Rongwu Zhu

Fault Detection and Localization Method for Modular Multilevel Converters

The modular multilevel converter (MMC) is attractive for medium- or high-power applications because of the advantages of its high modularity, availability, and high power quality. However, reliability is one of the most important issues for MMCs which are made of large number of power electronic submodules (SMs). This paper proposed an effective fault detection and localization method for MMCs. An MMC fault can be detected by comparing the measured state variables and the estimated state variables with a Kalman filter. The fault localization is based on the failure characteristics of the SM in the MMC. The proposed method can be implemented with less computational intensity and complexity, even in case that multiple SM faults occur in a short time interval. The proposed method is not only implemented in simulations with professional tool PSCAD/EMTDC, but also verified with a down-scale MMC prototype controlled by a real-time digital signal controller in the laboratory. The results confirm the effectiveness of the proposed method.

Virtual Damping Flux-Based LVRT Control for DFIG-Based Wind Turbine

This paper proposes a virtual damping flux-based low-voltage ride through (LVRT) control strategy for a doubly fed induction generator (DFIG)-based wind turbine. During the transient states of grid voltage drop and recovery, the proposed virtual damping flux-based strategy can suppress rotor current with a smooth electromagnetic torque. During steady-state faults, a negative sequence current compensation strategy is adopted to smooth the electromagnetic torque and reactive power for asymmetrical grid faults, while the conventional vector control is used to inject reactive power into the grid to support grid voltage for symmetrical grid faults. The effectiveness of the proposed strategies is examined by the simulation with a 2-MW DFIG in MATLAB/Simulink and verified by the experimental results from a scaled-down 7.5-kW DFIG controlled by a DSPACE1006. In addition, the impacts of the magnetic nonlinearity characteristics of a practical DFIG are investigated under asymmetrical grid faults. Although the magnetic nonlinearity characteristics degrade the control effects, the proposed strategies can still improve the DFIG performances during asymmetrical grid faults. The results clearly demonstrate that the proposed strategies can effectively improve the DFIG transient behavior and achieve LVRT performances.

Fault-Tolerant Approach for Modular Multilevel Converters Under Submodule Faults

The modular multilevel converter (MMC) is attractive for medium- or high-power applications because of the advantages of its high modularity, availability, and high power quality. The fault-tolerant operation is one of the important issues for the MMC. This paper proposed a fault-tolerant approach for the MMC under submodule (SM) faults. The characteristic of the MMC with arms containing different number of healthy SMs under faults is analyzed. Based on the characteristic, the proposed approach can effectively keep the MMC operation as normal under SM faults. It can effectively improve the MMC performance under SM faults but without the knowledge of the number of faulty SMs in the arm, without extra demand on communication systems, which potentially increases the reliability. The time-domain simulation studies with the PSCAD/EMTDC are conducted and a down-scale MMC prototype is also tested with the proposed approach. The study results show the effectiveness of the proposed approach.

Dual-Loop Control Strategy for DFIG-Based Wind Turbines Under Grid Voltage Disturbances

For a multimegawatts doubly-fed induction generator (DFIG), the grid voltage disturbances may affect the stator flux and induce the transient stator flux, due to the direct connection of the stator and the grid. The accumulation of the transient stator flux caused by the variations of the stator voltage may introduce harmful power and torque oscillations to the DFIG, and even lead to rotor overcurrent. For the conventional field-oriented vector control strategy, the design of the controller is based on the steady-state model of the DFIG, which neglects the dynamic of the stator flux, and, therefore, it cannot work well during the transient state to decay the transient flux and to suppress the flux accumulation. In this paper, a dual-loop control strategy, which includes the conventional current loop and an additional flux loop, is proposed to not only control the active and reactive power, but also decay the stator transient flux, and avoid the accumulation of the stator transient flux. Moreover, the proposed strategy can obtain nearly constant stator active power and electromagnetic torque, which may prolong the lifetime of the drive train. A case study on a typical 2-MW DFIG-based wind turbine demonstrating the effectiveness of the proposed control methods is verified with simulations in MATLAB/Simulink. The proposed control methods are also experimentally validated using a scaled-down 7.5-kW DFIG. The simulation and experimental results clearly validate the effectiveness and feasibility of the proposed strategy, and show the improved dynamic performances of the DFIG.

