Virtual synchronous generator (VSG) control technique is widely used for the grid-friendly integration of renewable energy. However, when VSG is attached to a weak grid, it is prone to voltage and frequency instability, which aggravates the voltage quality and frequency quality at the point of common coupling (PCC), and in turn, pollutes the supplying quality for the loads. On the one hand, focusing on voltage stability, this paper proposes the impedance modeling for VSG in the synchronously inertial reference coordinates (SIRC) to measure the voltage stability of grid-tied VSG and identify the interaction between VSG and the weak power grid. Studies show that weak grid strength can lead to a wide-band oscillation of VSG. On the other hand, focusing on frequency instability, the motion equation has been put forward to illustrate the oscillation and dynamic deviation of frequency. The relationship between synchronizing ability of VSG and grid stiffness is discussed. Besides, this paper proposes an inertia-damping-strengthened control method to improve frequency stability. Finally, the theoretical analysis is compared with simulations and experiments.

In recent years, the virtual synchronous generator (VSG) concept has been widely studied to integrate renewable energy sources. However, instability occurs due to the implementation of the dc-link voltage controllers under the weak grid, and its mechanism remains unclear, which is investigated in this article. At first, the wideband dq -frame impedance models of the VSGs with the dc-link voltage controllers for two cases are established. Then, the stability analyses of the VSGs are compared based on these impedance models. It is revealed that the interaction dynamics between the dc-link voltage loop and the active loop lead to the negative resistor behavior of the q–q channel impedances for both VSGs, which induces the emerging oscillations of the system in a weak grid. Besides, as a useful design guideline, the parameter design of the VSGs is proposed to enhance the system stability. Finally, the experimental results obtained from a 100-kW prototype system show good agreement with simulated results, validating the impedance models and theoretical analysis.

In this paper, the structure, modeling, parameter design, and control method are proposed for a new hybrid structure of a static var compensator in parallel with a hybrid active power filter (SVC//HAPF). The SVC part of the SVC//HAPF is used to dynamically compensate the reactive power. And, the HAPF part is used to provide harmonic power and small amount of reactive power compensation. Due to the large fundamental voltage drop on coupling the LC part, the active inverter part of the SVC//HAPF has low voltage rating. Meanwhile, the parallel-connected SVC acts as a current divider to reduce the active inverter current. Therefore, the proposed SVC//HAPF shows the great promise in compensating harmonic current and wide-range reactive power with a low (both) voltage and current rating active inverter part. To show the advantages of the proposed SVC//HAPF, simulation comparisons among the active power filter (APF), HAPF, SVC in series with HAPF (SVC-HAPF), and the proposed SVC//HAPF are provided. Finally, experimental results based on the laboratory-scaled hardware prototype are given to show the validity of the SVC//HAPF.

This paper proposes a new type DC/AC inverter named: hybrid-coupling grid-connected inverter (HGCI) for Photovoltaic (PV) active power generation with power quality conditioning, which consists of a full-bridge three-phase DC/AC inverter coupling to the power grid in series with a thyristor controlled LC (TCLC) filter. Compared with the conventional inductive-coupling grid-connected inverter (IGCI) and capacitive-coupling grid-connected inverter (CGCI), the proposed HGCI has distinct characteristics of wide operational range and low DC-link operating voltage. Based on these prominent characteristics, the system cost and operational cost can be reduced. Moreover, it can transfer the active power and compensate reactive power, unbalanced power and harmonic power simultaneously. In this paper, the analysis of the structure, parameter design and control method of the HGCI are proposed and presented. Finally, simulation and experimental results are provided to verify the effectiveness and performance of the proposed HGCI in comparison with the IGCI and CGCI.

The respective advantages of AC and DC microgrids lead to the blooming development of the hybrid AC/DC microgrid, which consists of AC and DC microgrid tied by an interlinking converter. A new structure of hybrid coupled interlinking converter (HCIC), which composed of a converter in series with a static VAR compensator (SVC), is proposed for hybrid AC and low voltage DC (LVDC) microgrids. Moreover, a new droop control method is correspondingly developed for the proposed HCIC. Its fundamental active and reactive power flow in the hybrid AC/DC microgrid is controlled by P-δ and Q-P-V-α droop control for converter part of the HCIC, and P-Q-α droop control is proposed for SVC part of the HCIC. Furthermore, a fast harmonic control is given based on a decoupled control strategy to improve the filtering characteristics of SVC part of the HCIC, suppress the harmonic power, and/or inject active power. Finally, simulation and experimental results are provided to verify the proposed HCIC for hybrid AC/LVDC microgrids has the ability of good power flow control and power quality compensation, which not only meets the power flow requirements with low rating system, but also greatly promotes the development of more robust hybrid AC/DC microgrids.

Electric vehicles (EVs) that offer grid services using the V2G concept essentially require an interface that allows for directional power flow. This paper presents a wireless bidirectional grid power interface for EVs that facilitates power flow between the grid, EV and homes with nonlinear loads. The proposed wireless vehicle-grid-home power interface (WVGH-PI) uses the grid side low frequency to DC converter, used in typical wireless bidirectional chargers only to facilitate the two-way energy flow, to improve power quality by compensating for the harmonic and reactive power of nonlinear household loads. To improve the overall efficiency of the interface, an adaptive DC-link voltage controller is also proposed. Operation of the proposed wireless interface is investigated under dynamic conditions as well as in different modes. Simulation and experimental results obtained from a 1kW prototype are presented, benchmarking against conventional control, to demonstrate the validity of the proposed concept.