外文翻译---发电机和变压器.docx

附录一英文原文Generator And Transformer The turbine turns the rotor of the electric generator in whose stator are embedded three windings. In the process mechanical power from the turbine drive is converted to three phase alternating current at voltages in the range of 11kV to 30kV line to line at a frequency of 60 Hz in the United States. The voltage is usually stepped up by transmission to remote load centers.A generator (also called an alternator or synchronous generator)is shown in longitudinal cross section; the transverse across section is approximately round. The roctoe is called round or cylindrical or smooth. We note that steam-driven turbine-generators are usually two-pole or four-pole, turning at 3600 rpm or 1800 rmp, espectively, corresponding to 60Hz.The high speeds are needed to achieve high steam turbine efficiencies. At these rotation rates, high centrifugal forces limit rotor diameters to about 3.5 ft for two pole and 7 ft for four-pole machines.The average power ratings of the turbine-generator units we have been describing have been increasing,scince1960s, from about 300MW to about 600MW，with maximum sizes up to about 1300MW.Inceased ratings are accompained by increased rotor and stator size, and with rotor diameters limited by centrifugal forces, the rotor lengths have been increasing. Thus in the larger sizes, the rotor lengths may be five to six times the diameters. These slender rotors resonate at critical speeds below their rated speeds and care is requied in operation to avoid sustained operation at these speeds.A Transformer is a device that changes ac electric energy at one voltage level into ac electric energy at another voltage level through the action of a magnetic field. It consists of two or more coils of wire wrapped around a common ferro magnetic core. These coils are not directly connected. The only connection between the coils is the common magnetic flux present within the core.One of the transformer windings is connected to a source of ac electric power, and the second(and perhaps third)transformer winding supplies electric power to load. The transformer winding connected to the power source is called the primary winding or input winding, and the winding connected to the power source is called the primary winding or output winding. If there is a third winding on the transformer, it is called the tertiary winding.Power transformer is constructed on one of two types of cores. One type of construction consists of a simple rectangular laminated piece of steel with the transformer windings wrapped around two sides of the rectangle. This type of construction is known as core form. The other type consists of a three-legged laminated core with the windings wrapped around the center leg. This type of construction is known as shell form. In either case, the core is constructed of thin laminations electrically isolated from each other in order to reduce eddy currents to a minimn.The primary and secondary windings in a physical transformer are wrapped one on top of the other with low-voltage winding innermost. Such an arrangement serves two purpose: (1)It simplifies the problem of insulating the high-voltage winding from the core. (2)It results in much less leakAge flux than would be the two windings were separated by a distance on the core.Power transformers are given a variety of different names, depending on there use in power systems. A transformer connected to the output of a generator and used to step its voltage to transmission levels is sometimes called a unit transformer. The transformer at the other end of the transmission line, which steps the voltage down from transmission levels, is called a substation transformer. Finally, the transformer that takes his distribution levels, is called a distribution transformer. All these devices are essentially the same-the only difference among them is their intended use.In addition to the various power transformer, two-special purpose transformers are used with electric machinery and power systems. The first of these special transformers is a device specially designed to sample a high voltage and produce a low secondary voltage directly proportional to it. Such a transformer is called a potential transformer. A power transformer also produces a secondary voltage directly proportional to its primary voltage; the different between a potential transformer and a power transformer is that the potential transformer is designed to handle only a very small current. The second type of special transformer is a device designed to provide a secondary current much smaller than but directly proportional to its primary current. This device is called a current transformer.Transformers come in many sizes. Some power transformers are as big as a house. Electronic transformers, on the other hand, can be as small as a cube of sugar. All transformers have at least one coil; most have two although they may have many more.The usual purpose of transformers is to change the level of voltage. But