An Adaptive Control Strategy for Virtual Synchronous Generator

The conventional synchronous generator (SG) can regulate the active and reactive power for frequency and voltage stability, when the system fluctuates. In renewable energy generation system (RES), the stability problem of RES can be solved by adding virtual inertia and damping in the control strategy of the grid-connected inverter. The grid-connected inverter and renewable energy system with this kind of control strategy, called as virtual synchronous generator (VSG), acts as a synchronous generator. In this paper, the topology of grid-connected PV array and energy storage system is proposed. The VSG used in grid-connected inverter, is modelled as stator and rotor model by stator voltage equation and rotor motion equation, respectively. Meanwhile, the VSG control mechanism, based on droop control and model of VSG, can be divided as active power-frequency and reactive power-voltage control. Besides, the system stability is analyzed based on small signal model, for the design of inertia and damping. The simulation model is designed by MATLAB/Simulink for the implementation of VSG. In simulation, it is compared that the dynamic regulation performance of VSG with different control coefficient, such as virtual inertia, virtual damping, droop coefficient and integral coefficient. The optimization for regulation performance in simulation provides an appropriate selection method of the control coefficient for VSG.

An enhanced second-order-consensus-based distributed secondary frequency controller of virtual synchronous generators for isolated AC microgrids

In recent years, the increasing penetration of distributed generation in microgrids challenges the control and coordination of energy resources. Especially in microgrids with virtual synchronous generator (VSG)-controlled converters and conventional synchronous generators (SG), the inherent inertia difference (i.e., the VSG and SG) results in a poor transient performance when the VSG and/or loads are cut in/out. Thus, this paper explores the transient performance of microgrids with parallel VSG and SG systems. More importantly, a novel pre-synchronization control method is proposed to eliminate the phase jump while meeting the requirements in case of closures or re-closures of generation units. A small-signal dynamic model is presented, and accordingly, the VSG inertia and its damping can be designed considering the capacity ratio of VSG and SG units. In addition, with the power angle stability analysis, an active power provision strategy is introduced to suppress the transient power oscillation due to the inertia difference. Finally, the feasibility of the proposed methods is verified by simulations on a microgrid consisting of parallel VSG and SG units.

Virtual synchronous generator (VSG) has been generally concerned in the field of new energy power generation because of their ability to imitate the operational mechanism of synchronous generator. In order to solve the problem of power and frequency fluctuation or even off-limit caused by load changes, this paper summarizes the control strategy for the stable operation of the electricity grid supported by VSG. It combines the changes of various physical quantities in the transient process. It fully uses VSG virtual inertia and damping coefficient flexible and regulable advantages and puts forward an adaptive control strategy for inertia and damping coefficient. The control strategy can efficiently suppress the fluctuation of VSG output power and frequency in dynamic processes. It can cut down the transient adjustment time significantly and promote the stability of the system. The correctness and effectiveness of the proposed control strategy are verified by simulation experiment results.

In order to improve the friendliness of the grid connection of new energy power generation, the new energy photovoltaic (PV) unit is equivalent to a synchronous generator in the power system and a virtual synchronous generator (VSG)-controlled PV energy storage complementary grid-connected power generation system model is established and studied to analyze the VSG. When power is supplied to the load together with the power grid, the energy storage unit inside the VSG will release and store the electrical energy according to the fluctuation of the PV output, which plays the role of the adjustment of the prime mover; in the case of load power fluctuations, and power grid assume the corresponding active power regulation according to their capacity. The amount of active power adjustment to jointly and maintain the power balance inside the system under the condition of fluctuating load power. The overall system architecture and control strategy of PV grid-connected inverter based on VSG algorithm are proposed. The PV-VSG proposed here not only takes into account the maximum power point tracking control but also has independent participation in the power supply. A series of characteristics of synchronous generators, such as network frequency modulation voltage regulation and inertia damping, can effectively improve the new energy PV power generation system and promote the new energy consumption. The results of system simulation and field demonstration operation fully show the effectiveness and correctness of the proposed control strategy based on VSG algorithm.

Virtual synchronous generator (VSG) was a control strategy for inverter generators (IGs) to overcome the shortcomings of small inertia and low damping by simulating the rotor operating mechanism of synchronous generators (SGs). The stability of VSG is very important for its application. At present, the related research mostly focuses on the VSG itself and its application in microgrid. There is few research to explain how VSG effecting the low frequency oscillation in transmission power system. This paper built a small-signal model of VSG-SG interconnected system, which is suitable for the study of low frequency oscillation damping characteristics of the transmission power grid. Based on the model, the effect of VSG on the low frequency oscillation were compared with the effect of SG to reveal the mechanism of how VSG affecting the damping characteristic.

