外文文献及翻译

1、山东建筑大学毕业设计外文文献及翻译PAGE 17 -外文文献：TRANSFORMER1. INTRODUCTIONThe high-voltage transmission was need for the case electrical power is to be provided at considerable distance from a generating station. At some point this high voltage must be reduced, because ultimately is must supply a load. The transform

2、er makes it possible for various parts of a power system to operate at different voltage levels. In this paper we discuss power transformer principles and applications.2. TOW-WINDING TRANSFORMERSA transformer in its simplest form consists of two stationary coils coupled by a mutual magnetic flux. Th

3、e coils are said to be mutually coupled because they link a common flux.In power applications, laminated steel core transformers (to which this paper is restricted) are used. Transformers are efficient because the rotational losses normally associated with rotating machine are absent, so relatively

4、little power is lost when transforming power from one voltage level to another. Typical efficiencies are in the range 92 to 99%, the higher values applying to the larger power transformers.The current flowing in the coil connected to the ac source is called the primary winding or simply the primary.

5、 It sets up the flux in the core, which varies periodically both in magnitude and direction. The flux links the second coil, called the secondary winding or simply secondary. The flux is changing; therefore, it induces a voltage in the secondary by electromagnetic induction in accordance with Lenzs

6、law. Thus the primary receives its power from the source while the secondary supplies this power to the load. This action is known as transformer action.A transformer is a device that alternating current electric energy at one voltage level into alternating current electric energy at another voltage

7、 level through the action of a magnetic field.It consists of two or more coils wire wrapped around a common ferromagnetic core.These coils are (usually)not directly connected. The only connection between the coils is the common magnectic flux presen within the core.One of the transformer windings is

8、 connected to a source of ac electric power,and the second(and perhaps third) transformer winding supplies electric power to loads. the transformer winding connected to the power souce is called the primary winding or input winding.and the winding connected to the loads is called the secondary windi

9、ng or input winding.If there is a third winding on the transformer,it is called the tertiary winding.Power transformer are 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 side

10、s of the rectangle.This type of construction is know as coreform .The other type consists of three-legged laminated core with the windings wrapped around the center leg .This type of construction is know as shell form.In either case,the core is constructed of thin laminations electrically isolated f

11、orm each other in order in order to reduce eddy currents to a minimum.The primary and secondary windings in a physical transformer are wrapped one on top of the other with the low-voltage winding innermost.Such an arrangement severs two purposes: 1.It simplifies the problem of insulating the high- v

12、oltage 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.ower transformer are given a variety of different names, depending on their use in power systems.A transformer connected to the output of a generator and used t

13、o step its voltage up to transformer levels is sometimes called unit transformer. The transformer ai the other end of the transformer line,which steps the voltage down from transmission levels to distribution levels,is called a substation transformer.Finally,the transformer that takes the distributi

14、on voltage and steps is down to the final voltage ai which the power is actually used is called a distribution transformer.All these devices are essentially the same-the only difference among them is their intended use.3. TRANSFORMER PRINCIPLESWhen a sinusoidal voltage Vp is applied to the primary w

15、ith the secondary open-circuited, there will be no energy transfer. The impressed voltage causes a small current I to flow in the primary winding. This no-load current has two functions: (1) it produces the magnetic flux in the core, which varies sinusoidally between zero and m, where m is the maxim

16、um value of the core flux; and (2) it provides a component to account for the hysteresis and eddy current losses in the core. There combined losses are normally referred to as the core losses.The no-load current I is usually few percent of the rated full-load current of the transformer (about 2 to 5

17、%). Since at no-load the primary winding acts as a large reactance due to the iron core, the no-load current will lag the primary voltage by nearly 90. It is readily seen that the current component Im= I0sin0, called the magnetizing current, is 90 in phase behind the primary voltage VP. It is this c

18、omponent that sets up the flux in the core; is therefore in phase with Im.The second component, Ie=I0sin0, is in phase with the primary voltage. It is the current component that supplies the core losses. The phasor sum of these two components represents the no-load current, orI0 = Im+ IeIt should be

19、 noted that the no-load current is distortes and nonsinusoidal. This is the result of the nonlinear behavior of the core material.If it is assumed that there are no other losses in the transformer, the induced voltage In the primary, Ep and that in the secondary, Es can be shown. Since the magnetic

