Transient Damping of Virtual Synchronous Generator for Enhancing Synchronization Stability during Voltage Dips

Virtual synchronous generators(VSGs)are widely introduced to the renewable power generation,the variablespeed pumped storage units,and so on,as a promising gridforming solution.It is noted that VSGs can provide virtual inertia for frequency support,but the larger inertia would worsen the synchronization stability,referring to keeping synchronization with the grid during voltage dips.Thus,this paper presents a transient damping method of VSGs for enhancing the synchronization stability during voltage dips.It is revealed that the loss of synchronization(LOS)of VSGs always accompanies with the positive frequency deviation and the damping is the key factor to remove LOS when the equilibrium point exists.In order to enhance synchronization stability during voltage dips,the transient damping is proposed,which is generated by the frequency deviation in active power loop.Additionally,the proposed method can realize seamless switching between normal state and grid fault.Moreover,detailed control design for transient damping gain is given to ensure the synchronization stability under different inertia requirements during voltage dips.Finally,the experimental results are presented to validate the analysis and the effectiveness of the improved transient damping method.

An Adaptive Control Strategy for Energy Storage Interface Converter Based on Analogous Virtual Synchronous Generator

In the DC microgrid,the lack of inertia and damping in power electronic converters results in poor stability of DC bus voltage and low inertia of the DC microgrid during fluctuations in load and photovoltaic power.To address this issue,the application of a virtual synchronous generator(VSG)in grid-connected inverters control is referenced and proposes a control strategy called the analogous virtual synchronous generator(AVSG)control strategy for the interface DC/DC converter of the battery in the microgrid.Besides,a flexible parameter adaptive control method is introduced to further enhance the inertial behavior of the AVSG control.Firstly,a theoretical analysis is conducted on the various components of the DC microgrid,the structure of analogous virtual synchronous generator,and the control structure’s main parameters related to the DC microgrid’s inertial behavior.Secondly,the voltage change rate tracking coefficient is introduced to adjust the change of the virtual capacitance and damping coefficient flexibility,which further strengthens the inertia trend of the DC microgrid.Additionally,a small-signal modeling approach is used to analyze the approximate range of the AVSG’s main parameters ensuring system stability.Finally,conduct a simulation analysis by building the model of the DC microgrid system with photovoltaic(PV)and battery energy storage(BES)in MATLAB/Simulink.Simulation results from different scenarios have verified that the AVSG control introduces fixed inertia and damping into the droop control of the battery,resulting in a certain level of inertia enhancement.Furthermore,the additional adaptive control strategy built upon the AVSG control provides better and flexible inertial support for the DC microgrid,further enhances the stability of the DC bus voltage,and has a more positive impact on the battery performance.

Coordinated control strategy for a PV-storage grid-connected system based on a virtual synchronous generator

Due to the characteristics of intermittent photovoltaic power generation and power fluctuations in distributed photovoltaic power generation,photovoltaic grid-connected systems are usually equipped with energy storage units.Most of the structures combined with energy storage are used as the DC side.At the same time,virtual synchronous generators have been widely used in distributed power generation due to their inertial damping and frequency and voltage regulation.For the PV-storage grid-connected system based on virtual synchronous generators,the existing control strategy has unclear function allocation,fluctuations in photovoltaic inverter output power,and high requirements for coordinated control of PV arrays,energy storage units,and photovoltaic inverters,which make the control strategy more complicated.In order to solve the above problems,a control strategy for PV-storage grid-connected system based on a virtual synchronous generator is proposed.In this strategy,the energy storage unit implements maximum power point tracking,and the photovoltaic inverter implements a virtual synchronous generator algorithm,so that the functions implemented by each part of the system are clear,which reduces the requirements for coordinated control.At the same time,the smooth power command is used to suppress the fluctuation of the output power of the photovoltaic inverter.The simulation validates the effectiveness of the proposed method from three aspects:grid-connected operating conditions,frequency-modulated operating conditions,and illumination sudden-drop operating condition.Compared with the existing control strategies,the proposed method simplifies the control strategies and stabilizes the photovoltaic inverter fluctuation in the output power of the inverter.

Consensus-Based Distributed Control with Communication Time Delays for Virtual Synchronous Generators in Isolate Microgrid

A consensus-based distributed control method of coordinated VSGs with communication time delays in isolate microgrid is proposed. When time delays are considered in communication, there are some effects on frequency restoration and active power output allocation. In the control structure, only local information exchange is needed, while the final frequency can be controlled to the nominal value and the VSGs can automatically share loads according to their rated values. An AC microgrid with three VSGs and some loads is implemented. The proposed control strategy is verified by MATLAB/ Simulink simulation results.

