it is necessary not only to ensure the stability of the output voltage and low THD of the current, but also to be able to respond to the fluctuation of the voltage and frequency of the power system and improve the inertia and damping at the interface. Thereby, the stability of the power grid is improved to a certain extent, and fast-charging is ensured while being grid-friendly. Therefore, this paper proposes to apply the virtual synchronous motor control to the Vienna rectifier to improve the adaptability of the grid to the large-scale access of electric vehicles. Build the topology of the Vienna rectifier in simlink and the modulation method using SVPWM to build a complete model on the basis of VSM.

Output impedance modeling and grid-connected stability study of virtual synchronous control-based doubly-fed induction generator wind turbines in weak grids

Virtual synchronous control (VSynC) technology that mimics the behavior of a conventional synchronous generator can be applied in wind turbines (WTs), where it can effectively improve equivalent inertia, damping coefficient, and wind power permeability. Given the unique partial-scale converter structure of a doubly-fed induction generator (DFIG) and its complicated DFIG-grid coupling effect, the grid-connected VSynC-based DFIG WTs have gained little attention. Moreover, under a weak grid condition, some unstable modes of VSynC can interact with the wind power system, which impacts the DFIG’s dynamic characteristics and system stability. To bridge the gap, this study proposes an output impedance model for a VSynC-based DFIG to analyze the grid-connected stability in weak grids. First, the output impedance mathematical model for a VSynC-based DFIG is derived and explained. On the basis of this novel model, the low-frequency characteristics and vital unstable factors of a VSynC-based DFIG are comprehensively investigated. The impact of different VSynC parameters and grid strengths are extensively studied. Finally, the proposed output impedance model is validated by direct current disturbing theory, frequency scanning method, generalized Nyquist criterion, and time-domain simulation (MATLAB/Simulink). All the obtained results are consistent with each other and prove the effectiveness and accuracy of the proposed impedance model.

Fault Ride Through Strategy of Virtual-Synchronous-Controlled DFIG-Based Wind Turbines Under Symmetrical Grid Faults

Virtual synchronous control (VSynC) in DFIG -based wind turbines (WTs) was studied. It utilizes its relatively “slow” inner potential to enhance the WT's dynamic frequency support capability. However, this will pose more severe challenges on the VSynC-based DFIG under symmetrical grid faults than the conventional vector current controlled one. This article mainly studies the fault ride through strategy of the VSynC-based DFIG under symmetrical grid fault. The fault current is limited and the dynamic reactive power support to help grid voltage recovery are studied. Firstly, the transient behavior of the VSynC-based DFIG is analyzed and the influence of the originally existing virtual resistance is studied. In the proposed method, the virtual resistance just works in the forced current component to limit the forced component of the fault current and accelerate the attenuation of the natural flux. Then the dynamic feedforward compensation is employed to trade-off to limit the natural component of the fault current and decay the natural flux component aroused by grid faults. In addition, the fast dynamic reactive power support of during grid faults according to grid codes. On the post-fault, the coordination control of active power is also implemented to limit steady-state rotor current. Finally, simulation and experiment results demonstrate the feasibility and correctness of the proposed fault ride through strategy for the VSynC.

“Grid”-Less Power Systems: A Vision for Future Structure of Power Networks

This paper proposes a new paradigm in the structure of power systems to facilitate the large scale move to renewables-based distributed generation necessary to help decarbonize the current electricity networks. Since the design of the incumbent power system topologies is to control large synchronous generators, critical control metrics degrade as the penetration of converter-based units increases. Specifically, the reduction in short circuit level, phase angle movement, and rate of change of frequency limit the wider adoption of converter-based units. This paper proposes structural changes and control that inherently solve such critical performance issues through physically decoupling all synchronous generators from the network. A set of back-to-back AC/DC/AC converters controlled by a universal virtual synchronous machine-based control algorithm, introduced in the paper, allows the repurposing of existing plant to enable the integration of more converter-based units. Despite being physically disconnected, this new structure/control still benefits from inertial capacities of synchronous generators to suppress the oscillations caused by disturbances. Moreover, the method enables further exploitation of synchronous generators as energy storage mechanisms. PSCAD/EMTDC simulations demonstrate the advantages of the proposed structure and control system in different normal and abnormal scenarios.

