英文翻译1

Consider the situation shown in Fig.1.The rate of electrical energy flow(power)from network A to network B is

fig1 Power transmission between two networks

Lowercase letters are used to indicate instantaneous valuesm, that is, that p, v, and I may vary with time. High power levels require high voltage and current values. For a given value of current, higher power flows may be obtained by increasing the voltage, and vice versa. Unfortunately, the existing technology sets practical upper limits onm allowable currents and voltages.

What are the limiting factors for current? We fabricate power conductors using materials with high conductivity, appropriate mechanical characteristics, and that are economical: aluminum is the most common choice, with copper used for some applications. The current-carrying capacity of a conductor is related to its maximum allowable current density and its cross=sectional area:

ImaxJmaxA

The maximum current density Jmaxis determined by the maximum conductor temperature that will not damage the conductor or its insulatinon system.

What are the limiting factors on voltage? The vundamental consideration is to provide electrical isolation(or insulation)between adjacent parts that can conduct current-that is, to confine current to the paths through which it waw intended to flow. When the voltage exceeds the breakdown strength for a given insulation system, undesirable conduction paths will be created and the system will be created and the system whll be either temporarily or permanently disabled. Fluid insulation tends to be “self-healing”(the system will recover from a breakdown if it is de-energized for a short time and then re-energized), whereas solid insulation is permanently damaged

by a breakdown.

The meaning lf “ground” is important; we quote from the IEEE Standard Dictionary of Electrical and Electronic Terms:

ground (earth)(electric system). A conducting connection, whether intentional or accidental, by which an electric circuit or equipment is connected to the earth, or to some conducting body of relatively large extent that serves in place of the earth. Note: It is used for establishing and maintaining the potential of the earth (or of the conducting body) or approximately that potential, on conductors connected to it ,and for conducting ground current to and from the earth(or the conducting body).

We understand this to mean that at a given location in the power system, accessible parts of power apparatus and earth constitute an equipotential surface when perfectly grounded. Insulation of conductors from ground is a basic problem.

Let us consider some different schemes for implementing the transmission line indicated in Figure 1. For a fair comparison we select constrainst that all schemes must satisfy we allow any number of conductors to be used ,as long as each scheme uses the same amount of conducting material, Given that networks A and B are separated by a fixed physical length this means that in viewing the limes in cross section we must observe the same cross-sectional conducting area (A) for all schemes. Also, we argue that no conductor shall carry current greater than that constrained by some maximum current density J0.

We require that at least one conductor be grounded and shall refer to such a conductor as the neutral, designated as “n”. If it is not required to conduct any appreciable current, we will not include its cross-sectional area in A. This condition is achieved under certain symmetrical loading conditions, referred to as “balanced” loading ,and can be maintained in a practical situation; therefore we allow all schemes to ground exceedV0.It is assumed that the reader has a background in basic circuit theory. The adjective “dc”, essentially means time invariant or constant with time. Recall that the term“ac”, which historically stood for “alternating current “, in modern usage means “sinusoidal steady state.”These terms are used to describe voltages and currents in time invariant (constant) steady-state and sinusoidal steady-state modes.

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 ofsugar. 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 ghey are used to isolate a load from the power source.

TYPES OF TRANSFORMERS

Standard power transformers have two coils. These coils are labeled PRIMARY and SECONDARY. The primary coil is the one connected to the source. The secondary is the one connected to 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 satation. 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 poinst where the electric power is used.

TRANSFORMER CONSTRUCTION

The coils of transformer are electrically insulated from 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 difference in their operation,however.

Coils are wound with copper wire. The resistance is kept as low as possible to keep losses low.

IDEALIZED TRANSFORMERS

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.

EXCITATION CURRENT

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.

Excitation current is made up of two parts.One part is in phase with the voltage. This is the current that 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 is power stored in the magnetic field and returned to the source twice each cycle.Magnetizing current is quadrature(90 degrees out of phase) with the applied voltage.

