Zhen Xie

Study on active damping methods for voltage source converter with LCL input filter

Compared with L type filters, used in the AC side of voltage source PWM converters (VSC), the third-order LCL filters can suppress the switching frequency current ripples more effectively. However, the underdamped characteristics of LCL filters will bring control system stability problems. In order to assure stable operation of VSC control system, a novel active damping control strategy based on feedforward control of the transient components of the AC-side capacitor voltages is proposed. Compared with the conventional lead-lag element based one, the proposed active damping control method not only can eliminate the capacitor voltage sensors, but also can improve the dynamic response of VSC system. In this reporter, the LCL-VSC is modeled and the resonance is analyzed firstly, and then, based on the mathematic models, the selection of the system control parameters and the effection of the deviation of LCL-filter parameters on the system stability are analyzed by Matlab tools. At last, to validate the designed control strategy, a 15kW LCL-VSC prototype is built and some experimental results are reported.

Response and Protection of DFIG System under Grid Fault

The use of doubly fed induction generators (DFIG) in large wind turbines has become quite common over the last few years. However, DFIG are very sensitive to grid disturbances, especially to voltage dips, therefore low voltage ridethrough(LVRT) technology for DFIG becomes one of the critical technology in the uninterrupted operation of wind power generation. In order to help the study of low voltage ride-through(LVRT) technology for DFIG, a detailed thoretical analysis of the dynamic behavior of DFIG during grid voltage drop and recovery has been proposed, and obtained the maximum instantaneous rotor voltage when the voltage drop and the maximum and minimum rotor voltage when voltage recovery, then given the relevant simulation waveform. At last, the method on crowbar design is proposed.

High voltage ride through control strategy of doubly fed induction wind generators based on active resistance

The control strategy of doubly fed induction generator (DFIG) under grid voltage dip has been a focus of discussion up to now. Grid voltage swell will cause more problems for DFIG. In order to study the characteristic and control strategy of high voltage ride through (HVRT) for DFIG, the dynamic behavior during voltage swell is analyzed. It is found that active resistance manages effective to suppress rotor current and torque oscillation, avoid the operation of crowbar circuit, enhance its high voltage ride through technology capability, compared to the conventional control strategy. Simulation and experimental results verify the effectiveness of the control strategy.

Sensorless control for PMSG in direct-drive wind turbines

Permanent magnet synchronous Generator (PMSG) based direct-drive WECS has been attracting wide attentions. For the special application, sensorless control for PMSG is desired. By widely studying the previous contributes, a novel estimator based on back-EMF is proposed. The estimator is composed of back-EMF observer and a phase-lock-loop (PLL) control to get the rotor-flux speed and position. The estimator not only can be used to interior or surface permanent magnet synchronous generators, but also has a compact and symmetrical structure, which makes it be beneficial for implementation. Compared with previous strategies, the EMF observer is independent of the PLL control, which would simplify the observer design. Meanwhile, the proposed estimator is less sensitive to parameter variations. Based on mathematic models of PMSG, the proposed estimator was analyzed in detail, and the realizing process was also presented. To validate the proposed estimator, the important experiment results are reported.

Dynamic analysis of doubly fed induction generator during symmetrical voltage swells

With the rapid increase of wind farms especially large offshore wind farms, a new problem associated with the response of doubly fed induction generator to temporary overvoltage has arisen. This paper develops a theoretical analysis of the dynamic behavior during symmetrical three phase voltage swell. Overvoltage may lead to reversal of power in the line side converter. Simulation results are in good agreement with those obtained theoretically.

A Study on Dynamic Model And Analyse of Wind Turbine Generation System

In order to study dynamic performances of wind turbine generator system (WTGS) in laboratory, a dynamic model of WTGS is necessary. Firstly, a dynamic wind turbine emulator (WTE) is designed, a novel dynamic torque compensation scheme is developed, which consider wind shear effect, tower shadow effect and big inertia of the actual wind turbine all together. Then, the mechanical transmission system of variable speed WTGS set based on doubly-fed induction generator is fully studied in this paper. For the dynamic model of the mechanical transmission, we take account of wind turbine and generator inertia, damping and stiffness coefficient of the shaft chain all together, which are used in studying the influence on the wind power generation system dynamic performances detailed. Then it points out that optimization of parameters between the wind turbine and the mechanical transmission is very important and necessary for designing of WTGS. After that, the results of the simulation are used to prove the correctness of theoretical analysis. The simulation results strongly support the theoretical analysis and simulation results.

Speed sensorless torque closed-loop vector control of IPMSM

The speed sensorless control method, which is based on the extended electromotive force (EEMF) model of interior permanent magnet synchronous motor, is difficult to make into use and its parameter dependence is strong. So a method to simplify the EEMF model was proposed and a simplified full order state and sliding mode observer (FOSSMO) was designed in order to achieve sensorless control. In addition, a simplified stator flux and electromagnetic torque observation method was presented. Then, torque closed-loop control system without speed sensor is established. The simulation and experimental research show that the proposed sensorless stator flux and electromagnetic torque observation method is independent of all the inductance and permanent magnet flux linkage parameters and the transient characteristic of the sensorless torque closed-loop control system is good.

Study on Control Strategy for DFIG-Based Wind Turbines Under Unbalanced Grid Voltages

Unbalanced input voltages would make doubly fed induction generator (DFIG)-based wind turbine operating performance deteriorate, such as shaft tremble, temperature increasing, and so on, even make it cut out of the power grid. Meanwhile, without proper control the power ripples generated from wind turbines may further aggravate power grid. Considering the unbalanced conditions, DFIG was modeled in dual synchronous reference frame (SRF), namely the positive one and the negative one, based on which the dual PI current controllers were designed. To implement the dual current control, the sensing variables were divided into positive and negative sequence components, which were controlled in positive and negative SRF respectively. At the same time, to synchronize with the positive and negative sequence voltage components, a phase latch loop (PLL) control was designed. Experimental results on 11kW DFIG wind turbine test bed validated the designed control system.

Study on vector control for double-fed induction machine with stator windings short circuited

Starting up and/or low speed operations are imperative for double-fed induction machine (DFIM) in DFIM-based driving system or DFIM-based wind turbine system under low speed wind conditions. Compared with others, the rotor-side driving control strategy with stator-winding short circuited can operate DFIM, without extra equipments, at low speed very well. The operation theory is analyzed, based on which the equivalent circuits and the corresponding mathematical models are presented and discussed. According to the models, built in the paper, vector control for rotor-side driving control strategy is proposed, with which, the decoupled control between the torque current component and the excited current component of DFIM rotor currents is achieved. As result, the dynamic and stable control performances of DFIM-based systems are improved. To validate the proposed control strategy, a 2MW DFIM-based driving system is simulated.

Notice of Retraction The Analyse of Maximum Power Point Tracking's Energy Losses in PV System

Increasing efficiency is the most critical issues for photovoltaic systems, this paper introduces the views of the energy losses to the maximum power point tracking, and analyzes the maximum power point trackings dynamic and static energy losses detailedly.Then proved by the simulation of static and dynamic characteristics for incremental method (IncCond). The simulation and the theory proved that IncConds energy losses in the process of tracking the maximum power point.