Coordinated Rotor-Side Control Strategy for Doubly-FedWind Turbine under Symmetrical and Asymmetrical Grid Faults

In order to solve the problems of rotor overvoltage,overcurrent and DC side voltage rise caused by grid voltage drops,a coordinated control strategy based on symmetrical and asymmetrical low voltage ride through of rotor side converter of the doubly-fed generator is proposed.When the power grid voltage drops symmetrically,the generator approximate equation under steady-state conditions is no longer applicable.Considering the dynamic process of stator current excitation,according to the change of stator flux and the depth of voltage drop,the system can dynamically provide reactive power support for parallel nodes and suppress the rise of DC side voltage and rotor over-current.When the grid voltage drops asymmetrically,the positive and negative sequence components are separated in the rotating coordinate system.The doubly fed generator model is established to suppress the rotor positive sequence current and negative sequence current respectively.At the same time,the output voltage limit of the converter is discussed,and the reference value is adjusted within the allowable output voltage range.In order to adapt to the occurrence of different types of power grid faults and complex operating conditions,a fast switching module of fault type detection and rotor control mode is designed to detect the type of power grid faults and voltage drop depth in real time and switch the rotor side control mode dynamically.Finally,the simulation model of the doubly fed wind turbine is constructed in Matlab/Simulink.The simulation results verify that the proposed control strategy can improve the low-voltage ride through performance of the system when dealing with the symmetrical and asymmetric voltage drop of the power grid and identify the power grid fault type and provide the correct control strategy.

SYMMETRICAL SISF IN DEFORMED Ni_3Al AND ITS FORMATION MECHANISM

The symmetrical superlattice intrinsic stacking faults(SISFs)formed on {111} planes are the predominant crystal defects in polycrystalline Ni_3AI after slight deformation at room temper- ature.The formation of this kind of SISF is resulted from the dipoles which are distributed on the neighboring {111} planes after reorientation,dissociation and annihilation under applied stress.

Optimization of Wind Farm Collection Line Structure under Symmetrical Grid Fault

A power-transmission collection line is connected to each wind turbine of a wind farm and then connected to the incoming switchgear at the low-voltage side of the booster station at a certain distance.Therefore,the grouping of the wind turbines in the wind farm determines the layout of the lines and affects the line impedance.The line structure and composition of the wind farm are analyzed,and the relationship between the impedance of the collection line and reactive power generated by the wind turbine at low-voltage ride through is derived.We conclude that the smaller the equivalent impedance of the wind farm collection line structure is,the greater the reactive power at the wind farm collection line point is.In addition,the economic aspect of the collection line needs to be considered in the design.The economic aspect and impedance values have contrasting characteristics.Therefore,according to the condition of minimum impedance and optimized economic aspect,an optimization model of a wind-farm collector-circuit structure is proposed.The optimal structure of the wind farm collector circuit is calculated using the Monte Carlo method,and the theoretical analysis is verified by simulation.

Adaptive concentric power swing blocker

The main purpose of power swing blocking is to distinguish faults from power swings.However,the faults occur during a power swing should be detected and cleared promptly.This paper proposes an adaptive concentric power swing blocker(PSB)to overcome incapability of traditional concentric PSB in detecting symmetrical fault during power swing.Based on proposed method,two pairs of concentric characteristics are anticipated which the first one is placed in a stationary position(outer of zone3)but the position of the second pair is adjustable.In order to find the position of the second pair of characteristic,Static Phasor Estimation Error(SPEE)of current signal is utilized in this paper.The proposed method detects the abrupt change in SPEE and puts the second pair of characteristic in location of impedance trajectory correspondingly.Second concentric characteristic records travelling time of impedance trajectory between outer and inner zones and compares to threshold value to detect symmetrical fault during power swing.If recorded time is lower than threshold,three-phase fault is detected during power swing.Intensive studies have been performed and the merit of the method is demonstrated by some test signals simulations.