Improved Virtual Synchronous Generator Control Research Articles

A Method to Improve Both Frequency Stability and Transient Stability of Virtual Synchronous Generators during Grid Faults

Transient stability and frequency stability of virtual synchronous generator (VSG)-controlled converters during grid faults are critical for the stable operation of high-share renewable energy power systems. However, the existing transient stability methods often overlook the consideration of frequency stability. This paper presents a method to improve both frequency stability and transient stability of VSGs during grid faults. The influence mechanism of inertia and damping on transient stability and frequency stability is comprehensively analyzed under two types of faults, with or without stable equilibrium points. To address the conflict between frequency stability and transient stability of VSGs with large inertia under the first type of fault, an improved VSG control with angular frequency deviation feedback is proposed. The transient response of this control method is analyzed using small-signal and large-signal models. Furthermore, to achieve optimal dynamic performance, this improved VSG control introduces the optimal damping ratio and presents a parameter design method. Finally, the correctness of the proposed method and its theoretical analysis is verified through experimental results.

Power stability control of wind-PV-battery AC microgrid based on two-parameters fuzzy VSG

Virtual synchronous generator (VSG) control addresses the issue of decreasing microgrid standby inertia caused by the rise in wind turbines and photovoltaic (PV) penetration. However, various types of perturbations occur frequently making the traditional constant parameter VSG control unable to meet the system performance requirements, and thus a two-parameters fuzzy VSG control is proposed to ensure that microgrid inertia and damping. Firstly, the device-level control of each generation unit in the microgrid is designed based on the wind-PV-battery alternating current (AC) microgrid architecture. Secondly, fuzzy VSG control uses fuzzy rules written in plain language to represent the relationship between the main VSG factors and the power and frequency. Then, the influence of virtual inertia and damping coefficient on the dynamic performance of the system is analyzed through the theory of small-signal model, and a reasonable variation range of VSG parameters are given. Finally, the simulation model of wind-PV-battery AC microgrid is built in MATLAB/Simulink, and compared with other improved VSG control strategies, the fuzzy VSG control proposed in this paper has better dynamic performance and safety stability. This research emphasizes the practicality and importance of utilizing fuzzy control to adjust VSG techniques for developing microgrid configurations incorporating more renewable energy sources to guarantee the reliability and efficiency of microgrid.

An improved VSG control strategy based on transient electromagnetic power compensation

Virtual synchronous generator (VSG) not only increases the inertia of grid-connected system, but also brings the problem of active power oscillation under grid disturbance. Therefore, VSG control strategy and system model order reduction method with transient electromagnetic power compensation are proposed. The closed-loop active power small signal model of the system is established, and the influence of transient electromagnetic power compensation on the power stability of VSG is analyzed based on root locus method. By removing the items which have little influence on the stability of the system in the small signal model, the order is reduced to obtain the equivalent second-order model of the system. According to the second-order model, the quantitative design criteria of the system parameters are given. The proposed transient electromagnetic power compensation strategy not only increases the transient equivalent damping of the system, but also does not affect the primary frequency modulation characteristics and will not cause large overshoot of the output active power. The experimental results are consistent with the theoretical analysis, which testify the effectiveness and correctness of the system control strategy and the model reduction method.

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.

A Virtual Synchronous Generator Control Strategy for Microgrid Based on Harmonic Current Bypass Control

To improve the voltage and frequency stability and suppress the voltage harmonics of a microgrid, a virtual synchronous generator (VSG) control strategy based on the harmonic current bypass control is proposed in this paper. First, a robust voltage control link is added to the virtual excitation component of the traditional VSG control strategy to reduce the amplitude error of the inverter’s driving voltage caused by the reactive power-voltage droop coefficient. A harmonic current bypass control link is then used to adjust the inverter’s voltage signal to reduce the total harmonic distortion of the microgrid. Finally, a simulation model of the microgrid is established using the MATLAB/Simulink software, and its stability is analyzed. In addition, the proposed control strategy is also compared with the traditional droop control and traditional VSG control strategies, focusing on the frequency instability and harmonic suppression performance of the microgrid. The numerical simulation results verified the effectiveness and superiority of the improved VSG control strategy proposed in this paper.

