Improved Control Strategy for DFIG-based Wind Energy Conversion System during Grid Voltage Disturbances

Research output: Book/ReportPh.D. thesis

Abstract

For the DFIG-based WECS, due to the direct connection between the stator and grid, the DFIG is sensitive to grid voltage disturbances. The effects from the power grid may produce the oscillations of the stator active and reactive power, oscillatory electromagnetic torque, and distorted stator and rotor current. The degraded performances of the DFIG may reduce the life time of the drive train, affect the system stability, and even destroy the power converters. To improve the system performance, stability and reliability, it must improve the control strategy for the DFIG experiencing non-ideal grid voltage. Before improving the performances of the DFIG, it should analyze and find the real intrinsic reason that degrades the DFIG performance. Therefore, the DFIG performances during the stator voltage consecutive variations are studied firstly. It demonstrates that the consecutive variations of the stator voltage can cause the transient stator flux, and then the transient stator flux may be enlarged due to the effects of the initial value. The amplitude of the transient flux is determined by both the instant and depth of stator voltage variation, and the decaying characteristic of the transient flux is affected by the control strategy of the rotor current and stator time constant. The stator transient flux is harmful to the DFIG, because it may cause oscillations of the electromagnetic torque, stator or rotor current, DC-link voltage, and stator active and reactive power. On the basis of stator flux performances and its effects on the DFIG, the improved FOVC and high stability vector-based DPC control are proposed, respectively, to fast decay the transient flux and to improve dynamic responses. By using Bode diagram and root locus, the improved FOVC strategy is designed to fast decay the transient flux and then obtain constant electromagnetic torque, and stator active and reactive power as well. In addition, the stator and rotor current fast becomes sinusoidal. For the vectorbased DPC, a SM controller is used to control the stator active and reactive power. Based on the SM controller, a small signal model is used to analyze the stability of the DFIG system, and then an improved vector-based DPC is proposed to control the DFIG and increase the stability margin. Under the improved vector-based DPC, the dynamic performances of the DFIG are improved and the dynamic period is reduced, during the stator voltage disturbances. Then, during the grid faults, the active and passive damping strategies are proposed to help the DFIG to fulfill the LVRT requirement, respectively.
The active damping strategy is based on the virtual damping flux to suppress the rotor current with a constant electromagnetic torque during grid faults. Therefore, the virtual damping flux based strategy not only can help the DFIG achieve the LVRT requirement, but also can reduce the mechanical stress on the drive train. On the other hand, on the basis of the decaying characteristic of the stator flux, the passive damping strategy using the series resistance in stator terminal helps the DFIG to suppress the rotor current and obtain improved performances. In addition, the passive damping strategy is easily implemented. Further, the simulation and experimental results both clearly validate that the virtual damping flux based active damping strategy and the stator series resistance based passive damping strategy can help the DFIG to fulfill the LVRT requirement, and improve the DFIG performances. Besides the previous active and passive damping strategies, the modified power converter and DFIG configurations are also investigated. Firstly, according to the passive generator side converter configuration, a DBRbased DFIG system, which can completely decouple the stator and the grid, and can be used for the DC-network, is presented. By connecting a full-scale power converter between the DC-link and the grid, the DFIG is easy to fulfill the LVRT requirement, because the generator is isolated from the grid by the DC-link. As the stator voltage is clamped by the DBR and is the square wave, it is not easy to directly measure the stator frequency.
Additionally, in practice, the existence of the parameter variation and control error leads to that the stator active and reactive power cannot be completely decoupled. Based on this, a stator frequency and phase control strategy is proposed to regulate the stator frequency and decouple the active and reactive power. The proposed strategy doesn’t need the PLL, which simplifies the control system and also reduce the effects of the PLL on system performance. Based on the simulation and experimental results, it clearly verifies that the proposed strategy can accurately control the stator phase angle and completely decouple the stator active and reactive power. Besides isolating the stator from the grid, the DFIG voltage can be compensated by the DVR. However, the configuration generally requires large capacity of the DVR system, which will increases the system cost. Therefore, an improved control strategy for the DVR-based DFIG WECS to reduce the capacity of the DVR is proposed and is studied in detail. The proposed strategy bases on the stator flux performances during the stator voltage consecutive variations. By the proposed strategy, the minimum output voltage amplitude and activation period of the DVR can be obtained. Therefore, the required capacity of the DVR can be significantly reduced.
The simulation results validate the correctness and feasibility of the theoretical analysis.
Lastly, the thesis is concluded and future work is also discussed.
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Details

