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电气专业毕业设计外文翻译 - -电力变压器工作原理

发布时间：2014-07-13 22:09:52 来源：文档文库

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Power Transformer Principles

1. INTRODUCTION

The 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 transformer 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 TRANSFORMERS

A transformer in its simplest form consists of two stationary coils coupled by a mutual magnetic flux. The 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 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. It sets up the flux φ 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 Lenz’s 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.

3. TRANSFORMER PRINCIPLES

When a sinusoidal voltage Vp is applied to the primary with the secondary open-circuited, there will be no energy transfer. The impressed voltage causes a small current Iθ 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 andword/media/image1.gifφm, where φm is the maximum 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 Iθ is usually few percent of the rated full-load current of the transformer (about 2 to 5%). 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 90º. It is readily seen that the current component Im= I0sinθ0, called the magnetizing current, is 90º in phase behind the primary voltage VP. It is this component that sets up the flux in the core; φ is therefore in phase with Im.

The second component, Ie=I0sinθ0, 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, or

word/media/image4.gif

It should be 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 flux set up by the primary winding，there will be an induced EMF E in the secondary winding in accordance with Faraday’s law, namely, word/media/image5.gif. 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 90º, therefore, they are 180º 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 sinusoidal; thus the induced quantities Ep and Es vary as a sine function. The average value of the induced voltage given by

Eavg = turns×word/media/image6.gif

which is Faraday’s law applied to a finite time interval. It follows that

Eavg = Nword/media/image7.gif = 4fNφm

which N is the number of turns on the winding. Form ac circuit theory, the effective or root-mean-square (rms) voltage for a sine wave is 1.11 times the average voltage; thus

E = 4.44fNφm

Since the same flux links with the primary and secondary windings, the voltage per turn in each winding is the same. Hence

Ep = 4.44fNpφm

and

Es = 4.44fNsφm

whereEp and Es are the number of turn on the primary 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 that

a = word/media/image8.gif = word/media/image9.gif

Assume that the output power of a transformer equals its input power, not a bad sumption in practice considering the high efficiencies. What we really are saying is that we are dealing with an ideal transformer; that is, it has no losses. Thus

Pm = Pout

or

VpIp × primary PF = VsIs × secondary PF

where PF is the power factor. For the above-stated assumption it means that the power factor on primary and secondary sides are equal; therefore

VpIp = VsIs

from which is obtained

word/media/image10.gif= word/media/image11.gif ≌word/media/image8.gif≌ a

It 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 raised 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 primary 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 transformation ratio a is greater than unity (a>1.0), while for a step-up transformer it is smaller than unity (a<1.0). In the event that a=1, the transformer secondary voltage equals the primary voltage. This is a special type of transformer used in instances where electrical isolation is required between the primary and secondary circuit while maintaining the same voltage level. Therefore, this transformer is generally knows as an isolation transformer.

As is apparent, it is the magnetic flux in the core that forms the connecting link between primary and secondary circuit. In section 4 it is shown how the primary winding current adjusts itself to the secondary load current when the transformer supplies a load.

Looking into the transformer terminals from the source, an impedance is seen which by definition equals Vp / Ip. From word/media/image10.gif = word/media/image11.gif ≌word/media/image8.gif≌ a , we have Vp = aVs and Ip = Is/a.In terms of Vs and Is the ratio of Vp to Ip is

word/media/image12.gif= word/media/image13.gif = word/media/image14.gif

But Vs / Isis the load impedance ZL thus we can say that

Zm (primary) = a2ZL

This equation tells us that when an impedance is connected to the secondary side, it appears from the source as an impedance having a magnitude that is a2 times its actual value. We say that the load impedance is reflected or referred to the primary. It is this property of transformers that is used in impedance-matching applications

电力变压器工作原理

1. 介绍

要从远端发电厂送出电能，必须应用高压输电。因为最终的负荷，在一些点高电压必须降低。变压器能使电力系统各个部分运行在电压不同的等级。本文我们讨论的原则和电力变压器的应用。

2. 双绕组变压器

变压器的最简单形式包括两个磁通相互耦合的固定线圈。两个线圈之所以相互耦合，是因为它们连接着共同的磁通。

在电力应用中，使用层式铁芯变压器(本文中提到的)。变压器是高效率的，因为它没有旋转损失，因此在电压等级转换的过程中，能量损失比较少。典型的效率范围在92到99%，上限值适用于大功率变压器。

