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外文翻译--变压器的类型及结构

2018-01-08 10页 doc 32KB 21阅读

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外文翻译--变压器的类型及结构外文翻译--变压器的类型及结构 Types and Construction of Transformer A Transformer is a device that changes ac electric energy at one voltage level into ac e1ectric energy at another vo1tage level through the action of a ma8netic field(It consists of two or more coil of wire wr...
外文翻译--变压器的类型及结构
外文翻译--变压器的类型及结构 Types and Construction of Transformer A Transformer is a device that changes ac electric energy at one voltage level into ac e1ectric energy at another vo1tage level through the action of a ma8netic field(It consists of two or more coil of wire wrapped around a common ferromagnetic ually) not directly connected(The only connection between core(These coils are (us 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 loads(The transformer winding connected to the power source is called the primary winding or input winding,and the winding connected to the loads is called the secondary winding or output 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 rectagu1arlaminated 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 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 serves two purposes: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 their use in power systems. A transformer connected to the output of a generator and used to step its vo1tage up to transmission levels is sometimes called a unit transformer(The transformer at the other end of the transmission line,which steps the voltage down 1 from transmission levels to distribution levels is called a substation transformer. Finally,the transformer that takes the distribution voltage and steps it down to the final vo1tage at which the power is actually used is called a distribution transformer(A11 these devices are essentially the same the only difference among them is their intended use( In addition to the various power transformers,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 vo1tage and produce a low secondary vo1tage directly proportional to it. Such a transformer is called a potential transformer(A power transformer also produces a secondary vo1tage directly proportional to its primary voltage; the difference 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. The Directional Protection Basis Early attempts to improve power-service reliability to loads remote from generation led to the dual-1ine concept(Of course,it is possible to build two 1ines to a load,and switch the load to whichever line remains energized after a di51urbance(But better service continuity will be available if both lines normally feed the load and only the faulted line is tripped when disturbances occur(Fig.1-l shows a sing1e-generator, two-1ine,single-load system with breakers properly arranged to supply the load when one line is faulted(For the arrangement to be effective it is necessary to have the proper relay application(Otherwise,the expensive power equipment will not be able to perform as p1anned(Consider the application of instantaneous and/or time delay relays on the four breakers.Obvious1y the type of the relay cannot coordinate for a11 1ine faults(For example, a fault on the line terminals of breaker D.D tripping should be faster than B, however , the condition reverses and B should be faster than D. It is evident that the relay protection engineer must find some characteristic other than time delay if relay coordination is to be achieved( 2 The magnitude of the fault current through breakers B and D is the same,regardless of the location of the fault on the line terminal of breaker B or D(Therefore relay coordination must be based on characteristics other than a time delay that starts from the time of the fault. Observe that the direction of current flowing through either breaker B or D is a function of which line the fault is on. Thus for a fault on the line between A and B, the current flows out of the load bus through breaker B toward the fault(At breaker D the current flows toward the load bus through breaker D(In this case breaker B should trip, but breaker D should not trip(This can be accomplished by installing directional relays on breakers B and D that are connected in such a way that they will trip only when current flows through them in a direction away from the