The grid forming strategy can be applied in the wind turbine system and offer better grid supporting service than traditional grid following strategy. A popular method is to implement the grid forming strategy in the grid side converter (GSC) system while the machine side converter (MSC) system controls the DC-link voltage. Compared to the grid following wind turbine (GFL-WT), the role of GSC and MSC are exchanged in the grid forming wind turbine (GFM-WT), therefore the electrical torque response of GFM-WT is different from the one of GFL-WT and the drivetrain oscillation caused by the incompatibility between electrical torque response and drivetrain model needs to be analyzed. In this paper, the drivetrain oscillation of the virtual synchronous generator (VSG) based type-IV wind turbine is investigated. The mechanism of drivetrain oscillation is explained by the linearized system in frequency domain. Then, a damping control strategy is proposed to suppress the drivetrain oscillation and its effectiveness is verified by time domain simulation.

With the increasing penetration of renewable energy, wind turbines based on doubly fed induction generators (DFIG) are becoming more and more important in the power system. The impedance reshaping control strategy based on the virtual impedance method can be utilized to solve the stability problem of DFIG-grid interconnected system under the weak grid. However, the stability issues caused by phase locked loop (PLL) is more complicated due to the frequency coupling characteristic. This paper presents an improved control strategy of DFIG system based on a symmetrical PLL, in which the frequency coupling characteristic can be avoided and the system can be simplified as a SISO system that facilitates the design of virtual impedance. The influence of symmetrical PLL on the operation of DFIG system is studied, and an improved impedance reshaping control strategy is proposed. Simulation and experimental results are conducted to verify the effectiveness of the proposed impedance reshaping control strategy.

Due to the limited overcurrent capability of semiconductor components, the grid-forming (GFM) inverters require additional overcurrent protection schemes. This article analyzes the interaction between the circular current limiter and different types of virtual impedance from the perspective of transient stability. The established unified equivalent circuit model reveals the mechanism of current limitation by the circular limiter, which is based on both the feedforward-virtual impedance (FVI) method and embedded-virtual impedance (EVI) method. Although with the same current-limiting effects, the power-angle characteristics of these two methods are different. The dominant factor influencing transient stability is demonstrated by the analysis of power-angle trajectory. This study observes that the EVI method exhibits superior transient stability compared to the FVI method under the same parameters. Further, a parameter-switching approach based on the EVI method is proposed, which aims to achieve an extended fault ride-through duration. Finally, theoretical analysis is validated through simulations and control hardware-in-the-loop experiments.

The loss of synchronization (LOS) is a main issue for dynamic behavior of grid-following converter (GFC) under inductive power grid. This letter reveals the underlying mechanism of insufficient damping or even negative damping during grid disturbance, and analyzes how the initial speed of angle-difference between PLL output and grid voltage affects the synchronization stability. Then a synchronization stability enhancement scheme to add a positive damping and a reverse angular frequency during transient process is proposed in this letter.

The synchronization stability of grid-following converter has attracted extensive attention. It has been reported that the transient synchronization behavior during a grid fault is dominated by the phase-locked loop (PLL), while the current loop is always approximated as a unity gain. Nevertheless, this letter notices that the reactive current dynamics will also influence the synchronization stability under ultra-weak grid, which increases the risk of losing synchronization, and causes the PLL output frequency to be outside the limitation. Accordingly, a refined fourth-order model is proposed to reveal the above underlying mechanism and quantify this coupling effect.

This paper proposes a modified self-synchronized synchronverter for unbalanced power grids. Small signal analysis of the conventional synchronverter shows that its stability margin around 50 Hz is very limited. Thus, power ripples will be caused at the frequency of 50 Hz. Filter- based current feeding loops are adopted in the conventional synchronverter in order to enhance its stability and eliminate power ripples. In addition, the characteristics of the conventional synchronverter in unbalanced power grids are analyzed, and an improved strategy using a resonant controller is proposed to restrain the current harmonics and power ripples. The parameter design is also studied for the proposed synchronverter. Experimental studies prove that the proposed strategy can achieve precise self-synchronization when the grid voltage is unbalanced, and the power-control performance is also improved significantly.