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Linear Active Disturbance Rejection Control and Stability Analysis for Modular Multilevel Converters Under Weak grid

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Journal of Modern Power Systems and Clean Energy 2023年第6期11卷 2028-2042页

作者： Hailiang Xu Mingkun Gao Pingjuan Ge Jiabing Hu College of New Energy China University of Petroleum(East China)Qingdao 266580China School of Electrical and Electronic Engineering Huazhong University of Science and TechnologyWuhan 430074China

The modular multilevel converters(MMCs) are popularly used in high-voltage direct current(HVDC) transmission systems. However, for the direct modulation based MMC, its complex internal dynamics and the interaction with the grid impedance would induce the frequency coupling effect, which may lead to instability issues, especially in the case of Weak grid. To effectively suppress the sub-and super-synchronous oscillations, this paper proposes a linear active disturbance rejection control(LADRC) based MMC control strategy. The LADRC mainly consists of the linear extended state observer(LESO) and the linear state error feedback(LSEF). And it is a potential method to enhance the system stability margin, attributing to its high anti-interference capability and good tracking performance. Thereupon, the system small-signal impedance model considering frequency coupling is established. And the effect of the introduction of the LADRC on the system stability is further investigated using the Nyquist criterion. Particularly, the influences of key control parameters on the stability are discussed in detail. Meanwhile, the impact of LADRC on the transient performance is explored through closed-loop zero poles. Finally, the correctness of the theoretical analysis and the effectiveness of the proposed control strategy are verified via electromagnetic simulations.

关键词： Modular multilevel converter(MMC) linear active disturbance rejection control(LADRC) sub-and supersynchronous oscillation stability Weak grid

The Resonance Suppression for Parallel Photovoltaic grid-connected Inverters in Weak grid

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International Journal of Automation and computing 2018年第6期15卷 716-727页

作者： Qiu-Xia Yang Kun Li Cui-Mei Zhao Hu Wang School of Electrical Engineering Yanshan University

Obvious resonance peak will be generated when parallel photovoltaic grid-connected inverters are connected to the Weak grid with high grid impedance, which seriously affects the stability of grid-connected operation of the photovoltaic system. To overcome the problems mentioned above, the mathematical model of the parallel photovoltaic inverters is established. Several factors including the impact of the reference current of the grid-connected inverter, the grid voltage interference and the current disturbance between the photovoltaic inverters in parallel with the grid-connected inverters are analyzed. The grid impedance and the LCL filter of the photovoltaic inverter system are found to be the key elements which lead to existence of resonance peak. This paper presents the branch voltage and current double feedback suppression method under the premise of not changing the topological structure of the photovoltaic inverter, which effectively handles the resonance peak, weakens the harmonic content of the grid current of the photovoltaic grid-connected inverter and the voltage at the point of common coupling, and improves the stability of the parallel operation of the photovoltaic grid-connected inverters in Weak grid. At last, the simulation model is established to verify the reliability of this suppression method.

关键词： Parallel photovoltaic grid-connected inverters Weak grid grid impedance resonance peak branch voltage and current double feedback.

Improved Model Predictive Direct Power Control of grid Side Converter in Weak grid Using Kalman Filter and DSOGI

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Chinese Journal of Electrical Engineering 2019年第4期5卷 22-32页

作者： Liang Chen Heng Nian Yunyang Xu College of Electrical Engineering Zhejiang UniversityHangzhou 310027China

When grid side converter(GSC)is connected to Weak grid,the small signal instability can happen due to terminal characteristics of the GSC that are incompatible with the grid impedance.This issue is not of wide concern in previous studies of direct power control(DPC)of GSC.By small signal analysis,it is shown that the impedance characteristic of the conventional direct power control GSC is not compatible with the inductive grid impedance in Weak grid due to its constant power load behavior.To solve the problem,instead of feeding the DPC controller with direct measured voltage and current,a Kalman filter(KF)is used to obtain filtered output current and a double second-order generalized integrator(DSOGI)is used to obtain filtered voltage at the point of common coupling(PCC).These strategies change the impedance characteristic of the GSC dramatically and make it suitable to operate in Weak grid where SCR is 2,while the rapid power response is preserved.The proposed strategy is verified through simulation and controller hardware in loop(CHIL)tests.

