This work evaluates the response of DFIG-based wind turbine under voltage dips caused by transformer energizing. The transformer energizing is part of the normal operation of the power system and it can cause even-harmonic distortion, which could impact the operation of the power converters on the wind turbine. Previous works only considered voltage dips caused by electrical faults, then other causes should be understood to guarantee sufficient fault-right through of wind power installations. In order to address this study, voltage dips measurements with distinct magnitude, harmonic distortion and voltage unbalance are applied to a DFIG model. The impacts on the rotor current, on the DC-link voltage, and on the DFIG active power are analyzed. 

This work compares the response of DFIG-based wind turbines under voltage dips caused by transformer-energizing and caused by an asymmetrical fault. Voltage dips with similar retained voltage and duration and the two distinct causes are applied to an equivalent model of a DFIG-based wind park. The dips are different in terms of harmonic content and unbalance during the recovery after the dip. Due to its slow recovery, the transformer-energizing dip results in longer fluctuation in torque, longer overshoot in DC-link voltage, and longer recovery to the pre-dip active power. Also, due to its harmonic content, the harmonic distortion in the rotor and stator current is higher for the transformer-energizing dip. As the impact is different between the transformer-energizing dip and the fault-caused dip, it is recommended to consider both events separately during low-voltage-ride-through studies.

Due to the increasing penetration of wind power, grid codes require that the wind farms remain connected to the grid during voltage dips. The harmonic distortion might change during the short-term reduction in the voltage magnitude. Although the distortion only occurs during the short period of the voltage dip, high levels of harmonic might cause mal-operation of the protective devices in a wind power plant. The main negative influence that harmonics can have on wind power plant protection is that it can act unnecessarily. This work analysis the changes in the harmonic content in the voltages and currents during dips in a DFIG-based wind power plant. The results showed that the total harmonic distortion for the voltages in the collector system and at the terminals of the wind turbines increases during asymmetrical dips. The currents were affected for both symmetrical and asymmetrical dips. The third and seventh harmonics were the voltage components that most increased. For the currents, the increase was in the third harmonic and interharmonics.

This study presents an optimum control scheme to maximize the output voltage level number of the cascaded H-bridge dynamic voltage restorer (CHB-DVR). The relationship between the modulation index and the output voltage level number is analyzed in detail. The compensation reference voltage value is adjusted with the voltage drop depth to obtain high-quality output voltage with an acceptable total harmonic distortion. Thus, the modulation index remains within a certain range and thus meets the requirements of the maximum level number technique (MLNT). In addition, an improvement method based on the MLNT is proposed to achieve minimum active power absorption from a direct current link of the CHB-DVR. The traditional in-phase compensation and optimum control strategies are implemented to analyze the output voltage quality for verifying the feasibility of the proposed approach. Simulation and experimental results show the effectiveness of the proposed control scheme.

In this paper, an improved control scheme is proposed to improve the voltage quality of sensitive loads using dynamic voltage restorer. The existing control strategies either put emphasis on the optimal control during steady operation stage of compensation or correct the phase angle jump in the initial stage of compensation. In current researches, the impact of phase jump characteristic of voltage sag on the load side after voltage sag recoveries is widely ignored, further, there are still drawbacks in existing energy self-recovery strategies of DVR. Therefore, to improve the overall voltage compensation time while correcting the phase jump and accelerate the energy recovery of dc link side, this paper aims to 1) Propose the strategies to minimum active power consumption during voltage compensation stage and maximum active power absorption during energy self-recovery stage. 2) Deliver smooth transition in dynamic process to ensure the flexible switching between two stages. Theoretical analysis of proposed strategy is been validated through simulation and experimental results.