摘要
随着电力系统中新能源占比的增加,传统电网的强度越来越弱,短路比降低,导致基于电压定向的跟网型电压源型换流器(VSC)的稳定性变弱。为了解决跟网型VSC连接弱网时的稳定性问题和功率传输极限问题,基于功率定向的构网型VSC得到广泛关注,尤其是采用功率同步控制(PSC)和虚拟同步机(VSG)控制的并网VSC,因结构简单而迅速发展。然而,考虑电网和线路等值电感动态时,构网型换流器在感性网络的等效电阻较小时易发生一种同步频率谐振(SFR),导致系统失稳。该文主要以PSC-VSC为研究对象,通过建立小信号模型分析其发生SFR的机理,发现除了线路电阻小之外,非最小相位效应是引发SFR的另一个原因。为了抑制这种同步频率谐振,该文提出采用陷波器的谐振抑制方法,与采用低通滤波器方法(等效VSG控制)相比,该文所提方法对控制参数更具有鲁棒性,同时能提高系统的带宽。最后,建立实验平台,通过物理实验验证该文所提出控制策略的有效性。
As an important interface between new energy power generation and the grid,the Voltage Source Converter(VSC)has been widely used in new power systems.At present,the grid following VSC based on current vector control has extensive application in engineering practice.However,with the increasing integration of new energy sources and the continuous addition of power electronic equipment,power grid stability weakens,leading to challenges in the stability of grid-following VSCs.The concept of grid-forming VSC is proposed to improve the stability,especially in weak grid scenarios and for power supply to isolated islands.Power Synchronization Control(PSC)and Virtual Synchronous Generator(VSG)control have gained significant attention due to their simplicity.It is found that PSC control with a first-order low-pass filter can be equivalent to VSG control when the influence of energy storage is ignored.However,an issue known as Synchronous Frequency Resonance(SFR)arises when VSCs controlled by PSC or VSG are connected to inductive grids with small equivalent resistances.Based on the control equation,VSG control and PSC control with a first-order low-pass filter can be equivalent to each other.The coupling effect between the active power control loop(APCL)and reactive power control loop(RPCL)is explored,and an equivalent mathematical model for their mutual coupling is deduced.Secondly,the influence of the voltage and current inner loop dynamics of the system is analyzed,demonstrating that the inner loop does not change the system characteristics near the synchronization frequency.Therefore,to simplify the analysis of the synchronous frequency resonance mechanism,the inner loop is considered fast enough to achieve ideal voltage tracking.Thirdly,based on the small signal model in the frequency domain,it is verified that a small equivalent resistance in an inductive grid may cause SFR,and power control loop parameters can affect system stability.The Nyquist criterion analysis indicates that the presence of a non-minimum phase system is a cause of SFR.Finally,the mechanism of the SFR phenomenon of the grid-forming converter is analyzed,and an SFR suppression strategy is proposed.The effectiveness of the proposed method is verified.The following conclusions can be drawn from the analysis:(1)A small signal model in the frequency domain of grid-forming PSC-VSC coupled with APCL and RPCL is established.According to the closed-loop system eigenvalue loci,it is verified that a small equivalent resistance of the inductive grid can cause SFR.The non-minimum phase effect is another reason for SFR.(2)Compared with the first-order low-pass filter(equivalent to VSG control),the proposed SFR suppression method can effectively suppress the resonance peak and require lower precision of parameter design.Meanwhile,it also has strong robustness in control parameters and a more substantial stability margin for the system.(3)The theoretical analysis is verified based on the established three-phase grid-connected converter experiment platform.
作者
熊小玲
李昕悦
周琰
罗博晨
Xiong Xiaoling;Li Xinyue;Zhou Yan;Luo Bochen(State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources,North China Electric Power University,Beijing,102206,China)
出处
《电工技术学报》
EI
CSCD
北大核心
2024年第12期3827-3839,共13页
Transactions of China Electrotechnical Society
基金
国家自然科学基金(52277176)
中央高校基本科研业务费专项资金(2022MS004)
《台达电力电子科教发展计划》资助项目。
关键词
构网型换流器
同步频率谐振
陷波器
稳定性
Grid-forming converter
synchronous frequency resonance(SFR)
notch filter
stability
