1、中文 3880 字 出处： Muljadi E, Butterfield C P, Romanowitz H, et al. Self-excitation and harmonics in wind power generationJ. Journal of solar energy engineering, 2005, 127(4): 581-587. Self-Excitation and Harmonics in Wind Power Generation E. Muljadi ， C. P. Butterfield National Renewable Energy Laborato
2、ry, Golden, Colorado 80401 H. Romanowitz Oak Creek Energy Systems Inc.,Mojave, California 93501 R. Yinger Southern California Edison,Rosemead, California 91770 Traditional wind turbines are commonly equipped with induction generators because they are inexpensive, rugged, and require very little main
3、tenance. Unfortunately, induction generators require reactive power from the grid to operate， capacitor compensation is often used. Because the level of required reactive power varies with the output power, the capacitor compensation must be adjusted as the output power varies. The interactions amon
4、g the wind turbine, the power network, and the capacitor compensation are important aspects of wind generation that may result in self-excitation and higher harmonic content in the output current. This paper examines the factors that control these phenomena and gives some guidelines on how they can
5、be controlled or eliminated. 1 Introduction Many of todays operating wind turbines have fixed speed induction generators that are very reliable, rugged, and low cost. During normal operation, an induction machine requires reactive power from the grid at all times. The most commonly used reactive pow
6、er compensation is capacitor compensation. It is static, low cost. Different sizes of capacitors are generally needed for different levels of generation. Although reactive power compensation can be beneficial to the overall operation of wind turbines, we should be sure the compensation is the proper
7、 size and provides proper control. Two important aspects of capacitor compensation, self-excitation and harmonics ,are the subjects of this paper. 2 Power System Network Description A diagram representing this system is shown in Fig(1). The power system components analyzed include the following: An
8、infinite bus and a long line connecting the wind turbine to the substation A transformer at the pad mount Capacitors connected in the low voltage side of the transformer An induction generator For the self-excitation, we focus on the turbine and the capacitor compensation only the right half of Fig.
9、 For harmonic analysis, we consider the entire network shown in Fig. 3. Self-Excitation 3.1 The Nature of Self-Excitation in an Induction Generator. Self-excitation is a result of the interactions among the induction generator, capacitor compensation, electrical load, and magnetic saturation. This s
10、ection investigates the self-excitation process in an off-grid induction generator, knowing the limits and the boundaries of self-excitation operation will help us to either utilize or to avoid self-excitation. Fixed capacitors are the most commonly used method of reactive power compensation in a fi
11、xed-speed wind turbine. An induction generator alone cannot generate its own reactive power； it requires reactive power from the grid to operate normally, and the grid dictates the voltage and frequency of the induction generator. One potential problem arising from self-excitation is the safety aspe
12、ct. Because the generator is still generating voltage, it may compromise the safety of the personnel inspecting or repairing the line or generator. Another potential problem is that the generators operating voltage and frequency may vary. Thus, if sensitive equipment is connected to the generator du
13、ring self-excitation, that equipment may be damaged by over/under voltage and over/ under frequency operation. In spite of the disadvantages of operating the induction generator in self-excitation, some people use this mode for dynamic braking to help control the rotor speed during an emergency such
14、 as a grid loss condition. With the proper choice of capacitance and resistor load, self-excitation can be used to maintain the wind turbine at a safe operating speed during grid loss and mechanical brake malfunctions。 3.2 Steady-State Representation. The steady-state analysis is important to unders
15、tand the conditions required to sustain or to diminish self-excitation. As explained above, self-excitation can be a good thing or a bad thing, depending on how we encounter the situation. Figure 2 shows an equivalent circuit of a capacitor compensated induction generator. As mentioned above, self-e
16、xcitation operation requires that the balance of both real and reactive power must be maintained. Equation （ 1） gives the total admittance of the system shown in Fig(2): SY+ MY+ RY=0 （ 1） where SY= effective admittance representing the stator winding, the capacitor, and the load seen by node M MY =
17、effective admittance representing the magnetizing branch as seen by node M,referred to the stator side RY = effective admittance representing the rotor winding as seen by node M, referred to the stator side Equation 1 can be expanded into the equations for imaginary and real parts as shown in Eqs.2and3: （ 2） Fig. 2 Per phase equivalent circuit of an induction generator under self-excitation mode
