PV Based Microgrid with Grid-Support Grid-Forming Inverter Control-(Simulation and Analysis)

Microgrid (MG) is a small entity of electrical network which comprises of various Distributed Generation (DG) sources, storage devices, and group of loads in various classes. MG provides reliable and secure energy supply to the critical loads of communities while operating either in on-grid or off-grid mode. In this study, a coordinated power management control strategy for a typical low voltage (LV) MG network with integration of solar Photovoltaic (PV) and storage facility has been developed and analysed in Matlab-Simu-link software environment at various modes (on-grid, off-grid, and on-grid to off-grid transition) of MG operation. Solar PV and battery power inverters are considered as grid-support grid-forming (GsGfm) Voltage Source Inverter (VSI) with the implementation of modified droop and virtual output impedance control strategies. Proposed control strategy supports coordinated control operation between PV units and battery storage, equal power sharing among the DG sources, and smooth MG mode transition with regulation of voltage and frequency level in MG network. In addition, voltage and current THD level were analysed and verified as per the standard of AS4777.

Zero Sequence Voltage and Current Control in Four-wire Grids-fed by Grid-forming Inverters

Low voltage three-phase four-wire AC distribution grids may experience high neutral current,mainly caused by asymmetrical distribution of single-phase loads in three phases.High neutral current will not only increase line losses but also result in neutral potential variations.For the LV AC distribution grid established by a grid-forming inverter(e.g.,uninterruptible power supply and solid-state-transformer),it also suffers from the same neutral current issues.Therefore,this paper comparatively studies several neutral current control approaches and their impacts on grid voltage balance,which is required by grid code.Then,this paper proposes an optimal neutral current control approach,which can obtain maximum neutral current suppression with less impact on grid voltage balance.The correctness of the theoretical analysis is validated through both simulation and experimental results.

太阳能船舶光伏系统构网型逆变器并网控制策略

[目的]旨在解决现有太阳能船舶光伏发电系统采用跟网型控制策略并网,而导致的船舶电力系统整体稳定性下降问题。[方法]结合船舶电力系统特点和运行特性,以跟网型恒功率控制和构网型下垂控制为研究基础,另外选取构网型虚拟同步发电机控制和构网型虚拟振荡器控制,共4种逆变器控制方法,分别构建太阳能船舶电力系统光伏并网控制策略。最后通过Matlab/Simulink建立太阳能船舶电力系统仿真模型,验证构网型控制理念和思路的有效性。[结果]研究结果表明,与跟网型控制相比,在负荷投切工况下,采用构网型控制策略可使频率波动极值平均减小26.86%、电压调节时间平均减小37.85%。在光伏输出功率切换工况下,频率波动极值平均减小39.95%。[结论]采用构网型控制策略能有效抑制光伏系统并网后所导致的船舶电力系统稳定性下降的问题。能够使光伏系统在完成供能的同时,主动维持船舶电力系统电力参数稳定,显著提高船舶电力系统稳定性。

Transient Voltage Support Strategy of Grid-forming Medium Voltage Photovoltaic Converter in the LCC-HVDC System

The participation of photovoltaic(PV)plants in supporting the transient voltage caused by commutation failure in the line-commutated-converter-based high voltage direct current(LCC-HVDC)system is of great significance,as it can enhance the DC transmission ability.However,it is found that the gridfollowing(GFL)PV converters face the problem of mismatch between reactive power response and transient voltage characteristic when the voltage converts from low voltage to overvoltage,further aggravating the overvoltage amplitude.Thus,this article proposes a transient voltage support strategy based on the gridforming(GFM)medium voltage PV converter.The proposed strategy takes the advantage of the close equivalent electrical distance between the converter and grid,which can autonomously control the converter terminal voltage through GFM control with adaptive voltage droop coefficient.The simulation results show that the proposed strategy can ensure the output reactive power of the PV converter quickly matches the transient voltage characteristic at different stages,indicating that the proposed strategy can effectively support the transient voltage.

Review of Grid-forming Inverters in Support of Power System Operation

The penetration of distributed energy resources in electrical grids has been steadily increasing in an effort to reduce greenhouse gas emissions.Inverters,as interfaces between distributed energy resources and grids,have become critical assets in modern power systems.In recent years,the development and application of grid-forming inverters have gained significant traction due to their capability of supporting power grid operations.A comprehensive review of grid-forming inverters is presented for power system applications.A comparison between grid-forming inverters and grid-following inverters is conducted in terms of their functionalities to highlight the potential of grid-forming inverter technologies in support of power system stability and resiliency.In addition,advanced control strategies integrated into grid-forming inverters under various operation conditions are presented through reviewing the innovations introduced in recent literature and in industrial applications.This paper is intended to provide an updated reference regarding grid-forming inverters for power system applications to researchers and practitioners in the field of power electronics.

