With the increasing penetration of wind power, reliable and cost-effective wind energy production is of more and more importance. The doubly-fed induction generator based turbine system is widely used and dominates the wind market. In this paper, an analytical approach to assess reliability for power capacitors, both the DC-link capacitor bank and AC-side filter capacitor bank, is presented considering the annual mission profile. Based on the electrical behavior at various loading conditions, the lifecycle of the single power capacitor can be predicted through its electro-thermal stresses. This percentile lifetime can be translated to the Weibull lifetime distribution of the power capacitor by considering the parameter uncertainties and tolerance variations. Thereafter, a reliability block diagram is used to bridge the reliability curves from the component-level of the individual capacitor to the system-level of the capacitor bank. A case study of a 2 MW wind turbine shows that the lifecycle is significantly reduced from the individual capacitor to the capacitor bank, where the DC-link capacitor bank dominates the lifetime consumption. Furthermore, the electrical stresses of the power capacitors are experimentally verified at a down-scaled 7.5 kW prototype.

Sub-synchronous oscillations in doubly-fed induction generator (DFIG)-based series compensated power systems are mainly caused by sub-synchronous control interaction (SSCI). SSCI is the most recently found type of sub-synchronous resonances. In this paper, SSCI is elaborated and investigated by performing eigenvalue analysis on a mathematically modelled DFIG system. The occurrence of SSCI is observed and the results of eigenvalue analysis are validated through a down-scaled 7.5-kW experimental setup of a grid-connected DFIG. Based on the analysis, the proportional control parameters of the rotor-side converter (RSC) are found to be very sensitive towards the sub-synchronous modes of the system. The results obtained from both the simulation and the experimental analysis show that if the sensitive proportional parameters of the RSC are tuned properly, then the DFIG system can become immune to the SSCI for any level of series compensation.

When operating in a micro or weak grid which has a relatively large network impedance, the doubly fed induction generator (DFIG)-based wind power generation system is prone to suffer high-frequency resonance due to the impedance interaction between the DFIG system and the parallel compensated network (series RL + shunt C). In order to improve the performance of the DFIG system as well as other units and loads connected to the weak grid, the high-frequency resonance needs to be effectively damped. In this paper, the proposed active damping control strategy is able to implement effective damping either in the rotor-side converter or in the grid-side converter, through the introduction of virtual positive capacitor or virtual negative inductor to reshape the DFIG system impedance and mitigate the high-frequency resonance. A detailed theoretical explanation on the virtual positive capacitor or virtual negative inductor has been given, and their parameters are also optimally designed. The proposed DFIG system damping control strategy has been validated by experimental results.

As the wind power technology develops, the Doubly Fed Induction Generator (DFIG) based wind power system, when connected to a weak network with large impedance, may suffer resonances, i.e., Sub- Synchronous Resonance (SSR) or High Frequency Resonance (HFR) when connected to the series or parallel compensated weak network. Besides these two resonances, a Middle Frequency Resonance (MFR) between 200 Hz and 800 Hz may appear when the Phase Locked Loop (PLL) with fast control dynamics is applied. In order to analyze the MFR, the DFIG system impedance considering the PLL is studied based on the Vector Oriented Control (VOC) strategy in Rotor Side Converter (RSC) and Grid Side Converter (GSC). On the basis of the established impedance modeling of the DFIG system, it is found that the PLL with fast control dynamics may result in the occurrence of MFR due to a decreasing phase margin. The simulation results of both a 7.5 kW small scale DFIG system and a 2 MW large scale DFIG system are provided to validate the theoretical analysis of the MFR.

During the past two decades, the doubly fed induction generator (DFIG) based wind farm has been under rapid growth, and the increasing wind power penetration has been seen. Practically, these wind farms are connected to the three-phase ac grid through long transmission cable which can be modeled as several Π units. The impedance of this cable cannot be neglected and requires careful investigation due to its long distance. As a result, the impedance interaction between the DFIG-based wind farm and the long cable is inevitable, and may produce high-frequency resonance (HFR) in the wind farm. This paper discusses the HFR of the large-scale DFIG-based wind farm connected to the long cable. Several influencing factors, including 1) the length of the cable, 2) the output active power, and 3) the rotor speed, are investigated. The transformer leakage inductances in the transmission system are taken into consideration when investigating the HFR. Simulation validations using MATLAB/Simulink have been conducted to verify the theoretical analysis.

As the wind power generation develops, the Doubly Fed Induction Generator (DFIG) based wind power system are more and more likely to operate in the emerging weak network rather than the conventional stiff network. Due to the comparatively large impedance of the weak network than the stiff grid, the DFIG system may be subject to the resonances due to the impedance interaction between the DFIG system and the weak network. Especially, when connected to a series π sections weak network, the Multiple High Frequency Resonances (MHFR) may occur and require careful studies. The impedance modeling of the DFIG system and the series π sections weak network is firstly demonstrated in this paper. Then, due to the multiple magnitude peaks of the series π sections of the weak network, the MHFR will be produced and can be theoretically explained based on the impedance modeling results. For the purpose of mitigating the MHFR, an active damping strategy which introduces a virtual impedance, including a phase leading compensation unit and a virtual positive resistance, is proposed and demonstrated. Simulations are conducted to validate the DFIG system MHFR as well as the proposed active damping strategy.