Optimal Combination Control Of Dual Converters In Doubly-Fed Wind Power Generation System

With energy and environment issues increasingly serious, as the current largest scale developmental renewable energy and with the most commercial prospects, wind power has been getting more and more concern and attention by all the countries in the world. In recent years wind power has obtained vigorous expansion, especially doubly-fed wind power system based on variable-speed constant-frequency (VSCF) technology has been one of the main stream models and the control technology of DFIG (Doubly-fed induction generator) wind power generation system has become the research hotspots worldwide due to its excellent advantages. This dissertation aims to discuss the optimal control of wind power and combination control of dual converters in detail based on the mathematical models of doubly-fed wind power generation under symmetric and asymmetric grid conditions. The main contents of this dissertation could be summarized as follows.1. The thesis deduces the precise mathematical models for three-phase voltage-source PWM converter in three-phase stationary reference frame and two-phase rotational reference frame, respectively. Based on the models, PWM converter's steady state characteristics are analyzed and its different output bridge voltage vectors are compared carefully. DC low limit voltage value of the converter is also calculated, which provides theoretical foundation for selecting doubly-fed wind power system parameters and designing converter control systems properly. The dissertation proposes the typical dual loops control structure of PWM converters in the synchronous rotational reference frame. This will provide the fundamental basis for studying control systems for grid side converter (GSC) and rotor side converter (RSC) further.2. The dissertation makes a comparative analysis of DFIG performances and capacities of the dual converters with several exciting modes, and makes a thorough research for utilizing dual converters capacities properly and bringing their ability of regulation and control into full play. Based on the equivalent circuit of the DFIG, its power characteristics during symmetric grid conditions are obtained. Considering generator losses, a novel reactive power control strategy for doubly-fed wind power system is proposed. This approach reallocates the exciting currents to the stator side and the rotor side. Consequently, the capacity of the GSC is the same as that of the RSC, which makes one converter could be a backup for another. This brings a lot of advantages, such as the less part categories, high usage and operational efficiency, et al. So this method has great practical significance. 3. The stator flux mathematical models in positive- and negative- synchronous reference frames during symmetrical and asymmetrical grid voltage faults are deduced. The dynamic characteristics of the DFIG are analyzed and the complete dynamic-analytical expressions of the stator flux under symmetric and asymmetric voltage fault conditions are provided. Through two typical voltage-fault simulations with the duration of 50ms and 100ms, respectively, this thesis points out that the stator flux will oscillate with the line frequency when the symmetrical gird fault occurs, and with twice the grid frequency under unsymmetrical grid voltage fault condition. Furthermore, under the unsymmetrical grid voltage fault condition with the duration odd multiple half-cycles, when the grid voltage recovers, the amplitude of the flux oscillations can reach twice the oscillating amplitude at the start of voltage sag. Simulations with various typical voltage faults are presented, and the results validate the correctness of the theoretical analysis.4. The dissertation models the doubly-fed wind power generation system, including RSC and GSC, in the positive- and negative- synchronous reference frames under asymmetric grid voltage conditions. The double-line-frequency components of the instantaneous active and reactive powers of the DFIG stator and the AC side of the GSC are analyzed and deduced, which provides a theoretical basis for designing the asymmetric control schemes for the dual converters. Based on the operational states of the doubly-fed wind power system under asymmetric grid voltage conditions, four different control strategies for RSC and GSC are proposed and evaluated, and various negative sequence references of rotor currents and grid side currents within non-identical control scheme are also calculated.5. This dissertation discusses fault ride through (FRT) technology of the DFIG wind power system under slight grid voltage fault conditions in detail. It is concluded that under the symmetrical grid voltage fault, the reactive power rapidly regulated capability of RSC and GSC should be made full use of to support the voltage of point of common coupling (PCC) during voltage fault, which helps to suppress the magnitude of the voltage dropping and helps the network to recover quickly after clearing the gird fault. Under the asymmetrical grid voltage faults, the combination control scheme utilizing rotor side converter and grid side converter based on double coordinate transformation method is proposed. And also the paper sets up, analyzes and evaluates several various performance indexes comparatively. Simulated results verify the correctness and effectiveness of the proposed combination control schemes for dual converters.