For the DFIG-based WECS, due to the direct connection between the stator and grid, the DFIG is sensitive to grid voltage disturbances. The effects from the power grid may produce the oscillations of the stator active and reactive power, oscillatory electromagnetic torque, and distorted stator and rotor current. The degraded performances of the DFIG may reduce the life time of the drive train, affect the system stability, and even destroy the power converters. To improve the system performance, stability and reliability, it must improve the control strategy for the DFIG experiencing non-ideal grid voltage. Before improving the performances of the DFIG, it should analyze and find the real intrinsic reason that degrades the DFIG performance. Therefore, the DFIG performances during the stator voltage consecutive variations are studied firstly. It demonstrates that the consecutive variations of the stator voltage can cause the transient stator flux, and then the transient stator flux may be enlarged due to the effects of the initial value. The amplitude of the transient flux is determined by both the instant and depth of stator voltage variation, and the decaying characteristic of the transient flux is affected by the control strategy of the rotor current and stator time constant. The stator transient flux is harmful to the DFIG, because it may cause oscillations of the electromagnetic torque, stator or rotor current, DC-link voltage, and stator active and reactive power. On the basis of stator flux performances and its effects on the DFIG, the improved FOVC and high stability vector-based DPC control are proposed, respectively, to fast decay the transient flux and to improve dynamic responses. By using Bode diagram and root locus, the improved FOVC strategy is designed to fast decay the transient flux and then obtain constant electromagnetic torque, and stator active and reactive power as well. In addition, the stator and rotor current fast becomes sinusoidal. For the vectorbased DPC, a SM controller is used to control the stator active and reactive power. Based on the SM controller, a small signal model is used to analyze the stability of the DFIG system, and then an improved vector-based DPC is proposed to control the DFIG and increase the stability margin. Under the improved vector-based DPC, the dynamic performances of the DFIG are improved and the dynamic period is reduced, during the stator voltage disturbances. Then, during the grid faults, the active and passive damping strategies are proposed to help the DFIG to fulfill the LVRT requirement, respectively.
The active damping strategy is based on the virtual damping flux to suppress the rotor current with a constant electromagnetic torque during grid faults. Therefore, the virtual damping flux based strategy not only can help the DFIG achieve the LVRT requirement, but also can reduce the mechanical stress on the drive train. On the other hand, on the basis of the decaying characteristic of the stator flux, the passive damping strategy using the series resistance in stator terminal helps the DFIG to suppress the rotor current and obtain improved performances. In addition, the passive damping strategy is easily implemented. Further, the simulation and experimental results both clearly validate that the virtual damping flux based active damping strategy and the stator series resistance based passive damping strategy can help the DFIG to fulfill the LVRT requirement, and improve the DFIG performances. Besides the previous active and passive damping strategies, the modified power converter and DFIG configurations are also investigated. Firstly, according to the passive generator side converter configuration, a DBRbased DFIG system, which can completely decouple the stator and the grid, and can be used for the DC-network, is presented. By connecting a full-scale power converter between the DC-link and the grid, the DFIG is easy to fulfill the LVRT requirement, because the generator is isolated from the grid by the DC-link. As the stator voltage is clamped by the DBR and is the square wave, it is not easy to directly measure the stator frequency.
Additionally, in practice, the existence of the parameter variation and control error leads to that the stator active and reactive power cannot be completely decoupled. Based on this, a stator frequency and phase control strategy is proposed to regulate the stator frequency and decouple the active and reactive power. The proposed strategy doesn’t need the PLL, which simplifies the control system and also reduce the effects of the PLL on system performance. Based on the simulation and experimental results, it clearly verifies that the proposed strategy can accurately control the stator phase angle and completely decouple the stator active and reactive power. Besides isolating the stator from the grid, the DFIG voltage can be compensated by the DVR. However, the configuration generally requires large capacity of the DVR system, which will increases the system cost. Therefore, an improved control strategy for the DVR-based DFIG WECS to reduce the capacity of the DVR is proposed and is studied in detail. The proposed strategy bases on the stator flux performances during the stator voltage consecutive variations. By the proposed strategy, the minimum output voltage amplitude and activation period of the DVR can be obtained. Therefore, the required capacity of the DVR can be significantly reduced.
The simulation results validate the correctness and feasibility of the theoretical analysis.
Lastly, the thesis is concluded and future work is also discussed.
Original languageEnglish
PublisherDepartment of Energy Technology, Aalborg University
Number of pages172
StatePublished - 2015
Publication categoryResearch

Bibliographical note

No public assess as Rongwu Zhu will be publishing some of the articles from the PhD

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