Search

Impedance-based approachis generally studied for stability analysis of the power electronic converter-based system. Considering multiple-frequency coupling effects, in this article, we establish the harmonic transfer function (HTF) based αβ-frame impedance models of droop inverters based islanded microgrid with unbalanced loads. Therein, the HTF-based αβ-impedance modeling methods of unbalanced loads, as well as droop inverters in parallel, are proposed. Afterward, the HTF-based αβ-frame impedance models are equivalent to single-input single-output (SISO) impedances for simpler impedance measurement and stability analysis, preserving the αβ-frame coupling and multiple-frequency coupling characteristics. Finally, the experimental measurement results verify the accuracy of the αβ-frame SISO impedances. Besides, the stability analysis based on the SISO impedance models accurately reveals low-frequency oscillations of the islanded microgrid with unbalanced loads, further verifying the effectiveness of the SISO impedance models. [ABSTRACT FROM AUTHOR]

For the distributed generation units (DGs) using virtual synchronous generator (VSG) control algorithm connected to the islanded microgrid with high penetration of nonlinear load, there is always a trade-off between harmonics sharing accuracy and voltage quality when using conventional harmonics sharing methods. To address this issue, an impedance-based adaptively reshaping method is proposed in this paper, which is based on harmonic current feedforward compensation control and impedance reshaping factor adaptive control, assigning a variable harmonic impedance reshaping factor for each VSG to improve the controllability of VSG harmonic impedance, thus simultaneously enhancing nonlinear load sharing and voltage quality. Moreover, a droop-based methodology for impedance reshaping factor tuning is presented to adaptively reshape harmonic impedance for each VSG without acquiring the line parameters in advance. The proposed method also enjoys the advantage of avoiding introducing additional virtual harmonic impedance, and thus it is immune from the inverter closed-loop gain throughout the dominant harmonic frequencies. Furthermore, the stability analysis and the key control parameter design are discussed in detail. Finally, the effectiveness of the proposed method is verified by simulation and experimental results. [ABSTRACT FROM AUTHOR]

Virtual synchronous machine (VSM) based high voltage dc systems enhance the inertia of the power system. However, the dynamic interactions between the VSM-based rectifier station, the inverter station, and the grid could induce the system oscillation, which has been investigated in this article. At first, the hybrid ac–dc impedance models of the VSMs considering the coupling between the ac and dc dynamics are established. Then, the relationships between the dc impedance, dq-frame impedance, and the hybrid ac–dc impedance are presented. It is found that the dc impedance of the VSM-based inverter station and the d-d channel impedance of the dq-frame impedance of the rectifier station behave as negative resistors in the low-frequency range. Moreover, a five-dimensional impedance stability criterion based on the hybrid ac–dc impedance and generalized inverse Nyquist criterion is proposed to assess the system stability. The analysis results show that the low-frequency oscillation occurs when the grid short-circuit ratio of the rectifier station is small. Finally, the simulation and experimental results verify the impedance models and the stability criterion. [ABSTRACT FROM AUTHOR]

Interactions of identical inverters in weak grids have received wide attentions. However, in a practical wind farm, parameters of grid-side inverters and lengths of collecting lines are not completely the same. In this case, a multi-inverter system is not symmetric, and thus the interaction mechanism could be more complicated than that of identical inverters. To investigate the interaction mechanism of differently parameterized inverters, this paper proposes a clustering-based modeling method, in which inverters with identical parameters are classified as one group. The clustering-based model allows for efficient stability assessment using three low-order transfer function matrices, instead of a full-order dynamic model. The root locus analysis is applied in the clustering-based model for stability analysis. Results show that the interconnection of distinct groups of inverters deteriorates common current stability, but has no impact on interactive current stability. The simulation and rtlab-based experimental results validate the effectiveness of the proposed model and the interaction mechanism of differently parameterized inverters. [ABSTRACT FROM AUTHOR]

The high penetration of renewable energy generation (REG) systems influences the dynamics of power systems. Accurate transient simulations and dynamic models are indispensable for the research on the dynamic behavior of REG systems. A large-scale REG system consisting of numerous grid-connected inverters consumes a large amount of computation time, so a dynamic aggregated model is needed for efficient simulation. The aggregated model established by coherent equivalence fits the detailed model well, and thus it is widely used in the aggregation of large-scale dynamic systems. Presently, the key challenge of the application of the coherent-equivalence method is to identify inverters that have similar dynamic characteristics. The Hamilton principle expressed by Hamilton's action has a simple form and indicates the comprehensive dynamic characteristics of inverters. Thus, this paper proposes a coherent-equivalence method of inverters based on Hamilton's action. The proposed method is applied to a wind farm of 16 permanent magnet synchronous generator-based wind turbines for demonstration. Compared with the existing equivalence method without considering coherency, the proposed method shows advantages in effectiveness and accuracy. [ABSTRACT FROM AUTHOR]

Converter-control-based generators exhibit different dynamic characteristics compared with traditional synchronous generators, and thus affect the oscillation modes and small-signal stability of power systems in a different manner. Simplified representation of a great number of grid-connected inverters is indispensable for efficient and accurate dynamic analysis of power systems integrated with large-scale renewable energy generation. This paper proposes a novel dynamic aggregation modeling method of grid-connected inverters using coherency-based equivalence, and validates the applicability of the aggregated model for small-signal stability analysis. Modal analysis and dynamic simulation are conducted on the detailed model and aggregated models of the test system. The frequency-domain results and the time-domain results from PSCAD/EMTDC both verify the adequacy of the coherent equivalence practice in the small-signal rotor angle stability analysis. [ABSTRACT FROM AUTHOR]