sometimes they are used to isolate a load from the power source.Standard power transformers have two oils. These coils are labeled PRIMARY and SECONDARY. The primary coil is the one connected to the source. The secondary is the one connected to the load .There is no electrical connection between the primary and secondary. The secondary gets its voltage by induction.The only place where you will see a STEP-UP transformer is at the generating station. Typically, electricity is generated at 13,800 volts. It is stepped up to 345,000 volts for transmission. The next stop is the substation where it is stepped down to distribution levels, around 15,000 volts. Large substation transformers have cooling fins to keep them from overheating. Other transformers are located near points where the electric power is used.The coils of transformer are electrically are electrically insulated from each other. There is a magnetic link, however. The two coils are wound on the same core. Current in the primary magnetizes the core. This produces a magnetic field in the core. The core field then affects current in both primary and secondary.There are two main designs for cores:1.The CORE type has the core inside the windings.2.The SHELL type has the core outside.Smaller power transformers are usually of the core type. The very large transformers are of the shell type. There is no different in their operation, however.Coils are wound with copper wire. The resistance is kept as low as possible to keep losses low.Transformers are very efficient. The losses are often less than 3 percent. This allows us to assume that they are perfect in many computations.Perfect means that the wire has no resistance. It also means that there are no power losses in the core.Further, we assume that there is no flux leakAge. That is, all of the magnetic flux links all of the turns on each coil.To get an idea of just how small the losses are ,we can take a look at the EXCITATION CURRENT. Assume that nothing is connected to the secondary. If you apply rated voltage to the primary, a small current flows. Typically, this excitation current is less than 3 percent of rated current supplies the power lost in the core. Core losses are due to EDDY CURRENTS and HYSTERESIS.Eddy currents circulating in the core result from induction .The core is, after all, a conductor within a changing magnetic field.Hysteresis loss is caused by the energy used in lining up magnetic domains in the core. The alignment goes on continuously, first in one direction, then in the other.The other part of the excitation current magnetizes the core. It is this magnetizing current that supplies the “shuttle power”. Shuttle power is power stored in the magnetic field and returned to the source twice each cycle. Magnetizing current is quadrature with the applied voltage.Excitation current is made up of two parts. One part is in phase with the voltage. The losses that occur in real transformers have to be accounted for in any accurate model of transformer behavior. The major items to be considered in the construction shuttle such a model are.(i)Copper losses. Copper losses are the resistive heating losses in the primary and secondary windings of the transformer. They are proportional to there turn square of the current in the windings.(ii)Eddy current losses. Eddyysteresis loss is current losses are resistive heating losses in the core of the transformer.(iii)Hysteresis losses. These losses are associated with the rearrangement of the magnetic domains in the core during each half-cycle.(iv)LeakAge flux. The fluxes which escape the core and pass through only one of the transformer windings are leakAge fluxes. These escaped fluxes produce a self-inductance in the primary and secondary coils, and the effects of this inductance must be accounted for英文译文发电机和变压器汽轮机驱动发电机的转子，通过嵌在其定子槽内的三相绕组将输入的机械能转变为三相交流电能。其额定电压在美国从11kV到30kV不等，频率为60 Hz。为了提高远距离输电的效率，要用变压器将电压升高。发电机（也一般称交流发电机或同步发电机）的纵向剖面图，其横断面近似圆形，因此转子也称为圆筒转子。我们注意到汽轮发电机一般是两极或四极的，转速为3600r/min或1800r/min，分别与60Hz的频率相对应。为了获得汽轮机的高效率，需要有很高的转速，而由此产生的较高离心力又限制了转子的直径，两极的转子直径极限大约为3.5英尺，四极的大约为7英尺。上述这种汽轮发电机组的平均额定自然功率自1960年以来一直在提高，从大约300MW到600MW，最大达1300MW，额定功率的增加要求转子和定子的尺寸增大，而由于转子 直径受离心力的限制，其长度必须增加，可能增至其直径的5倍到6倍。这些细长的转子在低于其额定转速的某个临界速度下会发生谐振，因此操作中应避免持续运行在这种转速下。 变压器是通过磁场作用将交流电从某一电压等级转换至另一电压等级的设备。它由两个或多个绕在铁体上的绕组构成。通常，绕组之间不直接相连，他们是通过铁芯内部的主磁通相连接的。变压器的一个绕组与交流电源连接，第二个绕组（也许第三个绕组）为负载提供电功率。与电源连接的绕组称为一次绕组或输入绕组。与负载连接的绕组称为二次绕组或输出绕组。如果有第三个绕组，称之为第三绕组。变压器的铁芯分为两类。一类是由绕组缠绕在一个简单的矩形钢片叠成的铁芯两边而构成。此类结构的绕组称为铁芯式结构。另一类是由三个分支的钢片叠成，绕组绕在中间的一个分支上。此类结构称为框式结构。铁芯不论是芯式还是框式，都是由薄薄的铁芯片做成的。铁心片之间相互绝缘，以最大限度的降低涡流。实际的变压器中，一次绕组和二次绕组一个在另一个的外面，低压绕组在最里面。此类结构安排有两个目的：（1）使高压绕组与铁芯之间相互绝缘；（2）使漏磁通较二个绕组相互隔开时少的多。在电力系统中，根据不同的用途，电力变压器有许多种不同的名称。与发电机连接并将其电压提高到电网电压的变压器被称为升压变压器。在输电线另一端，将电网电压降至配电电压的变压器称为降压变压器。最后，把电压降低到能实际应用量级的变压器称为配电变压器。以上变压器的结构基本相同，唯一的区别在于各自的实际用途不同。除了上述多种变压器之外，在电机与电力系统中还使用两种特殊用途的变压器。第一种专门设计的变压器是用来采样电压，并产生一个低的二次电压，该电压与所采样的电压成正比。此类变压器称为电压互感器。功率变压器中产生的二次侧的电压也与一次侧的电压成正比。但电压互感器与电力变压器的不同在于电压互感器设计为仅处理较小的电流。第二种专门设计的变压器设计成为用来提供比一次侧的电流要小的多的二次侧电流，且使二次侧的电流与一次侧的电流成正比。此类装置称为电流互感器。变压器的大小不一，有