In order to realize the grid-friendly wind turbine system (WTS), applying of the virtual synchronous generator (VSG) control strategy to WTS has become a trend of research, and it is also required to analyze the grid-connected operation characteristics of the VSG controlled WTS. In this paper, the impedance modeling and impedance-based stability analysis of the VSG controlled type-IV WTS are carried out. Different from existing impedance modeling method that the WTS is modeled by a high dimensional impedance model with complex expressions, a model to describe the shaping effects of dc-link dynamics is proposed, and combined with the space vector diagram to analyze the system impedance characteristics. It is found that the impedance of the VSG controlled type-IV can be capacitive below the fundamental frequency, which is inherited from its GSC impedance and the capacitive characteristics has been weakened by the influence of dc-link dynamics. Instable risk has also been found between the VSG controlled type-IV WTS and the connected weak grid, and the instability can be avoided if the capacitive characteristics of the VSG controlled type-IV WTS are weakened by reshaping its GSC impedance or by enhancing the dc-link dynamics. Experiments are also carried out as verifications.

In this article, we analyze the negative impact of phase-locked loop (PLL) on the stability of the doubly fed induction generators system. The small-signal disturbance of PLL mainly affects the coordinate transformation of the rotor current and introduces a negative resistance within PLL bandwidth. Two impedance reshaping methods are carried out to weaken this negative resistance. A virtual impedance in the conventional current control is added to the first reshaping method. The second reshaping method completely removes the coordinate transformation of the rotor current by switching the control target from current to power. The difference between these two reshaping methods is analyzed based on the equivalent single-input single-output impedance model. The experimental results validate the effectiveness of these two reshaping methods and the superiority of the second reshaping method at a lower short-circuit ratio.

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.

It has been found in existing research works that the dc-link dynamics are significant internal dynamics in the type-IV wind turbine system, which can shape system characteristics and influence system stability. If dc-link dynamics are weakly damped, this dynamic behavior may reduce system damping and worsen system stability. Considering that dc-link dynamics are determined by the interaction effects of controllers and different control structures can be adopted to regulate dc-link voltage in the type-IV wind turbine system, analytical models are needed to accurately evaluate and analyze dc-link dynamics under different control structures, which are lacked in existing publications. In this article, the mechanism of dc-link dynamics in the system is described at first. A single-in-single-out indicator function is proposed to model the dc-link dynamics, based on which a linearized state-space model of dc-link dynamics is obtained. Then, the eigenvalue sensitivity matrix is applied to determine the influencing factors of dc-link dynamics and possible causes of weakly damped dc-link dynamics. DC-link dynamics under different control structures are compared. The influenced system characteristics can also be modeled by the proposed model of dc-link dynamics and the system stability with weakly damped dc-link dynamics is also investigated with experiments based on control-hardware-in-loop platform.

Impedance measurement is an effective method to obtain the impedance characteristic of renewable power generation equipment (RPGE) for stability analysis. Frequency-coupling characteristic of RPGE will bring about the difficulty to measuring the impedance in a wide frequency band based on broadband signals. In order to guarantee the accuracy of the broadband impedance measurement considering the frequency–coupling characteristic, this article explores the method to accurately measure the frequency–coupling characteristic of the RPGE. The signal-to-noise ratio (SNR) issue in view of the measurement of the frequency–coupling characteristic is also investigated. Accordingly, an evaluation method for measurement accuracy is proposed. The evaluation results will be adopted to guide the design for the perturbation amplitudes. The redesigned multi-sine signal will be used in another group of measurements so as to achieve a targeted accuracy level. Finally, experiments based on the Typhoon control-hardware-in-loop (CHIL) platform are carried out to verify the reliability of the proposed evaluation method and the effectiveness of the proposed design method for the perturbation amplitudes.

Direct power control (DPC) can reduce the complexity of a controller and enhance the dynamic response for doubly fed induction generators (DFIG) system. The high-frequency resonance (HFR) issues will be analyzed for the DFIG system based on DPC under inductive weak grid in this article. Based on the simplified high-frequency impedance model of the DFIG system based on DPC and the Bode diagram of equivalent single-input single-output impedance, it is found that a strong frequency coupling related to the components of system delay in off-diagonal admittance will cause the high-frequency phase to change periodically for the DFIG system. Therefore, the DFIG system will show capacitive or even negative resistance to introduce potential risk of HFR at high frequency. The stability of a DFIG system will be further deteriorated as the system delay increases. An impedance reshaping control strategy is proposed to reduce the degree of frequency coupling at high frequency for eliminating the negative effects of system delay, so that HFR can be suppressed. The experimental results validate the availability of the proposed reshaping control strategy.