A Comprehensive Virtual Synchronous Generator Control Strategy for Harmonic and Imbalance Voltage Suppression of Multi-Inverter Parallel Microgrid

To reduce the impact of the imbalance of mixed non-linear loads on an inverter voltage output in the microgrid, we improve the disadvantage of the lack of damping and inertia for traditional droop control. This paper proposes a comprehensive virtual synchronous generator (VSG) control strategy for harmonic suppression and imbalance suppression of a multi-inverter parallel microgrid. On one hand, an improved VSG control strategy is proposed to increase the damping and inertia of distributed generations (DGs) in the microgrid, and secondary control is introduced to improve system stability. On the other hand, the frequency division suppression control strategy is used to eliminate the influence of harmonics, and the negative sequence component is compensated to eliminate the influence of imbalance. Then small-signal analysis is used for analysis of the stability of the strategy. Finally, we verify the comprehensive control strategy proposed in this paper through experiments. The experimental results suggest a significant improvement on the voltage, frequency, power optimization, handling of non-linear load and capacity distribution precision, as well as providing inertia support for the system.

Improved VSG control strategy based on the combined power generation system with hydrogen fuel cells and super capacitors

Due to their environmental protection and high power generation efficiency, the control technology of hydrogen fuel cells (HFCs) connected to the microgrid has become a research hotspot. However, when they encounter peak demand or transient events, the lack of power cannot be compensated immediately by HFCs which results in sudden changes of the voltage and frequency. The improved virtual synchronous generator (VSG) control strategy based on HFCs and supercapacitors (SCs) combined power generation system is proposed to overcome this shortcoming in this paper. The small-signal model for designing the combined system parameters is provided which are in accordance with the system loop gain, phase angle margin, and adjustment time requirements. Besides, the voltage and current double closed-loop based on sequence control is introduced in the VSG controller. The second-order generalized integrator (SOGI) is utilized to separate the positive and negative sequence components of the output voltage. At the same time, a positive and negative sequence voltage outer loop is designed to suppress the negative sequence voltage under unbalanced conditions, thereby reducing the unbalance of the output voltage. Finally, simulation results in MATLAB/Simulink environment verify that the proposed method has better dynamic characteristics and higher steady-state accuracy compared with the traditional VSG control.

Improved Vsg Control Strategy Based on the Combined Power Generation System with Hydrogen Fuel Cells and Super Capacitors

Hydrogen fuel cells (HFCs) occupy a dominant position in the development of renewable energy due to their environmental protection and high power generation efficiency. The control technology of their connection to the microgrid has become a research hotspot. However, the lack of power cannot be compensated immediately by HFCs when they encounter peak demand or transient events. In order to overcome this shortcoming, supercapacitors (SCs) are introduced as auxiliary power sources. The virtual synchronous generator (VSG) control strategy based on this combined power generation system is proposed to introduce virtual inertia to alleviate the impact of load disturbance on the microgrid in this paper. Through the small signal model, the influence of the control strategy parameters of VSG based on the HFCs and SCs combined power generation system on the system stability is studied. In addition, the parameters of the combined system are designed. In the VSG controller, the voltage and current double closed-loop is improved to suppress the negative sequence component in the output voltage under unbalanced conditions, thereby reducing the unbalance of the output voltage. Finally, simulation results in MATLAB/Simulink environment verify that dynamic changes of the load can be quickly tracked by the proposed control strategy and sufficient power is provided. Compared with the droop control, the rate of the change of frequency and voltage is greatly reduced. This could be explained by the virtual inertia of frequency and voltage responses. At the same time, the three-phase voltage of the microgrid under the improved VSG control strategy in different operating conditions can maintain balanced outputs. Compared with the traditional VSG control, it has better dynamic characteristics of the output voltage and higher steady-state accuracy.

