燕 山 大 学本科毕业设计（论文）英文翻译课题名称：含 风 电 场 的 电 力系 统 静 态 电 压 稳 定 性 研 究 学 院 （ 系 ） ： 电 气 工 程 学 院 年级专业：09 级电力四班 学生姓名：张建春 指导教师： 王珺 完成日期： 2013.5.8 一 The interduction of steady-State Characteristics the speed-torque characteristic is quite linear around synchronous speed. If the rotor speed is below synchronous speed，the induction machine is operating as a motor and if the rotor speed is above synchronous speed，the induction machine is running as a generator.the mechanical power and the mechanical torque are given by the slip,rotor resistance and the rotor current.The speed-torque characteristic of the induction machine is quite linear around synchronous speed.the torque is proportional to the inverse of the rotor resistance. This implies that it is possible to have external rotor resistances connected in series with the existing rotor resistances of a wound-rotor induction machine. By changing the value of the external rotor resistance it is possible to change the slope of the speed-torque characteristic. One disadvantage with this method is that it is only possible to increase the slip using the external rotor resistances. This implies that if the induction machine is running as a motor, then an increased rotor resistance will decrease the rotor speed. On the other hand, if the induction machine is running as a generator, then if the rotor resistance increases, the rotor speed will also increase.Before semiconductors were available, one way of adjusting the slip was to introduce external rotor resistances. The external rotor resistance will cause additional losses in the rotor circuit. When semiconductors became available it was possible to recover the slip otherwise dissipated in the external rotor resistance.Thus, the slip power can be recovered into mechanical or electrical energy; therefore,this method is called slip power recovery. The rotor current must be rectified with a diode rectifier. For motor operation, the rotor circuit will see the diode rectifier as a resistance and therefore this method will work approximately in the same way as for the external rotor resistances. Note that the diode rectifier cannot be used in generator operation. The rectified current could be converted to mechanical power using a dc motor coupled to the shaft of the induction motor or fed back into the grid .Since Kramer drive require an extra dc motor it is of no interest, while the Scherbius drive is still in use.The main advantage of this configuration compared to the external rotor resistance is that the losses of the external rotor resistance can be recovered.If both stator voltage and frequency can be adjusted by an inverter, the torque-speed characteristic can be easily changed. When the speed is increased so that the stator voltage reaches maximum voltage, there is need for field weakening, the stator voltage is kept constant while the frequency is still increased. 二 The interduction of doubly-Fed Induction MachinesDoubly-fed machines can be used in variable-speed constant-frequency applications,such as wind turbines. The main advantage of a doubly-fed machine compared to a singly-fed for a variable-speed system is the reduced rating of the converters power rating. The reduction in power rating is dependent on the speed range of the drive.The standard doubly-fed induction machine is a wound rotor induction machine equipped with slip rings. The stator circuit is connected directly to the grid while the rotor circuit is controlled by an inverter via slip rings.The cascaded doubly-fed induction machine consists of two doubly-fed induction machines with wound rotors, that are connected mechanically through the rotor and electrically through the rotor circuits. The stator circuit of one of the machines is directly connected to the grid while the other machines stator is connected via an inverter to the grid. Since the rotor voltages of both machines are equal, it is possible to control the induction machine that is directly connected to the grid with the other induction machine.It is doubtful whether it is practical to combine two individual machines to form a cascaded doubly-fed induction machine, even though it is the basic configuration of doubly-fed induction machine arrangement. Due to a large amount of windings, the losses are expected to be higher than for a standard doubly-fed induction machine of a comparable rating.三 Stability of a Power SystemPower system stability is understood as the ability to regain an equilibrium state after being subjected to a physical disturbance. three quantities are important for power system operation: (i) angles of nodal voltages , also called power or load angles; (ii) frequency; and (iii)nodal voltage magnitudes . These quantities are especially important from the point of view of defining and classifying power system stability. Hence power system stability can be divided into: (i) rotor (