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.

An Improved Feedforward Control Method Considering PLL Dynamics to Improve Weak Grid Stability of Grid-Connected Inverters

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.

/pdf/optimal-design-of-controller-parameters-for-improving-the-421xn1pcg8.pdf

Optimal Design of Controller Parameters for Improving the Stability of MMC-HVDC for Wind Farm Integration

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.

An Improved Design of Current Controller for LCL -Type Grid-Connected Converter to Reduce Negative Effect of PLL in Weak Grid

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.

Sequence Impedance Modeling and Stability Comparative Analysis of Voltage-Controlled VSGs and Current-Controlled VSGs

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.

Improved Design of PLL Controller for LCL -Type Grid-Connected Converter in Weak Grid

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.

Impedance-Phased Dynamic Control Method for Grid-Connected Inverters in a Weak Grid

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.

Sequence Impedance Modeling and Analysis of MMC in Single-Star Configuration

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.

Impedance modeling of three-phase grid-connected inverters and analysis of interaction stability in grid-connected system

In certain regions and countries, the static synchronous compensator (STATCOM) is required in large-scale wind farms to provide reactive power support. Due to the limitations and complexity in distributed wind power systems, the STATCOM is considered as an effective and cheap alternative for impedance shaping to mitigate resonance. However, there is still a lack of quantitative stability analysis for practical large-scale wind farms and an optimization approach for the STATCOM impedance to achieve so. Therefore, based on the sequence impedance matrix models of the Type-IV wind turbine generator (WTG) and STATCOM for the network modeling, this article presents a sequence impedance network-based stability analysis for practical wind farms to accurately reveal the stability of wind farms and quantitatively evaluate the damping provided by the STATCOM. A typical wind farm in Western China is exemplified to demonstrate the proposed stability analysis. Then, an intelligent parameter design-based optimization approach for the STATCOM impedance is proposed, which uses the heuristic intelligence algorithm to solve the STATCOM controller parameters to obtain the optimal stability margin of the wind farm under the constraints of desired performance. Finally, a comparison with the understanding-of-models-based optimization method confirms the efficacy of the proposed impedance optimization approach.

Intelligent Parameter Design-Based Impedance Optimization of STATCOM to Mitigate Resonance in Wind Farms

The distributed generation system (DGS) located far away from load centres leads to the existence of long distance transmission lines and several transformer devices. Consequently, the equivalent impedance is introduced to a grid, which is called a weak grid and shows high impedance characteristic. A weak grid will affect the control loop stability of grid-connected inverters in DGS. To study the effects of a weak grid, the small signal model of single-phase LCL-type grid-connected inverters is first derived in this study. Then, the stability of grid-connected inverters caused by the grid impedance is analysed. Second, two common control schemes of passive and active damping are further studied. Then, the effects of the grid impedance on the damping coefficients under different damping options are analysed in quantity. A novel hybrid-damping control scheme is thereby proposed. Meanwhile, an adaptive control scheme of grid-connected inverters based on the hybrid damping is introduced in combination with the grid impedance online measurement, which ensures the stable performance of grid-connected inverters with different grid impedances. Finally, a prototype is built to verify the effectiveness of the proposed adaptive control scheme.

Yang Zhang

Papers

Impedance-Phased Dynamic Control Method for Grid-Connected Inverters in a Weak Grid

Sequence Impedance Modeling and Analysis of MMC in Single-Star Configuration

Impedance modeling of three-phase grid-connected inverters and analysis of interaction stability in grid-connected system

Intelligent Parameter Design-Based Impedance Optimization of STATCOM to Mitigate Resonance in Wind Farms

Hybrid damping adaptive control scheme for grid-connected inverters in a weak grid