VSG Control Strategy Incorporating Voltage Inertia and Virtual Impedance for Microgrids

Virtual synchronous generator (VSG) control lacks voltage inertia and powerful decoupling capabilities. The voltage of the distributed generator (DG) unit controlled by the VSG will be easily affected by power fluctuations and high-frequency noise, and the DG coupling usually makes the VSG control effect unsatisfactory. In order to effectively reduce power fluctuations, the influence of high-frequency noise on voltage, the influence of coupling on the power regulation, and effectively improve the economy of the economic system, the improved VSG control that combines voltage inertia and virtual impedance is proposed in this paper. The second-order inertia in the proposed VSG control strategy can minimize the voltage change rate and filter high-frequency noise from the excitation and virtual impedance. The virtual impedance in the proposed VSG control strategy can simulate the actual impedance to change the line characteristics, so the coupling of the DG unit can be reduced. Experimental results based on the microgrid platform prove the feasibility of improving the VSG control.

Harmonic Current and Inrush Fault Current Coordinated Suppression Method for VSG Under Non-ideal Grid Condition

For the virtual synchronous generator, the voltage-controlled harmonic suppression method will seriously deteriorate the magnitude of inrush fault current in the moment of grid symmetrical fault, because the suppression effects of harmonic current and inrush fault current are mutually constrained from the perspective of its output impedance. For this issue, a harmonic current and inrush fault current coordinated suppression method is proposed to achieve both good results of harmonic current suppression in normal grid and inrush fault current limitation in fault grid, which mainly includes a passive branch, an active branch and an improved virtual synchronous generator (VSG) control. The passive branch is installed between the PCC and grid to introduce an additional voltage variable. Meanwhile, with the proper feed-forward of the PCC voltage and additional voltage, the active branch is used to increase the mid/high frequency impedance viewed from the grid fault point to VSG, and also decrease the harmonic impedance viewed from the PCC to VSG. Then, not only the transient inrush fault current is limited, but also the harmonic current introduced from the nonlinear load is mostly absorbed. Moreover, the improved VSG control is used to limit the steady-state fundamental frequency fault current. Therefore, the contradictory between harmonic current and inrush fault current suppression for VSG is well solved. Finally, the experiment results validate the proposed control method.

An improved control strategy of virtual synchronous generator under symmetrical grid voltage sag

The virtual synchronous generator (VSG) which mimics the synchronous generator enables the operation of inverter-interfaced distributed generations (DGs) more effectively. However, the insufficiency in the low-voltage ride-through (LVRT) capability of conventional VSG limits its extensive application during the voltage sag condition. In this paper, firstly the performance of conventional VSG under the condition of symmetrical voltage sag is elaborated, and then based on the reference dynamic compensation an improved VSG control strategy is proposed, where the new voltage reference generation algorithm is designated to accommodate to different degrees of voltage sag, avoiding controller switching. Then, a fault current limitation scheme is introduced to guarantee the transient peak current of VSG within an acceptable range, especially during serious system disturbances. When the grid voltage restores to normal, an additional phase angle control loop is implemented to facilitate the smooth and rapid transfer of VSG strategy while the voltage reference is switched back to the rated value. Furthermore, the details of design for the VSG algorithm and the phase angle controller are provided through the formulation of small-signal dynamic model, improving the overall system performance during grid voltage sag. Finally, the validity and effectiveness of the proposed strategy are verified by the hardware-in-the-loop (HIL) simulation results.

Improved virtual synchronous generator control for frequency regulation with a coordinated self-adaptive method

Power inverter adopting virtual synchronous generator (VSG) control can provide inertia support for distributed generation systems. However, it cannot take into account the dynamic regulation characteristics of frequency. Thus, when the system encounters a sudden change in load or disturbance, the dynamic process of frequency regulation will be greatly influenced. In view of this issue, an improved VSG control strategy based on a coordinated self-adaptive (CSA) method is proposed. The time domain analysis method is used to study the influences of virtual inertia and damping parameter perturbation on the system steady and dynamic performances. Further, in order to make the control strategy suitable for large load changes and suppress frequency variation beyond the limit, the secondary frequency modulation is introduced into the control loop. Through the coordinated adaptive control of virtual inertia, virtual damping and frequency modulation, the dynamic performance of VSG frequency regulation can be improved obviously. Simulation and experiment results have verified the effectiveness of the proposed CSA control strategy.