关键词： Direct power control Weak grid grid side converter stability

Voltage phase angle jump characteristic of DFIGs in case of Weak grid connection and grid fault

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Journal of Modern Power Systems and Clean Energy 2016年第2期4卷 256-264页

作者： Xinshou TIAN Gengyin LI Yongning CHI Weisheng WANG Haiyan TANG Xiang LI State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources(North China Electric Power University) Changping DistrictBeijing 102206China China Electric Power Research Institute Haidian DistrictBeijing 100192China

In the condition of connecting large scale doubly-fed induction generators (DFIGs) into Weak grid,the closely coupled interactions between wind generators and power grid becomes more severe.Some new fault characteristics including voltage phase angle jump will emerge,which will influence the power quality of power system.However,there are very few studies focusing on the mechanism of voltage phase angle jump under grid fault in a Weak grid with wind turbine integration.This paper focuses on the scientific issues and carries out mechanism studies from different aspects,including mathematical deduction,field data analysis and time domain simulation.Based on the analysis of transientcharacteristics of DFIGs during the grid fault,this paper points out that the change of terminal voltage phase angle in DFIGs is an electromagnetism transition process,which is different from conventional synchronous generator.Moreover,the impact on transient characteristics of voltage phase angle are revealed in terms of fault ride through(FRT) control strategies,control parameters of current inner-loop of rotor-side converter and grid strength.

关键词： DFIGs Weak grid Fault ride through Voltage phase angle jump

A Novel PLL Structure for Dynamic Stability Improvement of DFIG-based Wind Energy Generation Systems During Asymmetric LVRT

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Journal of Modern Power Systems and Clean Energy 2023年第4期11卷 1149-1164页

作者： Lei Guan Jun Yao State Key Laboratory of Power Transmission Equipment&System Security and New Technology School of Electrical EngineeringChongqing UniversityChongqing 400044China

The dynamic coupling effect,which is introduced by the dual-sequence phase-locked loops(PLLs)used in doublyfed induction generator(DFIG)based wind energy generation systems(WEGSs)during asymmetric low voltage ride-through(LVRT)in Weak grid,needs attention.In order to study this new dynamic coupling effect,an equivalent two-degree-of-freedom(2-DOF)spring damper particle model is used in this paper to develop a small-signal model for the dual-sequence PLLs.The dynamic interaction between the positive-sequence(PS)and negative-sequence(NS)PLLs is unveiled.Moreover,the impact of the dynamic coupling between the dual-sequence PLLs on the dynamic stability during the steady-state stage of an asymmetric fault is analyzed.The analysis results show that the dynamic coupling between the dual-sequence PLLs will cause drift in the frequency and damping for the PS and NS PLL modes.This will change the instability modal of the system and introduce the risk of dynamic instability.Hence,the effectiveness of existing control strategies for enhancing the dynamic stability will be decreased.Finally,a novel PLL structure is designed to improve the dynamic stability of the system during the steady-state stage of an asymmetric fault.The effectiveness of the proposed strategy is verified by simulations and experiments.

关键词： Doubly-fed induction generator(DFIG) phaselocked loop(PLL) dynamic stability dynamic coupling Weak grid asymmetric low-voltage ride through(LVRT)

Small-signal Stability Analysis and Improvement with Phase-shift Phase-locked Loop Based on Back Electromotive Force Observer for VSC-HVDC in Weak grids

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Journal of Modern Power Systems and Clean Energy 2023年第3期11卷 980-989页

作者： Yongqing Meng Haibo Wang Ziyue Duan Feng Jia Zhengchun Du Xiuli Wang State Key Laboratory of Electrical Insulation and Power Equipment Shaanxi Province Key Laboratory of Smart GridXi’an Jiaotong UniversityXi’an 710049China State grid Henan Electric Power Corporation Maintenance Company Zhengzhou 450000China IEEE

Voltage source converter based high-voltage direct current(VSC-HVDC)transmission technology has been extensively employed in power systems with a high penetration of renewable energy resources.However,connecting a voltage source converter(VSC)to an AC Weak grid may cause the converter system to become unstable.In this paper,a phase-shift phaselocked loop(PS-PLL)is proposed wherein a back electromotive force(BEMF)observer is added to the conventional phaselocked loop(PLL).The BEMF observer is used to observe the voltage of the infinite grid in the stationaryαβframe,which avoids the problem of inaccurate observations of the grid voltage in the dq frame that are caused by the output phase angle errors of the PLL.The VSC using the PS-PLL can operate as if it is facing a strong grid,thus enhancing the stability of the VSC-HVDC system.The proposed PS-PLL only needs to be properly modified on the basis of a traditional PLL,which makes it easy to implement.In addition,because it is difficult to obtain the exact impedance of the grid,the influence of shortcircuit ratio(SCR)estimation errors on the performance of the PS-PLL is also studied.The effectiveness of the proposed PSPLL is verified by the small-signal stability analysis and timedomain simulation.

