Research On The Key Techniques Of Fault Ride Through For Wind Turbines With Doubly Fed Induction Generator

The increased penetration of wind power into the grid is a challenge to safe operationand management of the grid. To this end wind, fault ride through capability is required bynew grid codes for wind power. So, it is becoming significant to study the FRT of windturbines. This will affect the further development of wind power and is one of keytechnologies for the penetration of large scale wind power into the grid. Among differentapplications of wind power generation to obtain variable speed, the system based on doublyfed induction generators (DFIG) is now a preferred configuration due to its advantages andits good behaviors during normal grid condition. However, the FRT capability of DFIG is alarge challenge, due to its sensitivity to grid disturbances, especially to voltage sags and thelimited rotor-side converter (RSC) capability. Therefore, improving their behaviors infaulty scenarios to satisfy the FRT requirements set by the grid codes has become an issueof special interest for wind turbines manufacturers.When the generators are subjected to a symmetrical dip of grid voltage, there are twomain problems that must be overcome in achieving the ride-through requirements of DFIG.The first one is the peak rotor current that may destroy the RSC, and the second one is thedc-link over voltage that may also destroy the RSC. However, when an asymmetrical dip ofgrid voltage occurs, in addition to attention to inrush stator currents, the more concern istorque ripple, power oscillations and dc-voltage ripple. So, a comprehensive, in-depth,systematic study on the concernd issues above is completed, the primary contents andoriginal contributions of this dissertation are as follows:1) Based on space vector, the mathematical models of DFIG are expressed in bothtwo-phase natural reference frame and two-phase synchronous rotating reference frame,respectively. Rotor-side converter (RSC), grid-side converter (GSC) and their controlmodels are also expressed in synchronous reference frame. As a basis of the research, adetailed model of grid-connected wind turbines with DFIG is implemented in the dedicatedpower system analysis tool PSCAD/EMTDC, which can be used to evaluate theeffectiveness and feasibility of the proposed FRT schemes for DFIG-based wind turbines.2) Based on the mathematical model of DFIG in stator (stationary) reference framerepresented by space vector, the transient behaviors of DFIG including stator flux, rotoropen voltage and rotor fault current are examined in generic way for symmetrical andasymmetrical voltage faults in the power system. Especially, rotor fault currents are investigated in detail, the voltages applied to the rotor winding by rotor side converter(RSC) and the differences of the rotor circuit impedance due to different frequencycomponents are considered, and finally, accurate expressions of rotor fault currents as afunction of time for both symmetrical and asymmetrical voltage faults in the power systemare obtained. The comparable simulations evaluate the expression of the rotor currents.This analysis helps to understand the causes of the problem and as a result, contributes toadapt reasonable approaches to enhance the FRT capability of DFIG wind turbines during avoltage fault.3) From soft control strategies perspective, based on conventional vector control (VC)for RSC, the effects of different feed-forward voltage compensation terms on the FRT ofDFIG are investigated，additional feed-forward transient terms compensating for thetransient effects of the stator on the rotor currents are further simplified, and as a result theinformation of stator resistance, which is often very difficult to estimate, is no longerrequired. Taking into considering the nonlinear characteristics of the DFIG electricaldynamics, the linear control scheme cannot properly work under large voltage dips, so thisproblem is addressed by means of a nonlinear controller named after hysteresis regulator.In fact, from energy balance perspective, the over currents in the stator and the rotor ofDFIG is due to the unbalanced enegy flow during voltage faults. To mitigate theunbalanced energy flow, and therefore enhance the FRT capability of DFIG-based windturbine, a de-load FRT scheme is investigated in detail. Simulation results show theircharacteristics of the FRT schemes proposed above, and all of them can enhance the FRTcapability of DFIG in different degree.4) Conventional crowbar (CB) protection techniques including the evaluation of theeffects of different bypass resistances on DFIG wind turbine, the optimization of bypassresistances, and control strategies of crowbar are investigated in detail. To address the issueof over speed of DFIG with crowbar protection, a coordinated FRT method based on thepitch-controlled method and the crowbar protection is presented. Against the temporarilylost controllability of DFIG with crowbar protection, the series resistances FRT schemesincluding rotor series resistances and stator series resistances are proposed. The effects ofdifferent current limiting resistances for the two methods on DFIG wind turbine areevaluated, and the advantages and disadvantages of both methods are compared. Finally,through the simulation, the effectiveness and feasibility of a combined FRT method basedon the rotor series resistances (RSR) is evaluated.5) The FRT scheme for DFIG wind turbine based on dynamic voltage restorer (DVR) is developed. Firstly, general issues involved in the application of DVR including itstopology, basic principle, voltage compensation strategy and so on are analyzed. For thepurposes of the specific applications for DVR in the improvement of the FRT capability ofDFIG-based wind turbine system, suitable topology and compensation strategy is selected.The determination methods of the main parameters of the main circuit of DVR areanalyzed. The vector control of DVR in different reference frame is investigated. Severalissues that need attention in the practical application are discussed. Simulation results showthe effectiveness the DVR in enhancing the FRT capability of DFIG and the transientbehaviors of DFIG are also large improved during both symmetrical voltage sags andasymmetrical voltage dips.