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Xin Chen

Bio: Xin Chen is an academic researcher from Nanjing University of Aeronautics and Astronautics. The author has contributed to research in topics: Electrical impedance & Output impedance. The author has an hindex of 6, co-authored 28 publications receiving 263 citations.

Papers
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Journal ArticleDOI
TL;DR: In this paper, a study of the stability of grid-connected inverters with high grid impedance based on impedance analysis is presented, where the effects of the PLL loop and the digital control delays on the output impedance characteristics have been taken into account.
Abstract: A power distribution grid exhibits the characteristics of a weak grid owing to the existence of scattered high-power distributed power-generation devices. The grid impedance affects the robust stability of grid-connected inverters, leading to harmonic resonance, or even instability in the system. Therefore, a study of the stability of grid-connected inverters with high grid impedance based on impedance analysis is presented in this paper. The output impedance modeling of an LCL -type single-phase grid-connected inverter is derived, where the effects of the PLL loop and the digital control delays on the output impedance characteristics have been taken into account. To enhance the stability of grid-connected inverters with different grid impedance, a novel impedance-phased compensation control strategy is proposed by increasing the phase margin of the grid-connected inverters. Specifically, a detailed implementation and parameter design of the impedance-phased compensation control method is depicted. Finally, an impedance-phased dynamic control scheme combined with online grid impedance measurement is introduced and also verified by the experiment results.

225 citations

Journal ArticleDOI
TL;DR: This paper presents sequence impedance modeling and analysis of modular multilevel converters (MMCs) in a single-star configuration for static VAR compensation (STATCOM) application to understand the similarities and differences in the impedance characteristics of the two configurations as well as the effects on stability.
Abstract: This paper presents sequence impedance modeling and analysis of modular multilevel converters (MMCs) in a single-star configuration for static VAR compensation (STATCOM) application. Prior work on impedance modeling of MMC is limited to the double-star configuration for high-voltage dc transmission applications. Compared to that, single-star configuration uses only a single arm of switching modules in each phase, eliminating the second-harmonic circulating current and the need for the associated circulating current control. On the other hand, the lack of connection to a stiff dc bus makes it essential to include module capacitor voltage control in the model. A special form of the multi-harmonic linearization method is applied to take advantage of the simpler harmonic spectra and to reduce the complexity of the resulting models. The developed models are verified by simulation and experiment, respectively, and used to understand the similarities and differences in the impedance characteristics of the two configurations as well as the effects on stability. A representative wind power plant is presented as an example to demonstrate the application of STATCOM impedance models on the system stability analysis.

32 citations

Journal ArticleDOI
Zheng Wei, Jie Chen, Xin Chen, Chunying Gong, Yi Fan 
TL;DR: In this paper, the saddle pulsewidth modulation (PWM) was obtained by injecting third harmonic current into sampled three-phase input currents, which can not only improve the utilisation of DC bus voltage and the efficiency of converter without sacrificing the advantages of the conventional OCC scheme, such as no phase-locked loop and constant switching frequency, but also is beneficial to suppress the ripple in the DClink voltage.
Abstract: The saddle pulse-width modulation (PWM) that can be obtained by injecting third harmonic current into sampled three-phase input currents is proposed to introduce in the conventional one-cycle control (OCC) strategy-based three-phase PWMrectifier on this study. In addition, for the three-level boost-type neutral-point-clamped structure rectifiers such as three-phase Vienna rectifier, the fluctuation in the neutral-point-potential (NPP) is analysed under modified OCC strategy. The proposed control scheme can not only improve the utilisation of DC bus voltage and the efficiency of converter without sacrificing the advantages of the conventional OCC scheme, such as no phase-locked loop and constant switching frequency, but also is beneficial to suppress the ripple in the DC-link voltage. Moreover, the neutral-point voltage loop is added in the proposed OCC scheme, and the neutral-point voltage controller is employed for controlling the unbalance DC voltage in the NPP which caused by the non-uniformity of parameters of components. The validity of the theoretical analysis and feasibility of the modified OCC strategy are verified by system simulation and experimental results based on a 5-kW prototype.

