Investigation On The Ride-Through Operation Of DFIG-based Wind Power Generation Systems During Grid Fault

As the increased penetration of wind power generations in power system, modern grid codes concerning grid-connected wind turbines are developed. As a result, the steady-state response and variable-speed constant-frequency (VSCF) performance of Doubly Fed Induction Generator-based (DFIG-based) wind turbines under normal grid conditions is well understood and applied. The fault ride-through (FRT) operation and control of the DFIG wind power system when network fault conditions has been the main subject of much recent research and development worldwide. The attractive research subjects are as follows, viz., modeling and control of wind turbine driven DFIG adjusting to network fault operation; impact of grid fault on the DFIG and the associated protection schemes; control strategies for ride-through operation of DFIG generation system during network fault. Among them, the most important is that research on the FRT operation has covered asymmetrical grid faults as well as symmetrical ones, which is a challenging and innovative research subject in the field of global wind power technologies.This dissertation intends to study the enhanced control and the fault ride-through operation of wind farms based on DFIG, connected to either a transmission system or embedded within a distribution system, when the network is faulted either symmetrically or asymmetrically. Comprehensive, thorough and detailed studies with respect to theoretical analysis, simulated operations and experimental verifications, are carried out. Keeping in step with advanced wind power technologies globally, some important conclusions and independent-innovative achievements are made and obtained.1. The precise mathematical model of DFIG's grid-side converter (GSC) and rotor-side converter (RSC) are created and expressed in the three-phase stator stationary reference frame, two-phase stator stationary reference frame and two-phase rotating reference frame at arbitrary angular speed, respectively. Based on the model, the dissertation presents an integrated system model with network, DFIG, GSC and RSC included in the two-phase synchronous reference frame, and analyzes the system's instantaneous active and reactive powers. Thereafter, a classical vector control scheme based on grid/stator voltage orientation (GVO/SVO) and composed of dual closed-loops, viz., DC-link voltage control and AC current control loops, for DFIG's GSC is introduced. While for RSC, a precise control model taking the stator voltage variation transients into account is constructed in the synchronous reference frame, and accordingly two improved vector control schemes are suggested based on stator voltage orientation (SVO) and stator flux orientation (SFO), respectively. Simulated analysis verifies the correctness and effectiveness of the proposed DFIG's control model and the system's new vector control schemes under relatively small grid voltage dips. It is clearly shown that the two proposed control model designs are useful tools for DFIG fault studies and can be used to determine the required converter rating and protection device settings.2. Under unbalanced network voltage conditions, evaluation of the DFIG's operation, dynamic modeling of DFIG's system and design of enhanced control strategies are emphasized. At first, evaluation of the impact of unbalanced network voltage on the DFIG and associated GSC and RSC are carried out. Secondly, via symmetric-component method, novel unified mathematical d-q models of DFIG, GSC and RSC in the positive and negative synchronously rotating frames under unbalanced grid voltage conditions are deduced. Finally, for GSC and RSC four different enhanced control targets are proposed, respectively. According to the positive and negative sequence active and reactive power orders, associated positive and negative sequence current orders are presented in the positive and negative synchronous reference frames.3. Under unbalanced grid voltage conditions, based on the precise models of entire DFIG system, including GSC and RSC, in terms of positive and negative sequence components, the dissertation proposes, discusses, verifies and evaluates four different control schemes for positive/negative sequence currents of DFIG-used GSC and RSC. The four control schemes are dual d-q PI current controllers implemented in the respective positive and negative synchronous reference frames, main and auxiliary current controllers in the positive and negative synchronous reference frames, proportional resonant (P-R) current controller in the two-phase stator stationary reference frame and proportional integral plus resonant (PI-R) in the positive synchronous reference frame. Once the positive and negative sequence currents are fully regulated, the proposed enhanced control targets for GSC and RSC are completely realized during network unbalance. Detailed simulations and comprehensive experiments verify the feasibility and performance of each current control schemes when the network voltage is unbalanced.4. The dissertation studies control strategies and protection schemes for DFIG wind generation systems under serious grid voltage dip conditions. The timing of rotor crowbar's switching on and off is optimized. The impact of value of resistor series-connected to rotor crowbar on faulted network recovery is analyzed. On the basis, a FRT operation scheme composed of a series-connected resistor rotor crowbar and an improved grid-side converter control strategy is proposed. Concerning the operation characteristics of wind turbine driven DFIG system during network unbalance, the dissertation analyzes the impact of limited ratings of GSC and RSC on the coordinated control strategies when the network voltage is relatively larger unbalanced. Thereafter, another improved unbalanced FRT control scheme for DFIG system by taking into account the limited converter's ratings is presented and simulated results are shown to confirm its feasibility.5. In order to verify the effectiveness and practicability of the enhanced control strategies for DFIG wind power generation systems during network voltage unbalance, the dissertation designs and develops a VSCF DFIG test rig based on GSC and RSC. On the test rig, detailed experiments are carried out under a few classical operation conditions when the network voltage is unbalanced. Several practical and independent-innovative achievements on key wind power technologies are obtained, viz., different enhanced control targets for GSC and RSC during network unbalance, various positive/negative sequence current control schemes and coordinated control strategies of GSC and RSC under unbalanced network voltage conditions. The experimental results further confirm the feasibility of the proposed basic theories and key technologies for AC-excited DFIG wind power system under both symmetrical and asymmetrical network fault conditions. As a result, theory and practice are achieved thoroughly.