Direct Power Control Of Wind-Turbine Driven Doubly-Fed Induction Generators Under Abnormal Grid Conditions

In China, as in other parts of the world, many of the best resources for wind power generation are located far away from load centers. A large number of wind farms, supplying distant load centers by long AC transmission lines, face numerous technical challenges. Since the Variable-Speed Constant-Frequency (VSCF) wind power generation system based on Doubly-Fed Induction Generator (DFIG) is very popular in the modern wind farms, the control strategies of DFIG, which dominates the dynamic performances of wind turbines attract more and more attentions to researchers and wind-farm managers . Recently, direct power control (DPC) of DFIG' based wind farms becomes one of the research highlights, especially in the filed of grid fault ride through (FRT) operation due to its simple structure and good dynamic performance. This dissertation is mainly focused on the DPC strategy of DFIG-based wind power systems under weak grid conditions.The dissertation disccusses the advanced grid codes and the characteristics of DFIG, particularly its FRT capability at first. The conclusion reveals that an improved control strategy for DFIG, which is capable of protecting the wind turbines and the AC excitation converter from damages, and even geting enhanced system performance during FRT operation, is the key issue to the investigation of wind turbines driven DFIG connected to the weak network. As a result, the main contributions of the dissertation can be summarized and highlighted as follows:1. The traditional DPC scheme is studied and analyzed in detail firstly. In the traditional DPC, the simple switching table and the section-dividing method produce periodic errors to the output voltage vector, which leads to abundant AC current harmonics and needs to be improved and optimized.2. The DPC of grid-side converter of DFIG is investigated in detail. A multi-switching-table DPC strategy and a constant switching frequency DPC with Space Vector Modulation (SVM) approach are proposed. The former needs a simple structure and lower hardware requirement since it calculates the switching table off-line and is implemented mainly by looking up the table. The latter keeps the switching frequency around the sampling frequency with SVM, which increases the complexity of the control algorithm properly but effectively simplifies the design of AC filter coupled with generators. 3. The DPC of rotor-side converter of DFIG is studied and three selectable DPC strategies with constant switching frequency by using SVM technique are presented. Based on the relationship between DFIG state-variables and control variables, these DPC strategies aim at the controlling of DFIG rotor flux (RF-DPC), rotor current (RC-DPC) and electromagnetic torque (EMT-DPC), respectively. The RF-DPC is able to reduce the system oscillating time, the RC-DPC is capable of depressing the magnitude of rotor oscillating current effectively and the EMT-DPC provides the capability of eliminating electromagnetic torque vibration and, consequently diminishing the stress on turbine shaft, which can be selected according to the different requirement of wind farms connected to the weak network.4. The influences of different faults on the transmission line connected to the DFIG's terminal of wind farms are analyzed by symmetrical component method. Under different types of fault conditions, positive and negative sequence components of the terminal voltage are measured. Besides, the fault angles leading to the maximum and the minimum stator dc flux respectively are estimated. On the above basis, the performance of conventional vector control scheme for DFIG's FRT operation is evaluated.5. A zero-time delay extraction method for positive sequence components based on the second-order notch filter is presented. As a result, during unbalanced network conditions, the proposed multi-switching table DPC, constant frequency DPC for the grid-side PWM converter and constant frequency DPC for the DFIG wind turbine system are improved, which makes them suitable for the unbalanced grid fault ride through operation.6. An integrated simulation mode of a DFIG wind power generation system is built-up by using MATLAB/SIMULINK. Various simulations on grid-side PWM converter and DFIG system are carried out to validate the proposed control strategies. Meanwhile, an experimental platform of 15kW DFIG system is re-built for the tests particularly under unbalanced network voltage conditions.