Research on Control Strategy of Grid-connected Brushless Doubly-fed Wind Power System Based on Virtual Synchronous Generator Control

The brushless doubly-fed wind power system based on conventional power control strategies lacks ‘inertia’ and the ability to support grid,which leads to the decline of grid stability.Therefore,a control strategy of brushless doubly-fed reluctance generator(BDFRG) based on virtual synchronous generator(VSG) control is proposed to solve the problem in this paper.The output characteristics of BDFRG based on VSG are similar to a synchronous generator(SG),which can support the grid frequency and increase the system ‘inertia’.According to the mathematical model of BDFRG,the inner loop voltage source control of BDFRG is derived.In addition,the specific structure and parameter selection principle of outer loop VSG control are expounded.The voltage source control inner loop of BDFRG is combined with the VSG control outer loop to establish the overall architecture of BDFRG-VSG control strategy.Finally,the effectiveness and feasibility of the proposed strategy are verified in the simulation.

基于CCIs/VSGs混合的分布式发电系统研究

针对现有电流源并网逆变器与弱电网之间的交互稳定性以及新能源高渗透率带来的系统低惯量问题,提出在单一电流源逆变器系统中加入一定比例的虚拟同步机(VSG)运行。分别对混合系统中加入电压源型和电流源型VSG的容量占比与短路比(SCR)稳定域的关系进行了研究,加入电压源型VSG的系统SCR稳定域随VSG容量增大而增大,而加入电流源型VSG的系统SCR稳定域随VSG容量增大而减小。由此提出在单一电流源逆变器系统中加入一定比例双模式VSG,以实现混合系统一定的惯量储备及全SCR范围稳定运行。

Research on Seamless Switching Control Strategy Based on Virtual Synchronous Generator

This paper studies the voltage, phase and current tracking strategy to eliminate voltage and current mutations when the virtual synchronous generator is switching between grid-connected and islanded. By using these strategies the inverter can realize secondary frequency regulation and voltage regulation. If the phase is near 0 or 2π a little disturbance may made the PLL output a big error, so a new PLL is proposed by this paper. A sine module is added in the PLL to avoid this error. In order to verify the strategy proposed by this paper a simulation model is built in Matlab/Simulink. The simulation results show that the control strategy can realize seamless switching.

Active Power Allocation of Virtual Synchronous Generator Using Particle Swarm Optimization Approach

In recent years, the penetration of renewable energy sources (RES) is increasing due to energy and environmental issues, causing several problems in the power system. These problems are usually more apparent in microgrids. One of the problems that could arise is frequency stability issue due to lack of inertia in microgrids. Lack of inertia in such system can lead to system instability when a large disturbance occurs in the system. To solve this issue, providing inertia support to the microgrids by a virtual synchronous generator (VSG) utilizing energy storage system is a promising method. In applying VSG, one important aspect is regarding the set value of the active power output from the VSG. The amount of allocated active power during normal operation should be determined carefully so that the frequency of microgrids could be restored to the allowable limits, as close as possible to the nominal value. In this paper, active power allocation of VSG using particle swarm optimization (PSO) is presented. The results show that by using VSG supported by active power allocation determined by the method, frequency stability and dynamic stability of the system could be improved.

The Novel Current Control for Virtual Synchronous Generator

In recent years, power generation using renewable energy sources has been developed as a solution to the global warming problem. Among these power generation methods, wind power generation is increasing. However, as the penetration level of wind power generation increases, the low inertia and lack of synchronous power characteristics of the penetrated power system can have a significant impact on the transient stability of the grid. The virtual synchronous generator provides the ability of virtual inertia and synchronous power to interconnected inverters. The interconnected inverter with the virtual synchronous generator ability uses, in general, PI control based current controller. This paper proposes a new current-control method and compares it with conventional methods. The proposed current control is a method that follows virtual synchronous generator model that changes every moment by solving the discrete-time linear quadratic optimal control problem for each sampling time interval. The new method follows the conventional method, and therefore the reactive power fluctuation can be suppressed and the interconnected inverter will be downsized.

A Study of Discrete-Time Optimum Current Controller for the Virtual Synchronous Generator under Constraints

In recent years, power generation using renewable energy sources has been increasing as a solution to the global warning problem. Wind power generation can generate electricity day and night, and it is relatively more efficient among the renewable energy sources. The penetration level of variable-speed wind turbines continues to increase. The interconnected wind turbines, however, have no inertia and no synchronous power. Such devices can have a serious impact on the transient stability of the power grid system. One solution to stabilize such grid with renewable energy sources is to provide emulated inertia and synchronizing power. We have proposed an optimal design method of current control for virtual synchronous generators. This paper proposes an optimal control method that can follow the virtual generator model under constrains. As a result, it is shown that the proposed system can suppress the peak of the output of semiconductor device under instantaneous output voltage drop.