Improved Virtual Synchronous Generator Control of DFIG to Ride-Through Symmetrical Voltage Fault

This article presents an improved virtual synchronous generator (VSG) control strategy of doubly-fed induction generator (DFIG) during symmetrical voltage fault. Based on VSG control, DFIG can behave like a conventional synchronous generator (SG), which can support frequency and voltage in grid-connected mode. Moreover, when microgrid disconnects from grid, DFIG can provide voltage and power for local loads. However, during symmetrical voltage fault, the transient components in DFIG cannot be controlled properly by VSG. To solve this problem, the air-gap flux feedback is designed in VSG control to accelerate the decay of transient components, and rotor current limiting method is also introduced in the article. In the proposed method, there is no need to switch the control structure or control targets. DFIG always works in the VSG control. Therefore, DFIG can operate in the standalone mode when the connection to grid is cut off. The decay time constant, the voltage control, current limiting control and parameter variation of the improved VSG control are analyzed in detail. Finally, the simulation and experimental results in both normal and fault grid condition are given to validate the effectiveness of proposed VSG control strategy.

An Improved VSG Control Strategy Based on the Amplitude-frequency Characteristics of Virtual Power

There are contradictions between the steady-index and dynamic characteristics of active power responses, when virtual synchronous generator (VSG) is connected to grid as a PQ source. The concept of virtual power was proposed for explaining the physical meaning of the control parameters in this paper. Furthermore, the amplitude-frequency characteristics of virtual power were analyzed and corrected to improve system characteristics, and a general expression of improved VSG control scheme is demonstrated. Afterward, the related control parameters were analyzed and designed for a specific case, and the implement steps were conducted. Finally, the effectiveness of the proposed control method was verified.

Adaptive-MPPT-Based Control of Improved Photovoltaic Virtual Synchronous Generators

The lack of inertia and damping mechanism of photovoltaic (PV) grid-connected systems controlled by maximum power point tracking (MPPT) poses a challenge for the safety and stability of the grid. Virtual synchronous generator (VSG) technology has attracted wide attention, since it can make PV grid-connected inverter present the external characteristics of a synchronous generator (SG). Nevertheless, traditional PV-VSG is generally equipped with an energy storage device, which leads to many problems, such as increased costs, space occupation, and post-maintenance. Thus, this paper proposes a two-stage improved PV-VSG control method based on an adaptive-MPPT algorithm. When PV power is adequate, the adaptive-MPPT allows the PV to change the operating point within a stable operation area to actualize system supply-demand, matching in accordance to the load or dispatching power demand; when PV power is insufficient, PV achieves traditional MPPT control to reduce power shortage; simultaneously, improved VSG control prevents the DC bus voltage from falling continuously to ensure its stability. The proposed control approach enables the two-stage PV-VSG to supply power to loads or connect to the grid without adding additional energy storage devices, the effectiveness of which in off-grid and grid-connected modes is demonstrated by typical simulation conditions.

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 Improved VSG Control Algorithm Based on Capacity-Limited Energy Storage System

A large scale of renewable energy employing grid connected electronic inverters fail to contribute inertia or damping to power systems, and, therefore, may bring negative effects to the stability of power system. As a solution, an advanced Virtual Synchronous Generator (VSG) control technology based on Hamilton approach is introduced in this paper firstly to support the frequency and enhance the suitability and robustness of the system. The charge and discharge process of power storage devices forms the virtual inertia and damping of VSG, and, therefore, limits on storage capacity may change the coefficients of VSG. To provide a method in keeping system output in an acceptable level with the capacity restriction in a transient period, an energy control algorithm is designed for VSG adaptive control. Finally, simulations are conducted in DIgSILENT to demonstrate the correctness of the algorithm. The demonstration shows: (1) the proposed control model aims at better system robustness and stability; and (2) the model performs in the environment closer to practical engineering by fitting the operation state of storage system.