Impedance Stability Analysis Research Articles

ADRC‐based symmetric phase‐locked loop structure for improving low‐frequency stability of grid‐connected inverters

AbstractWith a small short‐circuit ratio (SCR), the grid‐connected inverter is prone to low‐frequency oscillation instability due to the dynamic interaction between the phase‐locked loop (PLL) and the weak grid. To this end, an active‐disturbance‐rejection‐controller‐based symmetric PLL (ADRC‐based SyPLL) is proposed in this article to simplify the system modelling and improve the low‐frequency stability of the inverter system. Specifically, the ADRC technique is applied to phase‐regulated feedback control of a PLL, where an extended state observer (ESO) is used to estimate the lumped disturbance of the controlled system. A control loop symmetric to the q‐axis is further designed on the d‐axis of the PLL, and the d‐axis information is then added at the reference generation side to eliminate the coupled frequency term in the small‐signal model. A simple and accurate single‐input single‐output (SISO) model is thus obtained. The impedance stability analysis for grid‐connected inverters shows that the proposed ADRC‐based SyPLL can significantly improve the low‐frequency stability margin of the system compared with the conventional PI‐regulated PLL. Finally, the experimental results validate the effectiveness and superiority of the proposed method.

A data‐driven identification method for impedance stability analysis of inverter‐based resources

AbstractObtaining inverter controller information may be a premise for seeking its dynamic behaviour. But accurate knowledge of such information would be unrealistic for real functioning inverter‐interfaced generators (IIGs), which hinders the stability analysis of the IIG. A new data‐driven impedance identification method is proposed for stability analysis, which involves an improved sparse identification algorithm as an ancillary function within the system identification framework. It contains mainly two design stages. First, the transform basis matrix (TBM) is devised systematically as a prior knowledge library to contain the possibly existing control structures. In the second stage, a sparse identification algorithm is reformulated in order to extract the relevant structures in TBM while obtaining controller parameters. The authors demonstrate that the sparse vector between the TBM and output signal is closely related to the controller structure. The effectiveness of the proposed method is verified on grid‐connected inverters based on droop control and virtual synchronous machine control.

Analysis and Suppression of High-Frequency Resonance for Offshore Wind Power Grid-Connected Converter Considering Cable Capacitance Effect

Large-scale offshore wind farms have become the development trend in wind power generation. Long submarine cables are used to collect electrical energy in the collection networks of offshore wind farms. However, the distributed capacitance of cables cannot be ignored, which makes the impedance of the collection network complex and changeable. It is common to encounter high-frequency harmonic resonance (HFHR) problems when cables interact with wind turbine generators (WTGs). The HFHR may threaten the safe and stable operation of wind turbines. In order to solve this problem, firstly, the impedance of a collection network was constructed. Furthermore, the wind farm collection network was divided into the equivalent wind turbine subsystem (EWTS) and the remaining equivalent wind farm subsystem (REWFS). Then, the mechanism of HFHR was revealed based on the impedance stability analysis method. The effects of cable parameters, the number of connected WTG, and the grid impedance on the HFHR of the interconnected system were also analyzed. Finally, the hybrid damping control method, combining active damping control (ADC) and passive damping control (PDC), was proposed to suppress the resonance. A simulation and experiment were performed to verify the effectiveness of the analysis results and proposed control method.

Improved Fractional-order Damping Method for Voltage-controlled DFIG System Under Weak Grid

When a doubly-fed induction generator (DFIG) is connected to a weak grid, the coupling between the grid and the DFIG itself will increase, which will cause stability problems. It is difficult to maintain the tracking accuracy and ro-bustness of the phase-locked loop (PLL) in the weak grid, and the risk of instability of the current-controlled DFIG (CC-DFIG) system will increase. In this paper, a new type of voltage-controlled DFIG (VC-DFIG) mode is adopted, which is a grid-forming structure that can independently support the voltage and frequency with a certain adaptability in the weak grid. A small-signal impedance model of the VC-DFIG system is also established. The impedance of DFIG inevitably generates coupling with the grid impedance in the weak grid, especially in parallel compensation grids, and results in resonance. On the basis of the VC-DFIG, impedance stability analysis is performed to study the influences of the control structure and short-circuit ratio. Then, a feedforward damping method is proposed to modify the impedance of the VC-DFIG system at resonance frequencies. The proposed fractional order damping is utilized, which can enhance the robustness and rapidity of resonance suppression under parameter fluctuations. Finally, the experimental results are presented to validate the effectiveness of the proposed control strategy.