从交流电源流入电流的一侧被称为变压器的一次侧绕组或者是原边。它在铁圈中建立了磁通φ，它的幅值和方向都会发生周期性的变化。磁通连接的第二个绕组被称为变压器的二次侧绕组或者是副边。磁通是变化的；因此依据楞次定律，电磁感应在二次侧产生了电压。变压器在原边接收电能的同时也在向副边所带的负荷输送电能。这就是变压器的作用。

3. 变压器的工作原理

当二次侧电路开路是，即使原边被施以正弦电压word/media/image15.gif，也是没有能量转移的。外加电压在一次侧绕组中产生一个小电流word/media/image16.gif。这个空载电流有两项

功能：（1）在铁芯中产生电磁通，该磁通在零和word/media/image1.gifword/media/image17.gif之间做正弦变化，word/media/image17.gif是铁芯磁通的最大值；（2）它的一个分量说明了铁芯中的涡流和磁滞损耗。这两种相关的损耗被称为铁芯损耗。

变压器空载电流word/media/image16.gif一般大约只有满载电流的2%—5%。因为在空载时，原边绕组中的铁芯相当于一个很大的电抗，空载电流的相位大约将滞后于原边电压相位90º。显然可见电流分量word/media/image18.gif= I0sinθ0，被称做励磁电流，它在相位上滞后于原边电压VP 90º。就是这个分量在铁芯中建立了磁通；因此磁通φ与word/media/image18.gif同相。

第二个分量Ie=I0sinθ0，与原边电压同相。这个电流分量向铁芯提供用于损耗的电流。两个相量的分量和代表空载电流，即

word/media/image4.gif

应注意的是空载电流是畸变和非正弦形的。这种情况是非线性铁芯材料造成的。

如果假定变压器中没有其他的电能损耗一次侧的感应电动势Ep和二次侧的感应电压Es可以表示出来。因为一次侧绕组中的磁通会通过二次绕组，依据法拉第电磁感应定律，二次侧绕组中将产生一个电动势E，即E=NΔφ/Δt。相同的磁通会通过原边自身，产生一个电动势Ep。正如前文中讨论到的，所产生的电压必定滞后于磁通90º，因此，它于施加的电压有180º的相位差。因为没有电流流过二次侧绕组，Es=Vs。一次侧空载电流很小，仅为满载电流的百分之几。因此原边电压很小，并且Vp的值近乎等于Ep。原边的电压和它产生的磁通波形是正弦形的；因此产生电动势Ep和Es的值是做正弦变化的。产生电压的平均值如下

Eavg = turns×word/media/image19.gif

即是法拉第定律在瞬时时间里的应用。它遵循

Eavg = Nword/media/image7.gif = 4fNφm

其中N是指线圈的匝数。从交流电原理可知，有效值是一个正弦波，其值为平均电压的1.11倍；因此

E = 4.44fNword/media/image17.gif

因为一次侧绕组和二次侧绕组的磁通相等，所以绕组中每匝的电压也相同。因此

Ep = 4.44fNpword/media/image17.gif

并且

Es = 4.44fNsword/media/image17.gif

其中Np和Es是一次侧绕组和二次侧绕组的匝数。一次侧和二次侧电压增长的比率称做变比。用字母a来表示这个比率，如下式

a = word/media/image8.gif = word/media/image9.gif

假设变压器输出电能等于其输入电能——这个假设适用于高效率的变压器。实际上我们是考虑一台理想状态下的变压器；这意味着它没有任何损耗。因此

Pm = Pout

或者

VpIp × primary PF = VsIs × secondary PF

这里PF代表功率因素。在上面公式中一次侧和二次侧的功率因素是相等的；因此

VpIp = VsIs

从上式我们可以得知

word/media/image10.gif= word/media/image11.gif ≌word/media/image8.gif≌ a

它表明端电压比等于匝数比，换句话说，一次侧和二次侧电流比与匝数比成反比。匝数比可以衡量二次侧电压相对于一次恻电压是升高或者是降低。为了计算电压，我们需要更多数据。

终端电压的比率变化有些根据负载和它的功率因素。实际上, 变比从标识牌数据获得, 列出在满载情况下原边和副边电压。

当副边电压Vs相对于原边电压减小时，这个变压器就叫做降压变压器。如果这个电压是升高的，它就是一个升压变压器。在一个降压变压器中传输变比a远大于1(a>1.0)，同样的，一个升压变压器的变比小于1(a<1.0)。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。

从电源侧来看变压器，其阻抗可认为等于Vp / Ip。从等式word/media/image10.gif= word/media/image11.gif ≌word/media/image8.gif≌ 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