毕业设计论文 外文文献翻译 某变电所毕业设计 变压器 中英文对照.doc

附录 1：外文资料翻译 A1.1 原文 TRANSFORMER1. INTRODUCTION The high-voltage transmission was need for the case electrical power is to beprovided at considerable distance from a generating station. At some point this highvoltage must be reduced because ultimately is must supply a load. The transformermakes it possible for various parts of a power system to operate at different voltagelevels. 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 amutual magnetic flux. The coils are said to be mutually coupled because they link acommon flux. In power applications laminated steel core transformers to which this paper isrestricted are used. because the rotational losses so transformers are efficient andrelatively 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 largerpower transformers. The current flowing in the coil connected to the ac source is called the primarywinding or simply the primary. It sets up the flux in the core which variesperiodically both in magnitude and direction. The flux links the second coil called thesecondary winding or simply secondary. The flux is changing therefore it induces avoltage in the secondary by electromagnetic induction in accordance with Lenzs law.Thus the primary receives its power from the source while the secondary supplies thispower to the load. This action is known as transformer action.3. TRANSFORMER PRINCIPLES When a sinusoidal voltage V p is applied to the primary with the secondaryopen-circuited there will be no energy transfer. The impressed voltage causes a smallcurrent I to flow in the primary winding. This no-load current has two functions: 1 itproduces the magnetic flux in the core which varies sinusoidally between zero and m where m is the maximum value of the core flux and 2 it provides a component toaccount for the hysteresis and eddy current losses in the core. There combined losses arenormally referred to as the core losses. The no-load current I is usually few percent of the rated full-load current of thetransformer about 2 to 5. Since at no-load the primary winding acts as a largereactance due to the iron core the no-load current will lag the primary voltage by nearly90. It is readily seen that the current component Im I 0 sin 0 called the magnetizingcurrent is 90 in phase behind the primary voltage V P . It is this component that sets upthe flux in the core is therefore in phase with I m . The second component I e I0 sin 0 is in phase with the primary voltage. It is thecurrent component that supplies the core losses. The phasor sum of these twocomponents represents the no-load current or I0 Im IeIt should be noted that the no-load current is distortes and nonsinusoidal. This is theresult of the nonlinear behavior of the core material. If it is assumed that there are no other losses in the transformer the inducedvoltage In the primary E p and that in the secondary E s can be shown. Since themagnetic flux set up by the primary winding，there will be an induced EMF E in thesecondary winding in accordance with Faradays law namely EN/t. This sameflux also links the primary itself inducing in it an EMF E p . As discussed earlier theinduced voltage must lag the flux by 90 therefore they are 180 out of phase with theapplied voltage. Since no current flows in the secondary winding E s V s . The no-loadprimary current I0 is small a few percent of full-load current. Thus the voltage in theprimary is small and V p is nearly equal to E p . The primary voltage and the resulting fluxare sinusoidal thus the induced quantities E p and E s vary as a sine function. Theaverage value of the induced voltage given by change in flux in a given time E avg turns× given timewhich is Faradays law applied to a finite time interval. It follows that 2 m E avg N 4fN m 1/2 f which N is the number of turns on the winding. Form ac circuit theory the effective orroot-mean-square rms voltage for a sine wave is 1.11 times the average voltage thus E 4.44fN mSince the same flux links with the primary and secondary windings the voltage per turn 2in each winding is the same. Hence E p 4.44fN p mand E s 4.44fN s mwhere E p and Es are the number of turn on the primary and secondary windingsrespectively. The ratio of primary to secondary induced voltage is called thetransformation ratio. Denoting this ratio by a it is seen that Ep Np a Es Ns Assume that the output power of a transformer equals its input power not a badsumption in practice considering the high efficiencies. What we really are saying is thatwe are dealing with an ideal transformer that is it has no losses. Thus P m P outor V p Ip ×PF1 V s I s ×PF2where PF is the power factor. For the above-stated assumption it means that the powerfactor on primary and secondary sides are equal therefore V p Ip V s I sfrom which is obtained Vp Ip Ep a Vs Is Es 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 turnsratio. The turns ratio gives a measure of how much the secondary voltage is raised orlowered in relation to the primary voltage. To calculate the voltage regulation we needmore information. The ratio of the terminal voltage varies somewhat depending on the load and itspower factor. In practice the transformation ratio is obtained from the nameplate datawhich list the primary and secondary voltage under full-load condition. When the secondary voltage V s is reduced compared to the primary voltage thetransformation is said to be a step-down transformer: conversely if this voltage is raisedit is called a step-up transformer. In a step-down transformer the transformation ratio ais greater than unity agt1.0 while for a step-up transformer it is smaller than unityalt1.0. In the event that a1 the transformer secondary voltage equals the primaryvoltage. This is a special type of transformer used in instances where electrical isolationis required between the primary and secondary circuit while maintaining the samevoltage 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 linkbetween primary and secondary circuit. In section 4 it is shown how the