DQ Impedance Stability Analysis for the Power-Controlled Grid-Connected Inverter

For a grid-connected inverter requiring the ac voltage magnitude and the active power control, both vector control and power synchronization control can be applied. The stability comparison based on the dq impedance stability analysis between both control are carried out via three factors including the grid impedance, the inner current loop and the virtual impedance. The dq impedances of the inverter based on both control are derived. The determinant of the impedance ratio matrix is used for the stability analysis. The bode plot of the grid impedance and the inverter impedance are present to assist the stability analysis and explain their interactions. It is found that increasing the grid impedance and the cut-off frequency of the current loop stabilize the inverter with the power synchronization control, which is converse to the vector control. Furthermore, the inverter with the power synchronization control may suffer the instabilities when connecting to a strong grid. The virtual inductor and resistor are proposed to enhance the stability for the vector control and the power synchronization control respectively. The simulation validation using Matlab/Simulink is performed.

Impedance stability analysis and impedance remodelling of matrix converter grid‐connected system

Matrix converter has become one of the research hotspots of power electronic converter due to its excellent input and output characteristics and adjustable power factor. To study the stability of the matrix converter grid-connected system, the harmonic linearisation method is adopted to model the positive and negative sequence output impedance models of the matrix converter grid-connected system. The stability of the system is analysed by judging whether the plot of the ratio of the input impedance to the positive and negative sequence output impedance satisfies the Nyquist stability criterion. Also, an impedance remodelling method based on the positive sequence impedance is proposed to improve the stability of the system. Finally, the simulation verification is carried out by MATLAB/Simulink. The simulation results prove the correctness of the theoretical analysis and the feasibility of the impedance remodelling method.

Coupling Influence on the dq Impedance Stability Analysis for the Three-Phase Grid-Connected Inverter

The dq impedance stability analysis for a grid-connected current-control inverter is based on the impedance ratio matrix. However, the coupled matrix brings difficulties in deriving its eigenvalues for the analysis based on the general Nyquist criterion. If the couplings are ignored for simplification, unacceptable errors will be present in the analysis. In this paper, the influence of the couplings on the dq impedance stability analysis is studied. To take the couplings into account simply, the determinant-based impedance stability analysis is used. The mechanism between the determinant of the impedance-ratio matrix and the inverter stability is unveiled. Compared to the eigenvalues-based analysis, only one determinant rather than two eigenvalue s-function is required for the stability analysis. One Nyquist plot or pole map can be applied to the determinant to check the right-half-plane poles. The accuracy of the determinant-based stability analysis is also checked by comparing with the state-space stability analysis method. For the stability analysis, the coupling influence on the current control, the phase-locked loop, and the grid impedance are studied. The errors can be 10% in the stability analysis if the couplings are ignored.

A Novel Stability Analysis Approach for the Gird-connected Wind Generator Based on Impedance Ratio Criterion

A novel stability analysis approach is proposed for a gird-connected Wind generator system based on the impedance criterion, which can consider the dynamic characteristics of permanent magnet synchronous generator (PMSG). Firstly, the stability analysis criteria for the grid connected power system with electronic converters is deduced based on the impedance stability analysis principle. Secondly, the small signal model of a PMSG with consideration of the dynamic characteristics of PMSG is established by the state-space equation of PMSG, and the output impedance of a grid-connected PMSG system is derived. On this basis, the stability analysis method for the gird-connected PMSG is obtained. Numerical simulation are performed to validate the proposed control method.