load bus. Relay coordination for the system shown in Fig(1-l can now be achieved by the installation of directional over current time delay relays on breakers B and D(Breakers A and C can have non directional over current time delay relays(They may also now have instantaneous relays applied(The relays would be set as follows: The directional relays could be set with no intentional time delay. They will have inherent time delay(The time delay over current relays on breakers A and C would have current settings that would permit them to supply backup protection for faults on the load bus and load equipment faults(The instantaneous elements on breakers A and C would have current settings that would not permit them to detect faults on the load bus(Thus the lines between the generator and the load would have high-speed protection over a considerable portion of their length(It should be observed that faults on the line terminals of breakers A and C can co11apse the generator vo1tage(The instantaneous relays on breakers A or C cannot clear the circuit instantaneously,because it takes time for power equipment to operate(During this period there will be little or no current flow through breakers B and D(Therefore,B or D cannot operate for this fault condition until the appropriate breaker at the generating station has operated(This is known as sequential tripping. Usually, it is acceptable under such conditions( 3 A B Load G C D Fig.1-1 Direction of current flow on an a. c. system is determined by comparing the current In the systems of vector with some other reference vector,such as a voltage vector( Fig.1-1 the reference vo1tage vector would be derived from the voltages on the load bus(Direction of current or power flow cannot be determined instantaneously on a(c(systems whose lines and equipment contain reactance(This is apparent from the fact that when vo1tage exists, the lagging current can be plus or minus or zero, depending on the instant sampled in the voltage cycle(According1y,the vector quantities must be sampled over a time period(The time period for reasonably accurate sampling may be from one-half to one cycle(Work is proceeding on shorter sampling periods where predicting circuits are added to the relay to attempt to establish what the vectors will be at some future time(The process is complex, because it must make predictions during the time when electrical transients exist on the system(Usually, the shorter the time a11owed for determining direction,the less reliable will be the determination. 变压器的类型及结构 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氧体上的绕组构成(通常,绕组之间不直接相连,它 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 4 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。铁芯片之间相互绝缘,以最大限度地降低涡流。 在实际的变压器中,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。此类结构安排有二个目的:?使高压绕组与铁芯之间相互绝缘;?使漏磁通较二个绕组相互隔开时少得多。 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。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比例。此类装置称为电流互感器。 方向保护基础 早期,对于远离发电站的用户,为改善其供电可靠性提出了双回线供电的设想。当然,也可以架设不同的两回线给用户供电。在系统发生故障后,把用户切换至任一条正常的线路。但更好的连续供电方式是正常以双回线同时供电。当发生故障时,只断开故障线。图1—1所示为一个单电源、单负载、双回输电线系统。对该系统配置合适的断路器后,当一回线发生故随时,仍可对负载供电。为使这种供电方式更为有效,还需配置合适的继电保护系统,否则,昂贵的电力设备不能发挥其预期的作用。可以考虑在四个断路器上装设瞬时和延时起动继电器。显然,这种类型的继电器无法对所有线路故障进行协调配合。例如,故障点在靠近断路器D的线路端,D跳闸应比B快,反之,B应比D快。显然,如果要想使继电器配合协调,继电保护工程师必须寻求除了延时以外的其他途径。 5 无论故障点靠近断路器B或D的哪一端,流过断路器B和D的故障电流大小是相同的。因此继电保护的配合必须以此为基础,而不是放在从故障开始启动的延时上。我们观察通过断路器B或D的电流方向是随故障点发生在哪一条线路上变化的。对于A和B之间的线路上的故障,通过断路器B的电流方向为从负载母 ,电流通过断路器流向负载母线。在这种情况下,线流向故障点。对于断路器D 断路器B应跳闸,D不应跳闸。要达到这个目的,我们可在断路器B和D上装设方向继电器,该方向继电器的联接应保证只有当通过它们的电流方向为离开母线时才起动。 A B 负载 G C D 图1-1 对于图14—1所示的系统,在断路器B和D装设了方向过流延时继电器后,继电器的配合才能实现。断路器A和C装设无方向的过流延时继电器及瞬时动作的电流继电器。各个继电器整定配合如下:方向继电器不能设置延时,它们只有本身固有的动作时间。A和C的延时过流继电器通过电流整定使它们作为负载母线或负载设备故障的后备保护。断路器A和C的瞬时动作元件通过电流整定使它们在负载母线故障时不动作。于是快速保护可以保护发电机和负载之间线路长度的大部分。从图中我们还可看到,在断路器A或C的线路侧发生的故障使发电机电压崩溃,在断路器A和C上的瞬时继电器不能真正瞬时切除故陈,因为电力设备动作需要时间,在这个期间内,流过断路器B和D的电流很小甚至为0,因此在这种故障状态下,只有等到发电厂有关的断路器动作后,断路器B和D才动作。这就是我们所说的顺序跳闸,通常在上述情况下这样做是允许的。 在一个交流系统中,通过电流矢量与其他参考矢量(例如电压矢量)的比较,可以确定电流的方向。图1—1所示系统的参考矢量可从负载母线电压矢量推出。由于在该交流系统中,线路和设备含有电抗,电流和功率的瞬时方向不能确定,这是显而易见的,因为当有电压时,相位落后的电流取样的瞬时值取决于它在电压周期中的碑间,可能为正,也可能为负或为零。因此,电压、电流失量必须在 6 一个时间间隔内采样。为了较为准确的采样,时间间隔可从一个半周期到一个周期。目前正在进行更短时间的采样的研究工作。这个研究工作是给继电器加上一个预切电路,试图以此确定未来时间内矢量的情况。由于要在电力系统电磁暂态过程中预测,这项工作比较复杂。通常用于判断方向的时间越短,所做判断的可靠性越差。 7
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