Yuancan Xu

Transient angle instability and fault overcurrent are two serious problems affecting the safe operation of the virtual synchronous generator during grid faults. However, the two aspects are mostly treated as two independent problems in existing studies. In this article, taking both transient angle instability and fault overcurrent into consideration, a two-stage transient control collaboratively utilizing virtual impedance (VI) and switching PI is put forward. In stage one, the VI is switched to the boundary value determined by the iteration algorithm to avoid fault overcurrent. In stage two, the proportional loop or integral loop in switching PI, which can embed virtual damping or virtual torque into the traditional swing equation, respectively, is activated to prohibit two types of transient angle instability induced by VI. By adaptively tuning VI and activating different loops, fault current limitation and transient angle stability are guaranteed simultaneously under different fault degrees. Furthermore, inertia support is maintained, and transient performance is improved. The proposed control is also capable for a multi-inverter system. Simulation and experimental tests validate the effectiveness of the proposed control.

Similar to the traditional synchronous generator (SG), a drawback of the virtual synchronous generator (VSG) is the active power oscillation, especially in the system with parallel VSGs. In this paper, a decentralized transient damping (DTD) control strategy is proposed to suppress active power oscillation during load fluctuations in the parallel VSGs system, which is based on embedding the transient electromagnetic power compensation and transient damping power compensation (TDPCP) control into the original VSG active power controller, providing two new degrees of freedom for the flexible regulation of the system damping ratio without frequency measurement. In this way, the damping ratio of the parallel VSGs system can be adjusted more freely without affecting the steady-state characteristics of the system. Therefore, this flexible transient damping characteristic can facilitate the suppression of active power oscillation under load fluctuations. Moreover, the proposed method is immune to the parameter mismatch of the parallel VSGs. The design of key parameters and stability analysis are presented. Finally, the feasibility of the proposed control strategy is verified through the simulation and experimental results.

The impedance or admittance characteristics of the grid-connected inverter play an important role in its stability analysis. The traditional sequence-admittance measurement methods of the grid-connected inverter mostly rely on special admittance measurement equipment or adjacent grid-connected inverters for disturbance injection. Because of high hardware cost or inconvenience, the abovementioned methods are not suitable for price-conscious testers to some extent. To address the abovementioned issues, this article proposes a sequence-admittance measurement method of the grid-connected inverter with its control system disturbance. First, the sequence-admittance model of the measured grid-connected inverter when small-disturbance signal is superimposed on its modulation signal is deduced and the system equivalent circuit model is built. Then, the flow chart of the proposed sequence-admittance measurement method is designed. According to the injected small-disturbance signal, the output voltage and inductor current of the grid-connected inverter, the sequence-admittance characteristics in sub/supersynchronous frequency area can be obtained. The measurement process does not need the cooperation of external disturbance source, which saves costs and improves the measurement convenience. Finally, experiments verify the effectiveness of the proposed sequence-admittance measurement method.

Due to the multifrequency coupling effect ( m -FCE), precise modeling and effective stability assessment of an asymmetric grid-following converter (GFC) based ac system are complex to achieve. Traditional small-signal modeling methods and recently reported order-reduction methods could establish multi-input and multi-output (MIMO) and equivalent single-input and single-output (SISO) models to characterize m -FCE effectively. However, revealing the physical implications of coupling components and the generation mechanism of m -FCE are still challenging. Meanwhile, although the amplitude and phase dynamics have been considered in the existing modeling, a concrete conclusion illustrating these dynamics’ impact on m -FCE is still missing. To fill the gaps, this article thoroughly considers the impacts of amplitude and phase dynamics and finds that phase dynamics form additional positive feedback loops to enhance m -FCE. Based on this cognition, an accurate complex circuit modeling framework is proposed. It reveals that m -FCE is generated by the admittance mismatching between conjugated responses and ’chain reaction’ among coupling components. The proposed method has two merits: 1) it is a SISO modeling approach and helps reduce analysis complexity; 2) its graphical form helps provide a deep physical insight into m -FCE. Simulations and experiments verified the correctness of the proposed method.

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.

In islanded microgrids with complex feeder networks, distributed generation units (DGs) and loads can be freely connected to any node, leading to changes in the feeder impedance between nodes and the microgrid structure. However, traditional harmonic sharing control methods encounter difficulties with microgrids with complex topologies. Additionally, existing techniques essentially require constructing additional harmonic virtual impedance, possibly resulting in conflict between voltage quality and harmonic sharing effectiveness. To address this issue, this paper proposes an adaptive harmonic impedance reshaping method based on a consensus algorithm. The proposed method adaptively reshapes the inverter harmonic impedance by introducing harmonic current feedforward compensation control and impedance reshaping factor adaptive control, which can achieve accurate harmonic sharing while greatly improving the voltage quality of the microgrid. The proposed method avoids constructing additional harmonic virtual impedance and can be applied to microgrids with complex feeder networks and variable structures without acquiring prior microgrid configuration knowledge. Moreover, the proposed method is a fully distributed control, where the microgrid central controller (MGCC) is not required. Furthermore, the control parameter design method based on modeling and stability analysis is discussed in detail. Finally, simulations and experiments verify the feasibility and effectiveness of the proposed method.

In a single-phase grid-connected inverter, because the constructed orthogonal signals are not strictly orthogonal in broadband and the control of dq-axis is asymmetric, when the voltage disturbance with frequency of f p is injected in point of common coupling (PCC), not only the response current with frequency of f p will be generated, but also the response currents with frequencies of f p –2 f1 and f p +2 f1 will be coupled. In order to accurately establish the broadband impedance model of the single-phase grid-connected inverter, the frequency coupling mechanism is analyzed in this paper. Considering the influence of the constructed orthogonal signal, current loop, phase-locked loop (PLL) and frequency coupling, a broadband impedance model of the single-phase grid-connected inverter is established. Its broadband impedance characteristics are analyzed. The simulation results verify the accuracy of the established model.