primarywinding current adjusts itself to the secondary load current when the transformersupplies a load. Looking into the transformer terminals from the source an impedance is seen Vp Ip Epwhich by definition equals V p / I p . From a we have V p aV s Vs Is Esand I p I s /a.In terms of V s and I s the ratio of V p to Ip is Vp aVs a 2Vs Ip Is / a IsBut V s / I s is the load impedance Z L thus we can say that Z m a2Z LThis equation tells us that when an impedance is connected to the secondary side itappears from the source as an impedance having a magnitude that is a2 times its actualvalue. We say that the load impedance is reflected or referred to the primary. It is thisproperty of transformers that is used in impedance-matching applications.4. TRANSFORMERS UNDER LOAD The primary and secondary voltages shown have similar polarities as indicated bythe “dot-making” convention. The dots near the upper ends of the windings have thesame meaning as in circuit theory the marked terminals have the same polarity. Thuswhen a load is connected to the secondary the instantaneous load current is in thedirection shown. In other words the polarity markings signify that when positivecurrent enters both windings at the marked terminals the MMFs of the two windingsadd. Since the secondary voltage depends on the core flux 0 it must be clear that theflux should not change appreciably if E s is to remain essentially constant under normalloading conditions. With the load connected a current I s will flow in the secondarycircuit because the induced EMF E s will act as a voltage source. The secondary currentproduces an MMF N s I s that creates a flux. This flux has such a direction that at anyinstant in time it opposes the main flux that created it in the first place. Of course this isLenzs law in action. Thus the MMF represented by N s Is tends to reduce the core flux 0 . This means that the flux linking the primary winding reduces and consequently the 4primary induced voltage E p This reduction in induced voltage causes a greaterdifference between the impressed voltage and the counter induced EMF therebyallowing more current to flow in the primary. The fact that primary current Ip increasesmeans that the two conditions stated earlier are fulfilled: 1 the power input increases tomatch the power output and 2 the primary MMF increases to offset the tendency ofthe secondary MMF to reduce the flux. In general it will be found that the transformer reacts almost instantaneously tokeep the resultant core flux essentially constant. Moreover the core flux 0 drops veryslightly between n o load and full load about 1 to 3 a necessary condition if E p is tofall sufficiently to allow an increase in I p . On the primary side I p is the current that flows in the primary to balance thedemagnetizing effect of I s . Its MMF N p I p sets up a flux linking the primary only. Sincethe core flux 0 remains constant. I 0 must be the same current that energizes thetransformer at no load. The primary current I p is therefore the sum of the current I p andI0 . Because the no-load current is relatively small it is correct to assume that theprimary ampere-turns equal the secondary ampere-turns since it is under this conditionthat the core flux is essentially constant. Thus we will assume that I0 is negligible as itis only a small component of the full-load current. When a current flows in the secondary winding the resulting MMF N s I s creates aseparate flux apart from the flux 0 produced by I 0 which links the secondary windingonly. This flux does no link with the primary winding and is therefore not a mutual flux. In addition the load current that flows through the primary winding creates a fluxthat links with the primary winding only it is called the primary leakage flux. Thesecondary- leakage flux gives rise to an induced voltage that is not counter balanced byan equivalent induced voltage in the primary. Similarly the voltage induced in theprimary is not counterbalanced in the secondary winding. Consequently these twoinduced voltages behave like voltage drops generally called leakage reactance voltagedrops. Furthermore each winding has some resistance which produces a resistivevoltage drop. When taken into account these additional voltage drops would completethe equivalent circuit diagram of a practical transformer. Note that the magnetizingbranch is shown in this circuit which for our purposes will be disregarded. This followsour earlier assumption that the no-load current is assumed negligible in our calculations.This is further justified in that it is rarely necessary to predict transformer performanceto such accuracies. Since the voltage drops are all directly proportional to the loadcurrent it means that at no-load conditions there will be no voltage drops in eitherwinding. 6 A1.2 译文 变压器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。本文我们讨论的电力变压器的原理和应用。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 ，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 当二次侧电路开路时，即使原边被施以正弦电压V p ，也是没有能量转移的。 （1）在铁外加电压在一次侧绕组中产生一个小电流I 。这个空载电流有两项功能：芯中产生电磁通，该磁通在零和 m 之间做正弦变化， m 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I 一般大约只有满载电流的 25。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90。显然可见电流分量I m I 0 sin 0 ，被称做励磁电流，它在相位上滞后于原边电压V P 90。就是这个分量在铁芯中建立了磁通；因此磁通与Im 同相。 第二个分量I e I 0 sin 0 ，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?I0 Im Ie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E p 和二次侧的感应电压E s 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EN/t。相同的磁通会通过原边自身，产生一个电动势E p 。正如前文中讨论到的，所产生的电压必定滞后于磁通 90，因此，它与施加的电压有 180的相位差。因为没有电流流过二次侧绕组，E s V s 。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V p 的值近乎等于E p 。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E p 和E s 的值是做正弦变化的。产生电压的平均值如下 给定时间内磁通变化量 E avg n× 给定时间即是法拉第定律在瞬时时间里的应用。它遵循 2 m E avg N 4fN m 1/2 f 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 4.44fN m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 E p 4.44fN p m并且 E s 4.44fN s m其中N p 和E s 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 Ep Np a .