什么是发电机的短路比Kc，Kc与发电机结构有什么关系 What is the short-circuit ratio Kc of a generator? What is the relationship between Kc and the structure of the generator

短路比Kc是表征发电机静态稳定度的一个重要参数，Kc原来的意义是对应于空载额定电压的励磁电流下，三相稳态短路时的短路电流与额定电流之比，即Kc=Iko/IN。由于短路特性是一条直线，故Kc可表达为发电机空载额定电压时的励磁电流Ifo与三相稳态短路电流为额定值时的励磁电流Ifk之比，表达式为:Kc=Ifo/Tfk≈1/Xd。Xd是发电机运行中三相突然短路稳定时所表现出的电抗，即发电机直轴同步电抗(不饱和值)。

如忽略做饱和的影响，则短路比与直轴同步电抗Xd互为倒数。短路比小。说明同步电抗大相应短路时知路电流小，但是运行中负载变化时，发电机的电压变化较大，且并联运行时发电机的稳定度较美差，即发电机的过载能力小，电压变化率大，影响电力系统的静态稳定和充电容量。短路比大，则发电机组过载能力大，负载电流引起的端电压变化较小，可提高发电机在系统运行中的静态稳定性。但Kc大使发电机励磁电流增大，转子用铜量增大，使制造成本增加。短路比主要根据电厂输电距离，负荷变化情况等因数提出，一般水轮发电机的K，取0.9~1.3。结构上，短路比近似的等于。

可见，要使Kc增大，须减小A，即增大机组尺寸或加大气隙，须增加转子绕组安匝数。

The short-circuit ratio Kc is an important parameter that characterizes the static stability of a generator. The original meaning of Kc was the ratio of the short-circuit current to the rated current during three-phase steady-state short-circuit under the excitation current corresponding to the no-load rated voltage, that is, Kc=Iko/IN. Since the short-circuit characteristic is a straight line, Kc can be expressed as the ratio of the excitation current Ifo at the rated no-load voltage of the generator to the excitation current Ifk at the rated three-phase steady-state short-circuit current. The expression is: Kc=Ifo/Tfk ≈ 1/Xd. Xd is the reactance exhibited by the generator during sudden three-phase short circuit stability during operation, which is the generator's direct axis synchronous reactance (unsaturated value).

If the effect of saturation is ignored, the short-circuit ratio and the direct axis synchronous reactance Xd are reciprocal to each other. Short circuit ratio is small. When the synchronous reactance is large, the corresponding short-circuit current is small. However, when the load changes during operation, the voltage of the generator changes greatly, and the stability of the generator is poor when running in parallel, that is, the overload capacity of the generator is small and the voltage change rate is large, which affects the static stability and charging capacity of the power system. If the short-circuit ratio is large, the generator has a greater overload capacity, and the terminal voltage change caused by the load current is smaller, which can improve the static stability of the generator during system operation. But Kc increases the excitation current of the generator, increases the amount of copper used in the rotor, and increases the manufacturing cost. The short-circuit ratio is mainly proposed based on factors such as the transmission distance of the power plant and load changes. Generally, the K value of a hydroelectric generator is taken as 0.9~1.3. Structurally, the short-circuit ratio is approximately equal to.

It can be seen that in order to increase Kc, A needs to be reduced, that is, the size of the unit needs to be increased or the air gap needs to be increased, and the number of ampere turns of the rotor winding needs to be increased.