Virtual synchronous generator (VSG) control strategies have been widely researched and applied in grid-forming converters for utility grid inertia support under high level penetrations of renewable energies. Traditional VSG control encounters coupling issues between active and reactive power loops, especially in low voltage resistive feeder line conditions. This power coupling effect degrades not only the effectiveness of inertia support during transients, but also the system stability. In this article, the coupling mechanism between active and reactive power loops is analyzed and a full-state feedback control method is proposed to release the coupling issue in traditional VSG control. In the proposed control, the state feedback matrix is designed to eliminate the power coupling between active and reactive power control loops, and fully retain the capability for active and reactive inertia response. Disturbance injection is applied to avoid dependence on line parameters. Root locus analysis shows that the decoupled power control endows system with better stability and damping performance. Hardware experimental results indicate that the power decoupling VSG significantly releases the coupling between active and reactive power, contributing to better system stability and providing improved transient power inertia response to power grid.

A two-stage cascaded converter is formed by connecting two subconverters in series. Output impedance of one subconverter will, therefore, interact with input impedance of the other subconverter. Such interaction may affect the system dynamics and stability, especially when one subconverter is constant-power-controlled, and hence, has different impedance characteristics when its power reverses. To lessen such constant-power effects, a control scheme that can coordinate impedance behaviors of the two subconverters is proposed. The idea is to reshape the low-frequency negative impedance of the constant-power subconverter to a positive resistance needed for neutralizing stability differences between the two directions of power flow. A smooth power reversal can thus be ensured, as verified through simulations and experiments performed with a scaled-down 400-W laboratory prototype.

This paper presents an intelligent stability prediction method for high-frequency oscillation of grid-connected inverter considering time-varying parameters of power grid and inverter. A data-based analysis method based on radial basis function neural network (RBFNN) is first developed to identify and predict time-varying parameters of grid and inverter. Then, the oscillation characteristic represented by physical model is combined to predict real-time stability of grid-connected inverter. Furthermore, the stability prediction criterion is developed according to real-time parameter identification and physical model. Simulation and experimental results are given to validate the proposed intelligent stability prediction method. The proposed method is able to predict time-varying stability region and stability margin of grid-connected inverter considering parameters variation, which thus improves the self-learning capability and adaptivity of grid-connected inverter system.

In order to improve the phenomenon that a traditional phase-locked loop based on a double second-order generalized integrator (DSOGI-PLL) cannot track signal amplitude and phase accurately when the input signal contains DC components and high-order harmonics, the structure of a second-order generalized integrator-quadrature signals generator (SOGI-QSG) is modified. The paper establishes the impedance model considering the DSOGI-PLL structure of the inductor-capacitor-inductor-type (LCL-type) inverter grid-connected system adopting current control measured from the grid terminal in alternating current side, introducing voltage feedback control to enhance the stability of the system. Meanwhile, analyzing the influence of parameters on impedance according to the impedance model established preferable design parameters. The improvement in SOGI-QSG structure is good for PLL to lock the grid voltage phase more accurately and the retrofitting in control strategy based on the impedance is able to uplift the inverter output impedance phase which is conducive to system stability by increasing the phase margin of the system. The simulation in Matlab/Simulink is carried out to verify the effectiveness of the proposed control strategy.

The well-functioning of power factor correction (PFC) circuit plays a crucial role for the diode rectifier application. Traditional control strategies for boost-PFC, predominantly designed under the system's quasi-steady state mode, inherently possess nonlinear traits, which may lead to pronounced harmonics in grid-side current and electromagnetic interference. Such repercussions gravely compromise the total harmonic distortion and the overall power factor (PF). Addressing this challenge, this paper introduces a novel boost-PFC control methodology, rooted in the virtual DQ coordinate transformation. By formulating an equivalent grid-tied inverter control framework for the PFC circuit under virtual DQ coordinate system, it equates the dc inductor current to the ac grid side counterpart, facilitating a linearized control for the nonlinear PFC system. Empirical findings underscore the efficacy of the proposed method, highlighting its capability to improve the output voltage and PF performance of boost-PFC. Meanwhile the proposed control also provides improvements in mitigating grid-side current harmonics under gird distortions. The theoretical analysis and effectiveness of proposed control method have been validated by both MATLAB/Simulink simulations and experiment hardware test.

When facing the current sensor failures, the inverter system will be shut down or run in open-loop mode, and the controllability will be severely degraded. Therefore, the development of the current sensorless control strategy for the grid-connected inverter is worthy of academic attention. In order to solve this problem, a current observation method is proposed in this article, which is enhanced by the virtual impedance and operation data based error correction. The method combines the control scheme variables of dc side voltage and ac side grid voltage to observe the ac side inductor current. The output of current sensors can be replaced in the current closed-loop control, contributing to the current sensorless control. The research results show that the current observation method can effectively track the real-time current and the virtual impedance implementation ensures the accuracy of this current observation. Under the impedance refined current observations, dynamic variations in inductor current can be timely reflected, and the stable operation of the current sensorless closed-loop control can be ensured. Finally, the correctness and effectiveness of the proposed control strategy have been verified by MATLAB / Simulink simulation and experimental results.