Generalized stability regions of current control for LCL-filtered grid-connected converters without passive or active damping

This paper investigates the stability regions of current control for LCL-filtered grid-connected converters, where no active or passive damping is required to stabilize the closed-loop control system. It is already identified in the literature that if the LCL resonance frequency is placed within 1/6 to 1/2 of the system sampling frequency, the grid current control can be directly used without damping. If the resonance frequency is smaller than 1/6 of the sampling frequency, the converter current control should then be adopted. This paper further extends the analysis to the cases where the resonance frequency could be larger than 1/2 of the sampling frequency, and derives the complete stability regions for both grid and converter current control. Interestingly, it is found that for any given LCL-filter design, there will always be one stable current control design without any damping, which may greatly facilitate the system controller design. Two design examples are given to demonstrate the feasibility and effectiveness of the proposed LCL-filter and current controller design. These theoretical analyses are also supported with simulation and experimental results.

Closure to Discussion on “Virtual Damping Flux-Based LVRT Control for DFIG-Based Wind Turbine”

Presents a closure to the discussion on the paper, “Virtual damping flux-based LVRT control for DFIG-based wind turbine,” (Zhu, R., et al; IEEE Trans. Energy Convers., vol. 30, pp. 714–725, Jun. 2015.

Enhanced Control of DFIG Wind Turbine Based on Stator Flux Decay Compensation

For the doubly fed induction generator (DFIG)-based wind energy conversion system, the decaying flux and negative flux are the main reasons to cause the DFIG rotor overcurrent, during grid faults. The stator decaying flux characteristics versus the depth and instant of the stator voltage variation are analyzed first. On the basis of the stator flux performances, the enhanced control strategy is proposed to limit the decaying flux approximately into zero in half fundamental period by changing the stator voltage drop and recovery mode, and consequently the rotor transient current pulse is significantly reduced during grid faults. The experimental results based on the 7.5 kW DFIG setup are carried to validate the correctness and feasibility of the proposed strategy. Dynamic voltage restorer (DVR) can be one of the applications. With the proposed strategy, the DVR only works in a half fundamental period and its output voltage amplitude is half of the stator voltage variation, during the grid voltage drop and recovery, respectively. As a consequence, the DVR can be rated for lower power saving cost. The simulation results based on MATLAB/Simulink using a 2 MW DFIG and the experimental results based on the 7.5 kW DFIG validate the effectiveness and feasibility of the DVR-based low-voltage ride through strategy.

Zero-Sequence Voltage Modulation Strategy for Multiparallel Converters Circulating Current Suppression

A zero-sequence circulating current (ZSCC) is typically generated among the multiparallel converters that share the common dc link and ac side without isolated transformers under the space vector modulation (SVM), due to the injected third-order zero-sequence voltage (ZSV). This paper analyzes SVM techniques, studies the effects of the ZSV on the ZSCC control loop in detail, and then proposes an improved SVM scheme to suppress the impact of the ZSV on the ZSCC. The proposed strategy can effectively realize the ZSCC suppression, even though the parallel converter modules have asymmetrical current references and filter inductances. The simulation and experimental results based on the parallel converters clearly verify the effectiveness of the proposed control.

Diode rectifier bridge-based structure for DFIG-based wind turbine

This paper proposes a new structure for the doubly-fed induction generator (DFIG)-based wind turbine. The proposed structure consists of a DFIG controlled by a partial rated power converter in the rotor side, a three-phase diode rectifier bridge (DRB) connected to the stator, and a DC/AC full rated power inverter. As this structure could isolate the DFIG and the grid by a DC-link, the DFIG could avoid the direct influences from the grid, and achieve fault ride through requirement easily. Based on this structure, two control strategies are developed to control the DFIG and fulfill maximum power output. The simulation results based a MATALAB/Simulink using a 2MW DFIG compare the performances of DFIG system with the two control strategies. Further, the experimental results based on a scaled-down setup with a 7.5kW DFIG validate the correctness of the control strategies.

High order sliding mode control of doubly-fed induction generator under unbalanced grid faults

This paper deals with a doubly-fed induction generator-based (DFIG) wind turbine system under grid fault conditions such as: unbalanced grid voltage, three-phase grid fault, using a high order sliding mode control (SMC). A second order sliding mode controller, which is robust with respect to matched internal or external disturbances, fast transient response and finite reaching time, is employed to reduce chattering phenomenon caused by high frequency switching of SMC, which serious exists in lower order SMC, and to overcome parameter dependence of traditional proportional integral (PI) control. In order to improve control performance of the overall system, electromagnetic power and active power oscillations elimination strategies are proposed respectively. Lastly, the effective of the proposed control strategy is verified by the simulation results of a 2 MW DFIG system.