计及阻尼特性的构网型并网逆变器暂态同步稳定性分析

构网型并网逆变器(GFM-GCI)可以为电网提供电压和频率支撑,有利于提高电网稳定性。首先,针对GFM-GCI的暂态同步稳定问题,在考虑阻尼特性的基础上采用等面积法则,从能量角度定性分析GFM-GCI控制参数对暂态同步稳定性的影响机理。然后,采用摄动理论中的多尺度法得到功角非线性二阶微分方程的显式解析解,定量分析控制参数对GFM-GCI暂态同步稳定性的影响。接着,基于解析解和暂态同步稳定边界,给出一种面向GFM-GCI暂态同步稳定的控制参数设计方法,所提控制参数设计方法与现有传统暂态分析方法相比,保守性更低。最后,通过仿真和实验验证了理论分析、解析模型与所提控制参数设计方法的有效性。

Transient performance comparison of grid-forming converters with different FRT control strategies

Grid-forming converters(GFMs)are faced with the threat of transient inrush current and synchronization instability issues when subjected to grid faults.Instead of disconnecting from the grid unintentionally,GFMs are required to have fault ride through(FRT)capability to maintain safe and stable operation in grid-connected mode during grid fault periods.In recent studies,different FRT control strategies with distinguishing features and that are feasible for different operation conditions have been proposed for GFMs.To determine their application scope,an intuitive comparison of the transient performance of different FRT control strategies is presented in this paper.First,three typical FRT control strategies(virtual impedance,current limiters,and mode-switching control)are introduced and transient mathematical models are established.A detailed comparison analysis on transient inrush current and transient synchronization stability is then presented.The results will be useful for guiding the selection and design of FRT control strategies.Finally,simulation results based on PSCAD/EMTDC are considered to verify the correctness of the theoretical analysis.

Stability Comparison Between Grid-forming and Grid-following Based Wind Farms Integrated MMC-HVDC

Grid-forming(GFM)control based high-voltage DC(HVDC)systems and renewable energy sources(RESs)provide support for enhancing the stability of power systems.However,the interaction and coordination of frequency support between the GFM-based modular multilevel converter based HVDC(MMC-HVDC)and grid-following(GFL)based RESs or GFM-based RESs have not been fully investigated,which are examined in this study.First,the detailed AC-and DC-side impedances of GFM-based MMC-HVDC are analyzed.The impedance characteristics of GFL-and GFM-based wind turbines are next analyzed.Then,the influences of GFL-and GFM-based wind farms(WFs)on the DC-and AC-side stabilities of WF-integrated MMC-HVDC systems are compared and evaluated.The results show that the GFM-based wind turbine performs better than the GFL-based wind turbine.Accordingly,to support a receiving-end AC system,the corresponding frequency supporting strategies are proposed based on the GFM control for WF-integrated MMC-HVDC systems.The GFM-based WF outperforms the GFL-based WF in terms of stability and response time.Simulations in PSCAD/EMTDC demonstrate the DC-and AC-side stability issues and seamless grid support from the RESs,i.e.,WFs,to the receiving-end AC system.

Influence of Virtual Synchronous Generators on Low Frequency Oscillations

By simulating the operating dynamics of synchronous generators(SGs),the use of virtual synchronous gen-rators(VSGs)can help overcome inverter-based generators'shortcomings of low inertia and minimal damping for gridforming applications.VSGs'stability are very important for their solar and wind electricity applications.Currently,the related research primarily focuses on VSGs and their applications for microgrids.There has been little research to explore how VSGs effect low frequency oscillations in power transmission systems.This paper describes a small-signal model of a VSGSG interconnected system,which is suitable for studying low frequency oscillation damping in a power transmission grid.Based on this model,the effects of VSGs on low frequency oscillations are compared with the effects of SGs to reveal the mechanism of how VSGs infuence damping characteristics.The influence of each VSG control loop on oscillations is also analyzed in this paper.Then,the low frequency oscillation risks with different types of VSGs are described.Finally,experiments on a real-time laboratory(RT-LAB)platform are conducted to verify the small-signal analysis results.