21 citations

Proceedings ArticleDOI
22 May 2016
TL;DR: In this article, the frequency characteristic of phase-locked loop (PLL) was analyzed, and a design approach of PLL regulator parameters was proposed to make a theoretical analysis and experiment validation of the interaction stability of the grid connected system.
Abstract: In a distributed generation system, the stability of grid-connected inverters is directly related to the reliable operation of the grid-connected system. The impedance-based analysis method can be employed to effectively study the interaction stability between grid-connected inverters and grid, which means that it is necessary to obtain the impedance modeling of grid-connected inverters for the analysis of impedance stability. Based on the three-phase LCL-type grid-connected inverter, the harmonic linearization method was adopted in this paper to analyze the frequency characteristic of Phase-Locked Loop (PLL), and a design approach of PLL regulator parameters was proposed. Meanwhile, the impacts of the factors that include PLL and digital control delay on the impedance characteristic were considered, and the positive-sequence and negative-sequence impedance model of the grid-connected inverter was built by combining the harmonic linearization and symmetrical component methods. Then, the frequency characteristic of PLL was verified by the simulation results and the output impedance model of the grid-connected inverter was verified by the experimental results, which effectively proved the correctness of the theoretical analysis. Finally, based on the output impedance model of the grid-connected inverter, the impedance-based analysis method was adopted to make a theoretical analysis and experiment validation of the interaction stability of the grid-connected system.

20 citations

Journal ArticleDOI
TL;DR: The optimized design method of the PLL is proposed from the perspective of optimizing the inverter impedance characteristics to improve the system stability and confirm the effectiveness of the optimize design method.

18 citations


Cited by
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Journal ArticleDOI
TL;DR: It can be demonstrated that the proposed method can enhance the stability of grid-connected inverters under weak grid conditions and reduce the impact of PLL perturbations on grid-connecting current and low-order harmonics of the network current can be suppressed effectively.
Abstract: Phase-locked loop (PLL) is commonly used for three-phase grid-connected inverters to obtain the information of grid synchronization, and PLL dynamics are the key factors for stable operation of the inverters. Under weak grid conditions, the coupling between PLL and grid impedance can result in harmonic resonance, or even instability in the system. In this paper, the effect of PLL dynamics and grid impedance on the stability of three-phase grid-connected inverters is studied with the d – q impedance model. Besides, the variable transfer relationship is used to analyze the influence of PLL perturbations on output current under weak grid conditions. To suppress low-order harmonics of the network current caused by PLL perturbations under weak grid conditions, a novel feedforward control method is proposed to compensate PLL perturbations and revise the output impedance, where the operation of the inverter and PLL dynamics have been taken into account in the design process. By analyzing the impedance characteristic of the system, it can be demonstrated that the proposed method can enhance the stability of grid-connected inverters under weak grid conditions and reduce the impact of PLL perturbations on grid-connecting current. The experimental results indicate that the low-order harmonics of the network current can be suppressed effectively, which verifies the analysis.

170 citations

Journal ArticleDOI
TL;DR: An optimal design method for controller parameters is proposed in this paper in order to guarantee the small-signal stability of the interconnected system from a system point of view and time-domain simulations validate the effectiveness of the theoretical analysis and the proposed ideal design method.
Abstract: A subsynchronous oscillation (SSO) phenomenon has been observed in a modular multilevel converter-based high-voltage dc (MMC-HVDC) transmission system for wind farm integration in the real world, which is independent of the type of wind turbine generator. This kind of oscillation appears different from those in doubly fed induction generator-based wind farm with series-compensation line or wind farm integration through two-level voltage-source converter-HVDC transmission system, because the internal dynamics of the MMC may have significant impact on the oscillation. By far, however, very few papers have reported it. In this paper, the generation mechanism of the SSO phenomenon in an MMC-HVDC transmission system for wind farm integration is revealed from an impedance point of view. The harmonic state-space modeling method is applied to model the multifrequency behavior of the MMC, based on which, the ac-side small-signal impedance of the MMC is analytically derived according to harmonic linearization theory. As a general rule, the controller parameters of the wind power inverter and the HVDC converter are designed separately, to meet the performance requirements of the single converter under ideal conditions, but this practice does not guarantee the stability of the interconnected system. Therefore, an optimal design method for controller parameters is proposed in this paper in order to guarantee the small-signal stability of the interconnected system from a system point of view. Finally, time-domain simulations validate the effectiveness of the theoretical analysis and the proposed optimal design method.