20、flux set up by the primary winding，there will be an induced EMF E in the secondary winding in accordance with Faradays law, namely, E=N/t. This same flux also links the primary itself, inducing in it an EMF, Ep. As discussed earlier, the induced voltage must lag the flux by 90, therefore, they are 1

21、80 out of phase with the applied voltage. Since no current flows in the secondary winding, Es=Vs. The no-load primary current I0 is small, a few percent of full-load current. Thus the voltage in the primary is small and Vp is nearly equal to Ep. The primary voltage and the resulting flux are sinusoi

22、dal; thus the induced quantities Ep and Es vary as a sine function. The average value of the induced voltage given byEavg = turnswhich is Faradays law applied to a finite time interval. It follows thatEavg = N = 4fNmwhich N is the number of turns on the winding. Form ac circuit theory, the effective

23、 or root-mean-square (rms) voltage for a sine wave is 1.11 times the average voltage; thusE = 4.44fNmSince the same flux links with the primary and secondary windings, the voltage per turn in each winding is the same. HenceEp = 4.44fNpmandEs = 4.44fNsmwhere Ep and Es are the number of turn on the pr

24、imary and secondary windings, respectively. The ratio of primary to secondary induced voltage is called the transformation ratio. Denoting this ratio by a, it is seen thata = = Assume that the output power of a transformer equals its input power, not a bad sumption in practice considering the high e

25、fficiencies. What we really are saying is that we are dealing with an ideal transformer; that is, it has no losses. ThusPm = PoutorVpIp primary PF = VsIs secondary PFwhere PF is the power factor. For the above-stated assumption it means that the power factor on primary and secondary sides are equal;

26、 thereforeVpIp = VsIsfrom which is obtained = aIt shows that as an approximation the terminal voltage ratio equals the turns ratio. The primary and secondary current, on the other hand, are inversely related to the turns ratio. The turns ratio gives a measure of how much the secondary voltage is rai

27、sed or lowered in relation to the primary voltage. To calculate the voltage regulation, we need more information.The ratio of the terminal voltage varies somewhat depending on the load and its power factor. In practice, the transformation ratio is obtained from the nameplate data, which list the pri

28、mary and secondary voltage under full-load condition.When the secondary voltage Vs is reduced compared to the primary voltage, the transformation is said to be a step-down transformer: conversely, if this voltage is raised, it is called a step-up transformer. In a step-down transformer the transform

29、ation ratio a is greater than unity (a1.0), while for a step-up transformer it is smaller than unity (a1.0)，同样的，一个升压变压器的变比小于1(a1.0)。当a=1时，变压器的二次侧电压就等于起一次侧电压。这是一种特殊类型的变压器，可被应用于当一次侧和二次侧需要相互绝缘以维持相同的电压等级的状况下。因此，我们把这种类型的变压器称为绝缘型变压器。显然，铁芯中的电磁通形成了连接原边和副边的回路。在第四部分我们会了解到当变压器带负荷运行时一次侧绕组电流是如何随着二次侧负荷电流变化而变化的。从电

30、源侧来看变压器，其阻抗可认为等于Vp / Ip。从等式 = a中我们可知Vp = aVs并且Ip = Is/a。根据Vs和Is，可得Vp和Ip的比例是 = = 但是Vs / Is 负荷阻抗ZL，因此我们可以这样表示Zm (primary) = a2ZL这个等式表明二次侧连接的阻抗折算到电源侧，其值为原来的a2倍。我们把这种折算方式称为负载阻抗向一次侧的折算。这个公式应用于变压器的阻抗匹配。4. 有载情况下的变压器一次侧电压和二次侧电压有着相同的极性，一般习惯上用点记号表示。如果点号同在线圈的上端，就意味着它们的极性相同。因此当二次侧连接着一个负载时，在瞬间就有一个负荷电流沿着这个方向产生。换句

