Lihui Yang

Advanced Control Strategy of DFIG Wind Turbines for Power System Fault Ride Through

This paper presents an advanced control strategy for the rotor and grid side converters of the doubly fed induction generator (DFIG) based wind turbine (WT) to enhance the low-voltage ride-through (LVRT) capability according to the grid connection requirement. Within the new control strategy, the rotor side controller can convert the imbalanced power into the kinetic energy of the WT by increasing its rotor speed, when a low voltage due to a grid fault occurs at, e.g., the point of common coupling (PCC). The proposed grid side control scheme introduces a compensation term reflecting the instantaneous DC-link current of the rotor side converter in order to smooth the DC-link voltage fluctuations during the grid fault. A major difference from other methods is that the proposed control strategy can absorb the additional kinetic energy during the fault conditions, and significantly reduce the oscillations in the stator and rotor currents and the DC bus voltage. The effectiveness of the proposed control strategy has been demonstrated through various simulation cases. Compared with conventional crowbar protection, the proposed control method can not only improve the LVRT capability of the DFIG WT, but also help maintaining continuous active and reactive power control of the DFIG during the grid faults.

Oscillatory Stability and Eigenvalue Sensitivity Analysis of A DFIG Wind Turbine System

This paper focuses on modeling and oscillatory stability analysis of a wind turbine with doubly fed induction generator (DFIG). A detailed mathematical model of DFIG wind turbine with vector-control loops is developed, based on which the loci of the system Jacobians eigenvalues have been analyzed, showing that, without appropriate controller tuning a Hopf bifurcation can occur in such a system due to various factors, such as wind speed. Subsequently, eigenvalue sensitivity with respect to machine and control parameters is performed to assess their impacts on system stability. Moreover, the Hopf bifurcation boundaries of the key parameters are also given. They can be used to guide the tuning of those DFIG parameters to ensure stable operation in practice. The computer simulations are conducted to validate the developed model and to verify the theoretical analysis.

A Comprehensive LVRT Control Strategy for DFIG Wind Turbines With Enhanced Reactive Power Support

The paper presents a new control strategy to enhance the ability of reactive power support of a doubly fed induction generator (DFIG) based wind turbine during serious voltage dips. The proposed strategy is an advanced low voltage ride through (LVRT) control scheme, with which a part of the captured wind energy during grid faults is stored temporarily in the rotors inertia energy and the remaining energy is available to the grid while the DC-link voltage and rotor current are kept below the dangerous levels. After grid fault clearance, the control strategy ensures smooth release of the rotors excessive inertia energy into the grid. Based on these designs, the DFIGs reactive power capacity on the stator and the grid side converter is handled carefully to satisfy the new grid code requirements strictly. Simulation studies are presented and discussed.

Hopf bifurcation and eigenvalue sensitivity analysis of doubly fed induction generator wind turbine system

This paper first presents the Hopf bifurcation analysis for a vector-controlled doubly fed induction generator (DFIG) which is widely used in wind power conversion systems. Using three-phase back-to-back pulse-width-modulated (PWM) converters, DFIG can keep stator frequency constant under variable rotor speed and provide independent control of active and reactive power output. The oscillatory instability of the DFIG has been observed by simulation study. The detailed mathematical model of the DFIG closed-loop system is derived and used to analyze the observed bifurcation phenomena. The loci of the Jacobians eigenvalues are computed and the analysis shows that the system loses stability via a Hopf bifurcation. The eigenvalue sensitivity analysis with respect to machine and control parameters are performed to assess the impact of parameters upon system stability. Moreover, the Hopf bifurcation boundaries of the key parameters are also given that can facilitate the tuning of those parameters to ensure stable operation.

Fast-Scale Bifurcation Analysis in One-Cycle Controlled H-Bridge Inverter

This paper discusses the fast-scale instability in the one-cycle controlled H-bridge inverter which is widely used in practical applications. Simulations show that the fast-scale instability which occurs around the peak value of the inductive current is a type of local instability. The discrete-time model is derived to analyze the fast-scale instability. According to the Jacobian matrix derived from the discrete-time model, the theoretical analysis shows that the fast-scale instability manifests itself as a period-doubling bifurcation occurs, which reveals the intrinsic mechanism of the instability of one-cycle controlled H-bridge inverter. Furthermore, with the approximation of the matrix exponential in the discrete-time model and based on the Jacobian matrix, the stability boundary is obtained by the analytical expression which is helpful to select the proper values of parameters for avoiding the fast-scale instability. The experiments are carried out to validate the results of the simulation and theoretical analysis. It is shown that the analysis method is able to facilitate the design of the inverter.