Virtual Synchronous Generator Based Current Synchronous Detection Scheme for a Virtual Inertia Emulation in SmartGrids

Renewable energy sources, such as photovoltaic wind turbines, and wave power converters, use power converters to connect to the grid which causes a loss in rotational inertia. The attempt to meet the increasing energy demand means that the interest for the integration of renewable energy sources in the existing power system is growing, but such integration poses challenges to the operating stability. Power converters play a major role in the evolution of power system towards SmartGrids, by regulating as virtual synchronous generators. The concept of virtual synchronous generators requires an energy storage system with power converters to emulate virtual inertia similar to the dynamics of traditional synchronous generators. In this paper, a dynamic droop control for the estimation of fundamental reference sources is implemented in the control loop of the converter, including active and reactive power components acting as a mechanical input to the virtual synchronous generator and the virtual excitation controller. An inertia coefficient and a droop coefficient are implemented in the control loop. The proposed controller uses a current synchronous detection scheme to emulate a virtual inertia from the virtual synchronous generators. In this study, a wave energy converter as the power source is used and a power management of virtual synchronous generators to control the frequency deviation and the terminal voltage is implemented. The dynamic control scheme based on a current synchronous detection scheme is presented in detail with a power management control. Finally, we carried out numerical simulations and verified the scheme through the experimental results in a microgrid structure.

Low-Frequency Oscillation Analysis of Grid-Connected VSG System Considering Multi-Parameter Coupling

With the increasing integration of new energy generation into the power system and the massive withdrawal of traditional fossil fuel generation,the power system is faced with a large number of stability problems.The phenomenon of low-frequency oscillation caused by lack of damping and moment of inertia is worth studying.In recent years,virtual synchronous generator(VSG)technique has been developed rapidly because it can provide considerable damping and moment of inertia.While improving the stability of the system,it also inevitably causes the problem of active power oscillation,especially the low mutual damping between the VSG and the power grid will make the oscillation more severe.The traditional time-domain state-space method cannot reflect the interaction among state variables and study the interaction between different nodes and branches of the power grid.In this paper,a frequency-domain method for analyzing low-frequency oscillations considering VSG parameter coupling is proposed.First,based on the rotor motion equation of the synchronous generator(SG),a secondorder VSG model and linearized power-frequency control loop model are established.Then,the differences and connections between the coupling of key VSG parameters and low-frequency oscillation characteristics are studied through frequency domain analysis.The path and influencemechanism of a VSG during low-frequency power grid oscillations are illustrated.Finally,the correctness of the theoretical analysis model is verified by simulation.

基于功率微分项的双VSG有功功率振荡抑制策略

针对虚拟同步发电机(VSG)双机并联运行时产生的有功功率振荡问题,通过建立双VSG系统的小信号模型,分析了有功功率振荡产生的机理,提出一种基于改进虚拟阻抗的瞬态功率均分策略和一种基于功率微分项的自适应惯量和阻尼的功率振荡抑制策略,将有功功率微分项与角加速度结合起来实现惯量的自适应调节,同时将有功功率微分项引入阻尼控制环节以替代传统的阻尼项,实现阻尼的自适应调节。所提出的策略可以使动态时VSG之间的角频率差更快速地趋近于零,从而减小有功功率振荡的超调,改善频率动态响应性能。搭建了VSG并联的仿真模型,仿真结果表明:所提改进控制策略可以使瞬态时实现有功均分,使动态时的有功功率振荡超调减小41%,振荡过程减小了1秒。

孤岛微电网的虚拟同步发电机分布式协同二次调频控制

基于孤岛微电网中虚拟同步发电机一次控制存在的频率偏差问题,提出一种基于预定时系统的分布式二次频率控制策略。该策略通过预定时系统对一致性进行改进并适配虚拟同步发电机本体算法,使各逆变器仅依靠相邻单元的通信,即可实现系统有功缺额按容量分配与频率的无差控制。同时,预定时系统通过提前设定一致性的收敛时间,进一步提高频率的恢复速度,并且无需考虑初始容量与协议参数的影响,有利于分布式电源的即插即用,提高运行效率。最后通过仿真验证了所提控制策略的可行性和有效性。

