Correlation Between Multiple VSG Sources for Enhancing the Power Allocation in Microgrid

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.

Microgrids rely on both primary and secondary frequency control techniques to maintain system stability. Secondary frequency control effectively minimizes frequency fluctuations by adjusting the active power reference in each power inverter, but requires complex and costly interequipment communication. In this paper, we propose a distributed secondary frequency control strategy for microgrids containing multiple virtual synchronous generator (VSG) units based on a new maximum power loading factor (MPLF) concept. The MPLF algorithm facilitates power sharing by dynamically identifying the maximum VSG loading factor at each time instance, and then using this value as a unified reference signal for all the VSGs in the microgrid. The active power reference for each VSG will be adjusted based on the unified reference signal, subsequently the secondary frequency control can be realized. The proposed strategy does not require high-bandwidth communication since the MPLF data are transmitted among the VSGs using low-bandwidth communication. We also develop small-signal models for the control architecture to analyze the influence of major proportional–integral control parameters and communication latency. The MPLF control strategy is implemented using custom digital signal processor controllers, and experimentally validated using hardware in loop simulations. Finally, the new control paradigm demonstrates significant tolerance for communication delay or failure, which we purposely introduced in our investigation.

Virtual synchronous generator (VSG) control algorithm can simulate the characteristics of the traditional synchronous generator, inverter with control can provide the voltage and frequency support for large power grid, and the corresponding inertia and damping characteristics, improving the stability of large power grid. Aiming at the problem of multi inverter parallel circulation suppression, the island micro grid under the virtual synchronous generator (VSG) based on double closed-loop control strategy of virtual impedance, VSG parallel system simulation model built two units of the same capacity, the virtual impedance to suppress the circulation, the simulation results show that the proposed control strategy of circulation.

The trend of future power systems is rapidly changing towards renewable energy resources (RESs) due to the depletion of fossil fuels. Meanwhile, the integration of RESs leads to the instability of the power system due to the lack of inertia and frequency instability. In this paper, the virtual synchronous generator (VSG) model is originated from imitating the synchronous generator characteristics. The VSG control approach is systematically researched and studied for different inverter-interfaced systems that include RESs integrated with the power grid. Parallel multi VSG based Photovoltaic (PV) systems are analyzed to validate the frequency response and power stability under high penetration of PV system. An advanced master-slave droop control approach is proposed for frequency regulation. Additionally, three case studies are analyzed under various effects of irradiance and temperature on the PV-VSG based system based on a specific area during a year. Meanwhile, an advanced particle swarm optimization (PSO) technique is utilized which effectively optimized the optimal parameters of droop control of VSG and improved the frequency regulation.

Frequency damping control is a crucial aspect of islanded microgrids utilizing multiple virtual synchronous generators (VSGs). This paper studies the effects of line impedance mismatches and transient frequency out-of-sync leading to frequency interactive oscillation in VSGs. To address this issue, we propose a distributed interactive damping and zero-error control method that utilizes sparse communications. The objective of this method is to effectively suppress frequency oscillation by minimizing the differences in frequency dynamics among all VSGs. By implementing this approach, a low rate of change of frequency (RoCoF), accurate active power sharing, and zero frequency deviation are ensured. Through dynamic performance analysis, frequency characteristic analysis, and stability analysis based on LaSalle’s invariance principle, we demonstrate significant improvements in system stability, as well as dynamic and steady state performances. Finally, simulation and experimental results obtained under load changes, short-circuits, and communication faults validate the effectiveness of the proposed control method.

This paper investigates transient angle stability of parallel system of two virtual synchronous generators (VSGs) and hybrid system comprised by VSG and diesel generator (DG). Compared to single grid-connected VSG, the paralleled VSGs system and hybrid system are facing larger challenges on stability aspect during serious faults. Although there exist several publications on transient stability, the transient instability mechanism is rather ambiguous, especially for paralleled systems. Besides, swing equations of paralleled VSGs system and paralleled DG-VSG system are established to discuss the dynamic characteristics of the system during short-circuit fault. The increase of power angle between DG and VSG is caused by the different angular accelerations, which deteriorates the transient angle stability of paralleled DG-VSG system. Thus, paralleled DG-VSG system is more prone to instability than paralleled VSGs system, which can be illustrated by power angle (P-δ) characteristic curves. Finally, simulation results validate the correctness of theoretical analyses.

