This paper presents an improved virtual synchronous generator (VSG) control strategy of doubly-fed induction generator (DFIG) during symmetrical voltage fault. Based on VSG control, DFIG can behave like a conventional synchronous generator (SG), which can support frequency and voltage in grid-connected mode. Moreover, when microgrid disconnects from grid, DFIG can provide voltage and power for local loads. However, during symmetrical voltage fault, the transient components in DFIG cannot be controlled properly by VSG. To solve this problem, the air-gap flux feedback is designed in VSG control to accelerate the decay of transient components, and rotor current limiting method is also introduced in the article. In the proposed method, there is no need to switch the control structure or control targets. DFIG always works in the VSG control. Therefore, DFIG can operate in the standalone mode when the connection to grid is cut off. The decay time constant, the voltage control, current limiting control and parameter variation of the improved VSG control are analyzed in detail. Finally, the simulation and experimental results in both normal and fault grid condition are given to validate the effectiveness of proposed VSG control strategy.

This article presents the grid-forming control (GFC) for doubly-fed induction generator (DFIG)-based wind farm to enhance the stability of line-commutated converter (LCC) based high voltage direct current (HVDC) transmission system. GFC is a grid friendly control strategy to build or follow the grid voltage and provide inertia support for power grid. The GFC for DFIG can not only improve the stability of HVDC sending-terminal system, but also reduce the dependence of synchronous generator (SG) in HVDC sending-terminal system. Impedance analysis is used to analyze the stability of HVDC sending-terminal system. The impedance modeling process of the GFC for DFIG is introduced in detail. Based on the impedance analysis, the GFC for DFIG shows the better stability than the conventional vector control (VC) for DFIG in HVDC sending-terminal system. When the capacity of SG decreases and transmission line impedance increases, the GFC for DFIG can keep stable, while the VC for DFIG will lose the stability in HVDC sending-terminal system. Vector graph of impedance characteristic is applied to illustrate the mechanism of instability. The time domain simulations are presented to verify the impedance analysis. Moreover, the simulation in islanded condition is also given to illustrate the stability and effectiveness of the GFC for DFIG.

With the rapid growth of renewable energy and power electronic loads, voltage source converters (VSCs) have been commonly applied in different VSCs-based power systems, which also bring in stability issues due to their power electronic characteristics. The impedance-based stability theory and corresponding reshaping method can be used to analyze and solve system stability issue. However, dc stability issue in VSCs-based power systems becomes more complicated when considering ac/dc coupling relationship. This paper proposes an improved reshaping control strategy based on the detailed impedance model considering grid impedance and control delay. The influence of grid impedance, control delay and control parameters of VSC on dc stability are further analyzed to identify the cause of potential resonance risk. The proposed impedance reshaping method considers the trade-off between stability improvement and control performance, and compensates the impacts of grid impedance and control delay at the same time. The validity of stability analysis and reshaping method is further verified through the simulation and experimental results.

The primary objective of the grid connected DFIG-DC system is to achieve the accurate control of stator frequency and active power. In this letter, a simplified power control method is proposed by just controlling the magnitude of the rotor current vector, which can avoid the stator voltage and current sensors. The stator active power is calculated by the product of dc voltage and dc current. The stator frequency is simply controlled by the rotating speed of the rotor current vector, which is achieved through a given rotating frame. Furthermore, the parameter dependence and dc sampling offset problems can be eliminated because the voltage model or current model, which are usually used for acquiring stator frequency and stator flux angle, can be avoided. Therefore, the robustness and stability of the stator frequency and power control can be improved. Finally, experiments based on a 1 kW DFIG-DC setup are carried out to verify the proposed method.

This paper presents the analysis and mitigation of sub-synchronous resonance (SSR) for doubly fed induction generators (DFIG) under virtual synchronous generator (VSG) control, based on impedance methods. VSGs are considered to have grid-supporting ability and good stability in inductance-based weak grids, and are implemented in renewable power generations, including DFIG systems. However, stability analyses of VSGs for DFIG connecting with series capacitor compensation are absent. Therefore, this paper focuses on the analysis and mitigation of SSR for DFIG under VSG control. Impedance modeling of DFIG systems is used to analyze SSR stability. Based on impedance analysis, the influence of VSG control parameters and the configuration of damping factor of reactive power are discussed. Next, a parameter configuration method to mitigate SSR is proposed. Finally, time-domain simulation and fast fourier transform (FFT) results are given to validate the correctness and effectiveness of the impedance model and parameter configuration methods.