Model Predictive Control for Indirect Boost Matrix Converter Based on Virtual Synchronous Generator

Indirect boost matrix converter is potentially a great alternative to a back-to-back converter for permanent magnet synchronous generators based distributed generation since it can achieve a voltage-boost functionality without utilizing a bulky DC-link capacitor. Despite the success of the indirect boost matrix converter topology, there still exist some issues in the relevant control structure that must be resolved appropriately. First, the existing controls are grid-following controls, which is incapable of islanded operation. Secondly, the exiting controls generate a highly distorted current waveform, which needs to be suppressed by a passive damping resistor. Moreover, without an energy storage element, the distributed generations have no short-time power reserve unit for providing an inertial power to support the utility. In order to solve these issues, a novel approach based on a modified virtual synchronous control and a finite control set model predictive control scheme is proposed in this paper. The former is adopted to ensure proper operations in both grid-connected and islanded modes and to emulate the virtual inertial response by drawing inertial power from the input source. The latter utilizes multi-controls of real-time variables to avoid complicated coupling between the input side and the output side controls and to grant the indirect boost matrix converter with the capability of providing active filter resonance damping. Comparative studies between the proposed control and its existing counterpart are conducted with several simulations in PSCAD/EMTDC software to demonstrate the superior performances of the proposed strategy. Finally, the proposed control is verified in a scale-down experiment testbed.

Virtual synchronous generators for voltage synchronization of a hybrid PV-diesel power system

To ensure the synchronization process in a hybrid PV-Diesel generation system parallel topology, a phase-locked loop is generally used to estimate the angular frequency of the point of common coupling. However, the DG rotor inaccessibility imposes its operation mode as a master. This will therefore affect the system overall stability since the phase-locked loop is affected by oscillations, which are predicted by the power-angle curve concept interacting with the DG rotary inertia. In this context, this paper proposes a new control strategy based on virtual synchronous generators for a stand-alone PV-Diesel hybrid generation system to synchronize the output voltages without a phase-locked loop, while correcting the diesel generator rotor initial position and ensuring power sharing between the two sources, according to a desired power ratio. To prove the effectiveness of the proposed control strategy, simulation and experiments are carried out. The achieved results clearly illustrated the feasibility and the improvement brought by the proposed virtual synchronous generators-based control strategy.

Virtual synchronous generator model control of PV for improving transient stability and damping in a large‐scale power system

AbstractBecause of the significant changes in environmental policies and electric power deregulation in the last decade, a lot of photovoltaic generations (PV) have been and will be installed into the power system in Japan and the ratio of PVs to other synchronous generators will be increased. As a countermeasure against the decrease in the rotational inertia in the whole power system, a virtual synchronous generator (VSG) model control of the PV has so far been proposed. However, the system stabilization effect of the VSG in large‐scale power systems has been unclear. In this paper, a virtual step‐out blocking method of VSG for improving the transient stability is proposed. In addition, the necessity of governor control of VSG in a large‐scale power system is discussed. Finally, the rated kw and kwh capacities of the battery required for realizing the VSG‐model control are evaluated.

Virtual Synchronous Generator Grid Connected Control Method Based on Virtual Impedance

The virtual synchronous generator technology is used to simulate the working principle of the synchronous generator, which provides the inertia and damping support for the power grid. At present, the measurement of the phase, amplitude and frequency of the virtual syn-chronous generator is used to measure the output voltage of the distributed generation system. To solve this problem, this paper proposes a virtual synchronous machine grid virtual impedance control method based on the virtual impedance calculation of virtual current in the virtual synchronous machine from the network state and the grid between the assumption of power exchange, puts forward the pre synchronization strategy based on virtual impedance, and gives the simulation results. The results show that compared with the existing methods, the proposed method is more practical and more smooth.

Transient Frequency Response Improvement in Microgrid Using Virtual Synchronous Machine

Microgrid concept helps to provide reliable electric power by integrating distributed generators based on renewable energy into the power system network. Grid-connected and autonomous modes of operation of microgrids and power electronic converter-based distributed generators make the microgrids to operate in a highly flexible manner. Since many of the microgrids are dominated by converter-based generators as against the rotating synchronous generators, they have a lower amount of rotating inertia. This causes poor transient frequency response in microgrids. Virtual inertia can be provided to microgrids by virtual synchronous machine-based control of converters. In this method, the converter emulates the behaviour of a synchronous machine and provides virtual inertia with the help of the electrical energy storage. Swing equation is used to derive the equivalent power to be delivered by the inverter to the grid. Simulation of the virtual synchronous machine with the grid-connected inverter was carried out in MATLAB/Simulink platform, and the results indicate the satisfactory operation of the proposed control strategy. An experiment on the laboratory scale model of the grid-connected inverter with virtual synchronous machine control strategy was carried out. The control algorithm was implemented in OPAL-RT real-time simulator. The performance of the virtual synchronous machine is compared with droop control, and the difference in performance with the two schemes is analysed.