关键词： Phase-locked loop(PLL) small-signal model stability improvement voltage source converter based high-voltage direct current(VSC-HVDC) Weak grid

Online Measurements of High-band grid Impedance Using Variable Frequency Carrier Based PWM Schemes

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CSEE Journal of Power and Energy Systems 2021年第1期7卷 128-139页

作者： Zheyu Miao Hao Tong Wenxi Yao Zhengyu Lu the College of Electrical Engineering Zhejiang UniversityHangzhou 310027China

grid-connected power electronic devices, such asvoltage-source inverters (VSIs), are increasingly installed in thegrid for the utilization of renewable energy sources. Under highpenetration, the Weak grid contains plentiful harmonics and complexgrid impedance values, which have deep impacts on systemstability as well as its control performance. Online impedancemeasurement is needed for impedance based adaptive controland analysis. This paper proposes a variable carrier frequencyPWM (VCFPWM) based online grid impedance measurementtechnology. This method broadens the available working rangein the high frequency band without increasing the low bandperturbations. Through good analysis and design of the carrierfrequency, the VCFPWM method injects sufficient harmonicenergy at specified high frequency points and guarantees lowTHDs at the same time. The detailed design considerations ofthe carrier frequency are clarified and the characteristics of theVCFPWM based excitations are analyzed in this paper. Finally,the simulation and experimental results based on a three-phasegrid-connected VSI are presented to verify the effectiveness ofthe proposed method.

关键词： Carrier frequency harmonic impedance measurement voltage-source inverters Weak grid

Robust Dichotomy Solution-Based Model Predictive Control for the grid-Connected Inverters with Disturbance Observer

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CES Transactions on Electrical Machines and Systems 2021年第2期5卷 81-89页

作者： Huanyue Liao Xin Zhang Zhijun Ma School of Electrical and Electronic Engineering Nanyang Technological University College of Electrical Engineering Zhejiang UniversityHangzhou 310027China Hangzhou Global Scientific and Technological Innovation Center Zhejiang UniversityHangzhou 310058China National Maglev Transportation Engineering R&D Center Tongji UniversityShanghaiChina

This paper proposes a robust dichotomy-based model predictive control(DS-MPC)with a fixed switching frequency for the grid-connected inverter(GCI).The proposed fast dichotomy algorithm can select and deduce the optimal voltage vector dynamically through the space vector plane.Therefore,the proposed DS-MPC strategy could ensure dynamic performance and steady-state performance as well.Also,the current control robustness can be improved through DS-MPC with disturbance observer(DO)based on the extended Kalman filter(EKF).The novelty of this control is that the current control with fast dynamic response can be realized in the Weak grid,even if the grid voltages are greatly distorted.Simulation and hardware experiments on the Weak grid validate the effectiveness of the proposed DS-MPC with the EKF observer approach.

关键词： Dichotomy solution-based model predictive control(DS-MPC) three-phase grid-connected inverter(GCI) Weak grid voltage vector dichotomy

Resonance Active Damping and PCC Voltage Quality Improvement of DFIG System Connected to Parallel Compensated grid

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Chinese Journal of Electrical Engineering 2018年第4期4卷 33-40页

作者： Yipeng Song Nian Heng Frede Blaabjerg Department of Energy Technology Aalborg UniversityAalborg East 9220Denmark College of Electrical Engineering Zhejiang UniversityHangzhou 310027China

Under the connection to a Weak grid,the Doubly Fed Induction Generator(DFIG)based wind power system has potential risk from two operational issues.The first issue is the High Frequency Resonance(HFR)mode due to the impedance interaction between the DFIG system and the Weak grid.In order to ensure safe and reliable operation of the DFIG system,it is necessary to implement effective active damping strategies to mitigate the HFR.The second issue is low order voltage harmonic distortion at the Point of Common Coupling(PCC),where consequently the performance of other grid-connected devices may deteriorate.It could be advantageous that the DFIG system is able to improve the voltage quality at PCC by eliminating the low order harmonic components.In this paper,both of the above mentioned DFIG operational characters,i.e.,active damping of HFR and the improvement of voltage quality at PCC,will be achieved by implementing advanced control strategies in the Rotor Side Converter(RSC)and the grid Side Converter(GSC)respectively.Simulations are provided to verify the proposed control strategies for DFIG system connected to a Weak grid.

关键词： DFIG system Weak grid High Frequency Resonance(HFR) harmonic distortion active damping