A circulating current suppression strategy of VSG based on adaptive virtual complex impedance

When multiple virtual synchronous generators (VSGs) operate in parallel, severe power oscillation may be suffered. To deal with this issue, this article offers an adaptive virtual impedance method applied to a multi-VSG grid. The core idea is to change the output active power by adaptive virtual impedance. When the angular frequency of the local controller is greater than the angular frequency of the neighbors’ controllers, the output active power of local VSG falls, and thus <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dω_i/dt</i> rises. Then, as the angular frequency is lower than neighbors’, the output active power rises, and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dω_i/dt</i> falls. As a result of this, the method facilitates the consistency of response curves of all angular frequencies. Besides, the stability of the offered algorithm is demonstrated. A final proof of the efficacy of the proposed control method is shown by simulation and hardware-in-loop results.

In recent researches on inverter-based distributed generators, disadvantages of traditional grid-connected current control, such as no grid-forming ability and lack of inertia, have been pointed out. As a result, novel control methods like droop control and virtual synchronous generator (VSG) have been proposed. In both methods, droop characteristics are used to control active and reactive power, and the only difference between them is that VSG has virtual inertia with the emulation of swing equation, whereas droop control has no inertia. In this paper, dynamic characteristics of both control methods are studied, in both stand-alone mode and synchronous-generator-connected mode, to understand the differences caused by swing equation. Small-signal models are built to compare transient responses of frequency during a small loading transition, and state-space models are built to analyze oscillation of output active power. Effects of delays in both controls are also studied, and an inertial droop control method is proposed based on the comparison. The results are verified by simulations and experiments. It is suggested that VSG control and proposed inertial droop control inherits the advantages of droop control, and in addition, provides inertia support for the system.

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.

In several countries, the wind power penetration increased tremendously in recent years. To ensure the proper functioning of the power system, some grid operators already require the capability to provide inertial response or primary control with wind turbines. This paper discusses the emulated inertial response with wind turbines by means of the synthetic inertia and the droop control strategy. The behavior of the synthetic inertia strategy is determined by the inertial and the droop constant, whereas droop control only has a droop constant. When these strategies are used, it is important to tune the control parameters depending on the power system to which the wind turbines are connected. Simulations show that it is possible to enhance but also to deteriorate the frequency response of the system, dependent on these parameters. For different power system compositions, the optimal inertial constant is always close to zero. This way, the synthetic inertia strategy reduces to a fast droop control strategy. This is an important outcome, as it means that no differentiation of the frequency is needed to obtain an optimal inertial response from the wind turbines, which is beneficial in terms of robustness. Consequently, droop control is a viable alternative for the synthetic inertia.

The future of the machine-controlled power system is advancing to an inverter-based power system with a huge integration of renewable energy sources (RES) like photovoltaic (PV) and wind energy. The penetration of RES leads to instability of frequency due to the lack of inertia. Recent development has been made in inverter’s based virtual inertia control for frequency regulation in virtual synchronous generator-based systems. Inverters connecting to the microgrid are provided with virtual inertia and droop control by the virtual synchronous generator (VSG) system. In this study, three parallel VSGs based PV systems are used to investigate the frequency response and voltage stability. An advanced genetic algorithm (GA) optimization method is utilized to attain the optimum parameters of the droop control used in the VSGs. Furthermore, two different case studies with respect to load change are analyzed in detail with comparisons with the classical droop controller. The simulation results observed many improvements of the proposed controller over the classical controller.

The technology of Virtual Synchronous Generator (VSG), which can make the inverter connected to distributed generation work like traditional synchronous generator, plays an important role in improving the stability of the power system because of its good voltage and frequency supporting capability. In this paper, an improved VSG control method is proposed, which can ensure seamless transfer between grid-connected and stand-alone modes, and achieve accurate tracking control of active and reactive power. The control topology and mathematical model of VSG are constructed firstly based on the rotation equation and electrical equation of synchronous generator; then, the corresponding VSG active-frequency droop control, reactive-voltage droop control and inner loop voltage-current control are designed, and the influences of key parameters are analyzed through frequency domain method. Finally, two simulations with different operation modes and loads are conducted to verify the effectiveness in Matlab/Simulink environment, and the results show that the performance is well.

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.

Research on multi-VSG fast pre-synchronization grid connection with dual power loop