Research On The Variable-Speed-Operation Dual Stator-Winding Induction Generator System

Induction machines(IMs), as motors, are widely used in many kinds of application fields, such as speed adjustment, electric traction, electric drive, etc., because of its many advantages, such as, simple structure, reliable operation, low cost and easy maintenance, and so on. Based on the reversible principle of electrical machines, IMs, of course, can be used as generators for generation. Two widest and most important fields where the generating technology of IMs is applied are the wind power generation and stand-alone power system. In recent years, both of the above two fields develop very fast, and have been paid more and more attention, with many new demands. It not only brings much more opportunities, but also raises some new challenges for induction generator systems.To meet the developments of the offshore wind power generation and lager airplane variable frequency ac(VFAC) power system and satisfy their new demands on the generator system, this paper studies the novel variable-speed-operation dual stator-winding induction generator(DWIG) system deeply and carefully. The rotor of this DWIG is squirrel-cage type, and it has a natural brushless structure. There are two stator windings embedded in the stator slots. One of them, connected with the excitation capacitors, is referred to as the power winding which directly outputs constant voltage VFAC power or outputs dc power after rectifying, and the other one, connected with the static excitation converter(SEC) through the filter inductances, is termed as the control winding which is used to regulate the generator excitation.The concrete research contents and achievements are as follows:(1) Mathematical models of the DWIGThe mathematical models of DWIG in the three-phase static A-B-C coordinates and three-phase orthogonal rotating d-q-0 coordinates is established, and these mdels can meet with research requirements for the DWIG system with different outputs.(2) Control strategy of the variable-speed-operation DWIG generating systemTo further enrich and perfect the voltage control theory of the DWIG generating system, and widen its application fields, a suitable slip frequency control strategy for this system is proposed. In this control strategy, the SEC on the control-winding side is used to fast change the frequency and amplitude of the voltage applied on the control winding, so as to quickly regulate the rotating speed and amplitude of the control-winding flux, and then change the slip frequency and excitation of the DWIG. It not only can regulate generator reactive power, but also can modulate the change of the generator active power actively, for the fast balance of input and output power, so as to improve system static and dynamic performances. The implement of this control strategy is relatively simple, without the complex coordinate transformation and current loop which usually impacts the dynamic system. It is suitable for not only the DWIG generating system with the dc voltage output, but also the DWIG generating system with the VFAC voltage output. The experimental results from the prototype show that, the proposed control strategy has better static and dynamic performances than the existing three kinds of the control strategies.(3) Optimal selection of the excitation capacitor and filter inductance for the variable-speed operation DWIG generating systemAfter the analysis of change law of the control-winding reactive current, the general principle of the excitation capacitor optimization is obtained for the variable-speed operation DWIG generating system, namely, guaranteeing the absolute values of the positive maximum value and the negative minimum value of the control-winding reactive current are equal. However, because the control-winding reactive current has some differences when the DWIG generating system is applied in different application fields, the concrete optimal principles should also change correspondingly. And this is also analyzed.The selection of the filter inductance for the variable-speed operation DWIG generating system should guarantee the SEC providing enough large reactive current besides meeting the two basic requirements of the fast current response and good ability to suppress the current fluctuation. Its concrete optimal methods are that its maximum value is determined according to the current response speed and maximum reactive current; its minimum value is determined according to the current suppressing ability.The experimental results from prototype show that, the excitation capacitor and filter inductance selected according to the proposed optimal methods, not only can guarantee the system operating stable in the speed range, but also minimize the control reactive capacity and reduce the SEC capacity effectively.(4) Wide-speed-range-operation DWIG dc generating system and its application study for wind power generationFor the urgent needs to enlarge the operating speed range of the generating system while outputting the rated voltage, a wide-speed-range-operation DWIG dc generating system is proposed. The topology structure and the working principle of its high-speed and low-speed operating mode are both analyzed. The turn ratio of the control winding and power winding is derived and obtained. The boosting principle and control method of the output voltage in the low-speed operating mode are analyzed. The wide-speed-range-operation control strategy based on the orientation of control-winding flux is proposed, and the compatibility and smooth transition of the control strategy for the high-speed and low speed operating modes are overcome.To effectively utilize the wind energy in the low wind speed range, the efficiency optimization is studied for the wide-speed-range-operation DWIG dc generating system with the light load under the low speed. According to the basis of the principle of the DWIG efficiency optimization control, an efficiency optimization control strategy based on the loss model of the DWIG, and the concrete implementation method is also given.The experimental results from the prototype show that, this generating system can output the constant rated voltage in the wide speed range including the low speed range; the proposed efficiency optimization control strategy can greatly improve the efficiency for the system with the light load under the low speed; the wind power system using this generating system not only can output the electricity in the wide wind speed range, but also can utilize the wind energy in the low speed range effectively.(5) Operation performance of the DWIG VFAC generating system with different loads for large airplane applicationFor the characteristics and requirements of the large airplane VFAC power system, a DWIG VFAC generating system for large airplane application is proposed, and its operation performance with different loads is studied. To improve the operation performance, especially the dynamic performance for the DWIG VFAC generating system with inductive and capacitive loads, a slip frequency control strategy with the information of the load reactive power is proposed, and the concrete implementation method is also given. The experimental results from the prototype show that, using the proposed control strategy, the system with inductive and capacitive loads has good static and dynamic performances, and the performance can meet the requirements of American military standard MIL-STD-704 F.For the particularity of the DWIG VFAC generating system with unbalance loads, according to the symmetrical component method, an asymmetrical analysis method based on the load three-port network model is proposed, and the concrete implementation method is also given. Based on this method, the quantitative analysis for the system with unbalance loads is implemented. Under the typical resistance-capacitor composite unbalance loads, the experimental results from the prototype show that, the system has the good adaptability for unbalance loads, and the performance can meet the requirements of American military standard MIL-STD-704 F. In addition, under the above unbalance loads, the impact of the key generator parameters on the output voltage unbalance is studied using the proposed asymmetrical analysis method.