31、话说，极性的标注可以表明当电流流过两侧的线圈时，线圈中的磁动势会增加。因为二次侧电压的大小取决于铁芯磁通大小0，所以很显然当正常情况下负载电势Es没有变化时，二次电压也不会有明显的变化。当变压器带负荷运行时，将有电流Is流过二次侧，因为Es产生的感应电动势相当于一个电压源。二次侧电流产生的磁动势NsIs会产生一个励磁。这个磁通的方向在任何一个时刻都和主磁通反向。当然，这是楞次定律的体现。因此，NsIs所产生的磁动势会使主磁通0减小。这意味着一次侧线圈中的磁通减少，因而它的电压Ep将会增大。感应电压的减小将使外施电压和感应电动势之间的差值更大，它将使初级线圈中流过更大的电流。初级线圈中的电流Ip

32、的增大，意味着前面所说明的两个条件都满足：（1）输出功率将随着输出功率的增加而增加（2）初级线圈中的磁动势将增加，以此来抵消二次侧中的磁动势减小磁通的趋势。总的来说，变压器为了保持磁通是常数，对磁通变化的响应是瞬时的。更重要的是，在空载和满载时，主磁通0的降落是很少的（一般在）1至3%。其需要的条件是E降落很多来使电流Ip增加。在一次侧，电流Ip在一次侧流过以平衡Is产生的影响。它的磁动势NpIp只停留在一次侧。因为铁芯的磁通0保持不变，变压器空载时空载电流I0必定会为其提供能量。故一次侧电流Ip是电流Ip与I0因为空载电流相对较小，那么一次侧的安匝数与二次侧的安匝数相等的假设是成立的。因为在

33、这种状况下铁芯的磁通是恒定的。因此我们仍旧可以认定空载电流I0相对于满载电流是极其小的。当一个电流流过二次侧绕组，它的磁动势（NsIs）将产生一个磁通，于空载电流I0产生的磁通0不同，它只停留在二次侧绕组中。因为这个磁通不流过一次侧绕组，所以它不是一个公共磁通。另外，流过一次侧绕组的负载电流只在一次侧绕组中产生磁通，这个磁通被称为一次侧的漏磁。二次侧漏磁将使电压增大以保持两侧电压的平衡。一次侧漏磁也一样。因此，这两个增大的电压具有电压降的性质，总称为漏电抗电压降。另外，两侧绕组同样具有阻抗，这也将产生一个电阻压降。把这些附加的电压降也考虑在内，这样一个实际的变压器的等值电路图就完成了。由于分支

34、励磁体现在电流里，为了分析我们可以将它忽略。这就符我们前面计算中可以忽略空载电流的假设。这证明了它对我们分析变压器时所产生的影响微乎其微。因为电压降与负载电流成比例关系，这就意味着空载情况下一次侧和二次侧绕组的电压降都为零。5.关于处理变压器短路事故的几点思考.处理变压器短路事故，首先要通过检查、试验找出问题实质所在；其次处理过程还应 注意相关问题。具体思考如下：首先，变压器短路事故后的检查、试验。变压器在遭受突发短路时，高低压侧都将受很大的短路电流，在断路器来不及断开的很短时间内， 短路电流产生与电流平方成正比的电动力将作用于变压器的绕组，此电动力可 分为辐向力和轴向力。在短路时，作用在绕组

35、上的辐向力将使高压绕组受到张力，低压绕组 受到压力。由于绕组为圆形，圆形物体受压力比受张力更容易变形，因此，低压绕组更易变 形。 在突发短路时产生的轴向力使绕组压缩和使高低压绕组发生轴向位移， 轴向力也作用于 铁芯和夹件。因此，变压器在遭受突发短路时，最容易发生变形的是低压绕组和平衡绕组，然后是 高中压绕组、铁芯和夹件。因此，变压器短路事故后的检查主要是检查绕组、铁芯、夹件以 及其它部位。绕组的检查与试验由于变压器短路时，在电动力作用下，绕组同时受到压、拉、弯曲等多种力的作用， 其造成的故障隐蔽性较强， 也是不容易检查和修复的， 所以短路故障后应重点检查绕组情况。变压器直流电阻的测量根据变压器直流电阻的测量值来检查绕组的直流电阻不平衡率及与以往测量值相比 较，能有效地考察变压器绕组受损情况。例如，某台变压器短路事故后低压侧C向直流电阻增加了约 10，由此判断绕组可能有新股情况，最后将绕组吊出检查，发现C相绕组断 1 股。（2）变压器绕组电容量的测量。绕组的电容由绕组匝间、层