The increasing penetration of renewable energy, such as photovoltaic generation, makes it essential to enhance power system dynamic performance through improved grid-forming control strategies. In the grid-forming control system, the virtual synchronous generator control (VSG) is currently widely used. However, the inertia (J) and damping (D) in the traditional VSG are fixed values, which can result in large overshoots and long adjustment times when dealing with disturbances such as load switching. To address these issues, this paper proposes an adaptive virtual synchronous generator (VSG) control strategy for grid-side inverters, which is accomplished by adaptively adjusting the VSG’s inertia and damping. Firstly, we established a photovoltaic-storage VSG grid-forming system; here, the photovoltaic power is boosted through a DC-DC converter, and the energy storage is connected to the common DC bus through a bidirectional DC-DC converter. We analyzed how J and D shape the system’s output characteristics. Based on the power-angle characteristic curve, the tanh function was introduced to design the control function, and a JD collaborative adaptive control (ACL) strategy was proposed. Finally, simulation experiments were conducted under common working conditions, such as load switching and grid-side voltage disturbance, to verify the results. From the results shown, the proposed strategy can effectively improve the response speed of the system, suppress system overshoot and oscillation, and, to a certain extent, improve the dynamic performance of the system.

Due to various implementations of the voltage control, a virtual synchronous generator (VSG) may have different behaviors. This paper compares several voltage control strategies used in the VSG as well as the technology applied in a traditional synchronous generator. The performances are evaluated by the small-signal model and frequency-domain analysis, where the relationship and differences among various control strategies are revealed. The experimental results verify the theoretical analysis. The outcome of this paper is a reference to properly design the voltage control of a VSG and to understand the differences between a VSG and an actual synchronous generator.

As one of the most promising control strategies, virtual synchronous generators (VSG) control is studied worldwide and impedance models have been built for VSG by some papers, but most of them overlook the inner loops which can make significant difference to impedance of VSG. This paper establishes the sequence impedance models of VSG with and without inner loops and demonstrates the negative influence of inner loops on dynamic performances in the range of low frequency by the comparison of impedance models of VSG with and without inner loops. Then the paper gives suggestion that virtual resistance should be adopted to the VSG with inner loops to improve stability. Finally, all the theoretical analysis is verified by simulation results.

Virtual Synchronous Generator technology improves the frequency dynamic performance of the system by introducing virtual inertia and damping coefficients into the power electronic inverter. In order to further suppress power oscillations and solve the problem that constant virtual parameters cannot take into account the maximum frequency overshoot and response speed, firstly, a small-signal model of active and reactive power rings of virtual synchronous machine is established to study the influence of virtual inertia on the frequency stability and power equalization effect of parallel connection of multiple machines. Secondly, based on the fuzzy control method in variable universe, the control strategy of coordinated adaptive adjustment of the virtual inertia of the parallel system according to the output angular frequency is proposed, and at the same time, the automatic adjustment of the virtual damping coefficient with the inertia is realized by combining with the appropriate damping ratio. Finally, the parallel parameter matching principle is designed so that the output power can be distributed according to the capacity ratio. Two different virtual synchronous machine control strategies are compared and analyzed by matlab/simulink to verify the correctness and feasibility of the proposed adaptive control strategy.

Integration of distributed generations (DGs) in the power grid poses major technical challenges. Many of the DGs comprise of power electronic converters which is resulting in low system inertia and is creating the challenge of voltage and frequency control during micro-grid switching events. This paper investigated hybrid micro-grid frequency and voltage control techniques, in particular multivariable droop and virtual synchronous generator (VSG) for transient load sharing between inverter based DGs and a low inertia synchronous generator based DG. Numerical simulation results show that frequency control performances are better achieved with the implementation of virtual synchronous generators compared with droop based control.

Virtual synchronous generator (VSG) is one of the inverter control methods which can provide extra virtual moment of inertia and achieve frequency support by mimicking the output characteristics of a rotating synchronous generator (RSG), which makes VSG particularly suitable for multi-access point applications, or called the multi-VSG system. However, frequency fluctuations may often occur in the case of power variation. In terms of this issue, this study presents the small-signal state-space model of VSG and analyzes the cause of frequency fluctuation, first. And then, a novel VSG control method is proposed for frequency fluctuation attenuation. The proposed method is especially fit for the parallel multi-VSG system because it does not take grid angular frequency into computation so that it can get rid of the ill effect introduced through PLL. The damping power item is reconstrued with a new means to judge whether the system is in a steady state or not. At the same time, the parametric design method of the proposed method is also figured out. Finally, the simulation experiments are performed, and the results verify that the proposed method performs better than the conventional one in terms of dynamic response and power-sharing among the multi-VSG system.

A comparative study of single-loop VSG and multi-loop VSG

The high penetration of renewable energy sources (RES) results in a low‐inertia, weak power grid. To mitigate this and restore system inertia, it has been widely proposed to operate the inverters of RES units to mimic synchronous generators; this technology is known as a virtual synchronous generator (VSG). In weak grids there is, however, strong coupling between active power () and reactive power (), and any VSG technique therefore requires decoupling in order to operate effectively. This article proposes a new decoupling technique based on the transformation of the power circle of a VSG connected to a weak grid: first, the power circle is translated from its designed position to that of a conventional synchronous generator (SG) connected to a strong grid, achieving partial decoupling. Then, to achieve full decoupling, the authors propose further to modulate the radius of the translated power circle; this is achieved using a series resistance‐capacitance–inductance () circuit which is virtually implemented in the VSG controller. The efficacy of the proposed scheme is validated using a modified synchronverter connected to the weak grid in representative loading scenarios. It is demonstrated that the technique achieves a decoupled control for the synchronverter connected in a weak grid. Moreover, the modified synchronverter is capable of supporting frequency and voltage regulation in the grid without inducing large transient grid currents during mild frequency and voltage variations.

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.