基于功角保持的VSG低电压穿越控制策略

虚拟同步发电机(virtual synchronous generator,VSG)控制通过模拟同步发电机的运行特性,达到并网逆变器为电网提供惯性支撑的目的。传统VSG在电网发生短路故障时容易导致输出电流越幅与功角振荡,无法为电网提供无功支撑,使逆变器不具备低电压穿越能力。文章提出一种VSG低电压穿越控制策略,故障期间保持功角稳定,故障恢复期间通过基于虚拟功率的功角保持环节实现预并列,故障恢复后输出功率能够快速恢复至给定值,运行模式之间能够平滑快速切换,实现低电压穿越。仿真验证了该控制策略的正确性与有效性。

基于虚拟电力系统稳定器的低频振荡抑制策略

基于虚拟同步发电机(VSG)控制的新能源机组虽可增强电网惯量及频率稳定性,但可能继承与同步机类似的低频振荡问题。针对该问题,首先基于单台VSG并网的线性化模型,构建了适用于VSG的Phillips-Heffron模型,借助阻尼转矩法挖掘了VSG低频振荡的诱发机理。进一步提出一种附加于VSG无功环节的虚拟电力系统稳定器(VPSS),并给出了控制结构及基于相位补偿法的参数设计过程。研究结果表明:VSG发生低频振荡是由于系统受扰后,虚拟功角的振荡信号经过虚拟励磁调节器对VSG机电振荡回路产生了负阻尼转矩。将所提VPSS接入系统可在几乎不影响振荡频率的前提下,实现0.1~3 Hz下VSG低频振荡的有效抑制。

孤岛微电网多虚拟同步发电机频率无差协调控制策略

传统虚拟同步发电机的频率控制为有差控制,且虚拟惯量的引入使得虚拟同步发电机呈现二阶振荡特性,导致孤岛微电网多虚拟同步发电机并联系统在受到负荷扰动时面临频率偏移和有功超调的问题。针对此,提出了一种多虚拟同步发电机频率无差协调控制方法。该方法包含了兼具有功精确分配的频率无差调节控制和阻尼改进控制两部分,通过利用母线频率全局一致的“通信”效果,以分散式的形式协调系统内各台虚拟同步发电机,可主动消除微电网频率稳态偏差且实现各台虚拟同步发电机按照容量承担负荷有功功率,并抑制系统的有功超调。最后,通过仿真和实验验证了所提控制方法的有效性。

考虑储能约束的神经网络VSG参数自适应控制策略

虚拟同步发电机(Virtual Synchronous Generator, VSG)控制策略常用于处理高比例新能源并网导致系统惯量、阻尼缺失的问题,储能单元的配置制约着VSG惯量阻尼的设定。首先,建立考虑储能荷电状态(State of Charge, SOC)约束的VSG小信号模型,计算多约束条件下惯量阻尼取值范围。然后,借助径向基函数(Radial Basis Function, RBF)神经网络算法处理连续非线性函数的优点,提出一种计及储能约束的RBF神经网络VSG惯量阻尼自适应控制策略。该策略取频率偏差量和频率变化率为输入,输出为VSG转动惯量,根据二阶系统最优阻尼比确定阻尼系数取值。最后通过Matlab/Simulink仿真验证,结果表明所提计及多约束的VSG自适应控制策略能够快速响应系统频率变化,优化系统动态调节能力;减小直流母线电压跌落,增强系统鲁棒性。

考虑可再生能源接入的多端MMC交直流混合系统协调控制

模块化多电平变换器的零转动惯量特性,使其无法对交流电网的频率波动提供支撑,导致交直流混合系统的整体惯性下降,影响并网系统的动态性能与稳定性。文章将改进功率-电压下垂控制与虚拟同步机技术相结合,提出一种新型功率协调控制策略。在系统直流电压稳定的情况下,对交流电网进行频率调节,同时对各变换器间的功率进行合理分配,无需通讯系统即可建立有效的能量管理体系。与传统双闭环控制进行对比试验,结果表明,所提控制策略有效解决了系统由低惯量、欠阻尼引起的动态性能与稳定性的问题。

基于角频率暂态前馈的VSG控制策略

针对虚拟同步发电机(VSG)并网时,输出功率的稳态性能与动态性能之间存在耦合,无法同时满足抑制有功振荡和电网一次调频的需求,提出基于角频率暂态前馈的控制策略,通过补偿输出功率,减小Δω,进而减小振荡过程。该控制策略可以有效抑制有功振荡,并且实现了稳态性能与动态性能间的解耦,不会影响一次调频特性,同时提高了响应速度。首先建立VSG有功闭环小信号模型,分析了输出功率稳态性能与动态性能之间的矛盾,并根据同步发电机功角特性分析了有功振荡机理;在此基础上提出了基于角频率暂态前馈的控制策略,利用闭环零极点与Bode图分析了所提控制策略对系统的影响;最后通过仿真和RT-LAB半实物平台验证了所提控制策略的可行性与优越性。