130 citations

Journal ArticleDOI
TL;DR: An improved design of current controller, i.e., proportional–integral controller and capacitor-current-feedback active damping, is proposed to reduce the negative effect of PLL on current control and has a strong robustness against wide-range variation of grid impedance.
Abstract: For three-phase LCL -type grid-connected converter, when it is attached to weak grid, current control interacts with phase-locked loop (PLL) via point of common coupling voltage. Consequently, PLL dynamic might deteriorate grid current control and even result in system instability. However, the conventional design method of current controller neglects the impact of PLL, and therefore, it is hard for the current controller to mitigate the negative effect of PLL. In this paper, an improved design of current controller, i.e., proportional–integral controller and capacitor-current-feedback active damping, is proposed to reduce the negative effect of PLL on current control. First, a small-signal impedance model is developed to analyze the impact of PLL on current control. Then, the effect of current controller parameters on the converter output impedance is analyzed, and a design guideline to improve the current controller parameters is presented. With the improved parameters, in precondition of satisfying system stability margin under both stiff and weak grid, the negative effect of PLL on current control can be effectively mitigated without employing additional control strategies. Furthermore, the current control has a strong robustness against wide-range variation of grid impedance. Finally, the experiment demonstrates the effectiveness of the proposed design method.

114 citations

Journal ArticleDOI
TL;DR: The voltage-controlled VSG is more suitable than the current-controlledVSG for grid-connected renewable energy generation in an ultraweak grid from the point of view of system stability and experiment results validate the stability analysis.
Abstract: The premise that the virtual synchronous generator (VSG) can actively support the weak grid is that the VSG can operate stably. In this paper, the small-signal sequence impedance models of the voltage-controlled VSG and the current-controlled VSG are built and the sequence impedance characteristics of these two types of VSGs are compared and analyzed. The sequence impedance of the voltage-controlled VSG, being consistent with the grid impedance characteristics, is generally inductive. By contrast, the sequence impedance of the current-controlled VSG is mainly negative impedance in the middle- and high-frequency area and the impedance amplitude is quite low. Based on the built sequence impedance models and the impedance stability criterion, the influence of the weakness of the grid on the stability of these two types of VSG grid-connected system is analyzed. The results of stability analysis show that the current-controlled VSG is prone to instability while the voltage-controlled VSG remains stable. Therefore, the voltage-controlled VSG is more suitable than the current-controlled VSG for grid-connected renewable energy generation in an ultraweak grid from the point of view of system stability. Finally, experiment results validate the stability analysis.

113 citations

Journal ArticleDOI
TL;DR: An improved design of PLL controller parameters is proposed to mitigate the negative effect of P LL on the current control in weak grid, and not only the dynamic and static response performance of Pll independent system can be maintained, but also the negative influence of Poll dynamic on theCurrent control can be effectively reduced inWeak grid.
Abstract: When LCL -type converter is attached to weak grid, its current control and phase-locked loop (PLL) will interact with each other, via the point of common coupling voltage. Unfortunately, the conventional PLL controller design methods are mainly for PLL independent systems, regardless of the aforesaid interaction. As a consequence, PLL dynamic might deteriorate the grid current control and even leads to instability problem. For this issue, this article proposes an improved design of PLL controller parameters to mitigate the negative effect of PLL on the current control. First, the small-signal model of the current control system considering PLL effect is established, and the system stability with the conventional PLL controller design method is analyzed. Then, an improved design of PLL controller parameters is proposed, and the design guideline is given in detail. With the method, not only the dynamic and static response performance of PLL independent system can be maintained, but also the negative influence of PLL dynamic on the current control can be effectively reduced in weak grid. Moreover, the grid current control has a strong robustness to the grid impedance variation. Finally, the proposed method is validated by the simulation and experiment.

109 citations