Study Of Small-Signal Modeling And Analysis Of Grid-Connected Doubly-Fed Wind Generator During Riding Through Grid Fault

Common to most grid codes,wind farms are required to inject reactive current during riding through grid faults.Stability of grid-connected wind turbine during grid fault is not only the precondition of reliable reactive current response during grid fault but also the basic of active power gradual recovery after grid fault is cleared,and thus is of importance.It is known that the stability of grid-connected equipment is related to the grid condition closely.However,existing control strategies of fault riding-through are designed at the assumption that the grid at connection point is strong enough,and the high grid impedance resulted by the large-scale centralized development and long-distance transmission of wind power has not been fully considered yet.Therefore,with the increasing penetration of wind power,the risk of instability of grid-connected wind turbines during grid faults has not been fully evaluated yet,and the existing studies do not cover the asymmetrical fault condition of more common.Since the duration of grid faults can be as long as several hundred milliseconds or even several seconds,electromagnetic transient quantity will be almost completely attenuated and wind turbine will be in quasi steady state during grid faults,and thus small signal method can be employed to investigate the stability of grid-connected wind turbine.With power electronic technology employed,wind turbine has fast current dynamics and this characteristic should be considered in the stability modeling and analyzing.In this thesis,based on deep symmetrical/asymmetrical fault scenario of high-impedance grid and with the current dynamics considered,the mechanism of low-frequency instability(i.e.,10-100 rad/s approximately)for doubly-fed induction generator(DFIG)based wind turbine during grid faults is explored.With special attention paid to the rotor-side converter's parameter,the objective of this work is to provide suggestion for the designing of grid fault riding-through control strategy in engineering practice and the possible further research.Details are as follows:(1)Based on the general modeling idea,the DFIG equipment and the grid during general asymmetrical grid fault are modeled by means of small-signal method with current dynamics considered,respectively.Based on the external characteristics of DFIG equipment,the internal voltage and reactance of DFIG equipment with current dynamics considered are defined.Then the small-signal model of DFIG in frequency domain is developed.And the small-signal model of grid under general asymmetrical fault condition in frequency domain with current dynamics considered is developed.Moreover,the state equations of the fault grid and the DFIG equipment during general asymmetrical grid fault with current dynamics considered in time domain are developed.According to the developed small-signal model in frequency domain,the main two categories of instability issue of grid-connected DFIG during faults of weak AC grid can be identified.(2)Based on the scenario of deep symmetrical fault of weak AC grid,the mechanism of the first-category low-frequency instability issue of grid-connected DFIG system during grid fault,which is induced by the interaction between phase-locked loop(PLL)and rotor current controller(RCC)inside positive-sequence subsystem,is analyzed.Firstly,the intra-sequence interaction in positive sequence subsystem resulted by the impedance of positive-sequence grid is elaborated.As indicated by the modal analysis results,the poor damping modes are dominated by PLL,thus the complex torque coefficient method is generalized to investigate the phase motion of PLL and the damping and restoring coefficients are obtained to quantify the influence of the PLL-RCC interaction on phase locked loop.Based on these coefficients,key factors related with the stability are analyzed and thus the mechanism of the first-category instability issue induced by the intra-sequence interaction in positive sequence subsystem during deep symmetrical fault of weak AC grid is revealed.(3)Based on the scenario of deep asymmetrical fault of weak AC grid and under the precondition that both positive-and negative-sequence subsystem itself are stable enough,the mechanism of the second-category low-frequency instability issue,which is induced by the interaction between positive-and negative-sequence subsystems,is analyzed for gridconnected DFIG system during grid fault.Firstly,the transfer characteristics of the DFIG equipment and the fault grid,in the form of vector,are studied and the key factors affecting the sequence coupling are made clear.As indicated by modal analysis results,the influences of the inter-sequence interaction on the positive-and negative-sequence subsystem are different,thus the inter-sequence networks are equivalently referred to negative sequence subsystem.Then the relationship between the stability of grid-connected system and the transfer characteristics of basic units is researched.Based on the concept of complex torque coefficient method,the stability of the sequences-coupling system can be evaluated by the damping and restoring coefficients,and the influence of inter-sequence grid on the stability of negative-sequence subsystem can be quantified by damping and restoring coefficients.Finally,the key factors affecting the inter-sequence interaction under asymmetrical grid fault condition are analyzed,and thus the mechanism of the second-category instability issue induced by the inter-sequence interaction during deep asymmetrical fault of weak AC grid is revealed.