电能输送

考虑图1所示的情况。从网络A送到网络B的功率为：公式

图1 电力系统网络图

小写字母表示瞬时值，即p,v和i会随时间而变化。传输大容量功率需要高电压和大电流。对于一给定电流值，升高电压可以提高传输容量，反之亦然。不幸的是，现有技术使额定电流和额定电压存在上限。

电流的因素是什么?q 我们用具体有高导电性和适当机械性能的材料制造导体，从经济方面考虑：通常选用铝，有时也选用铜。导体传导电流的大小与其最大允许电流密度和截面积有关：ImaxJmaxA。

最大电流密度Jmax取决于不损坏导体及其绝缘系统所允许的最大导体温度。 电压的因素是什么？基本考虑是在可以传导电流的相邻部件间提供电气隔离？（或绝缘）即：使电流在规定的路径上流通。当电压超过所给绝缘系统的击穿电压值时，电流将通过不希望导通的路径使系统暂时或永久损坏。

液体绝缘物能“自恢复”（如果短时去激励后又重激励，系统会同击穿状态恢复到绝缘状态），而固体绝缘物击穿后是永久性损坏。

“接地”的意义是重要的；从IEEE电气和电子标准术语词典中可得到解释：接地（电力系统）是一种导体联接形式，导体无论是有意或无意通过电路或设备连接到地上，或连接到可作为地用途的相当大的导体上。

说明：接地用于建立或维持地的电位，（或导体电位）或所连的导体对地的大概电位，并用以导通流入和流出大地（或导体）的电流。

我们对此的理解是电力系统的给定位置，当接地良好时，电力仪表的可触及部分和地之间构成一个等势面。导体与地之间的绝缘是一个基本问题。

让我们考虑一些不同类型作为对图1所示办输电线路的补充。为了公平比较，我们选择所有类型都满足的条件，并允许使用任何数量的导体，只要每种类型都使用等量的导体原料。假设用固定的物理距离将网络A和网络B分隔

开，这意味着在察看线路截面积时我们必须观察所有类型同样的导线截面积（A）。此外，可以说所有导体传导的电流均不大于其电大电流密度J0。

至少需要一个导体接地并称该导体为中性点，记为“n”。若中性点无需传导任何电流，则不需要截面积A。这种情况在某种称为“平衡”负荷的对称负荷条件下出现，也可在实际情况下得到。因此允许对所有类型作这种假设。同时要求对于所有的类型其对地电压不超过V0。

假定读者已具有电路理论的基本知识。“dc”主要含义是时间不变的或不随时间变化。回想一下“ac”，以往代表“交流电流”，现在用于表示“正弦稳定状态”。这些术语用来描述电压和电流随时间是恒定不变的稳定状态和正弦稳定状态。

变压器大小不一。有的变压器像房子一样大。而电子变压器可能小如糖块。所有的变压器至少有一只线圈；大多数变压器具有两个或两个以上的线圈。

通常使用变压器的目的是改变电压等级，但有时也用于隔离电源和负荷。 变压器的类型

标准电力变压器有两只线圈。这些线圈标记为一次绕组（一次侧）和二次绕组（二次侧）。一次绕组连接电源，二次绕组连接负荷。在一次和二次绕组间没有电的连接。二次绕组通过感应获得电压。

大概只有在发电厂才能见到升压变压器。典型的发电机出口电压为13800V，升压至345000V用来输送。下一站是变电所，把电压降到配电电压等级，约15000V。大型的变电所变压器具有冷却叶片，以防止变压器过热。有的变压器安装的位置靠近用户。

变压器的结构

变压器的线圈之间是绝缘的，但存在磁的联系。两只线圈绕在同一个铁芯上。一次绕组电流使铁芯磁化，并在铁芯中产生磁场。铁芯磁场影响一次和二次绕组的电流。

铁芯主要有两种方式： 1.芯型。铁芯在线圈内部。 2.壳型。铁芯在线圈外部。

小型电力变压器通常为芯型。特大型变压器为壳型，但是在运行时二者并无区别。线圈用铜线绕制而成。电阻应尽可能的小，从而保持低损耗。

理想变压器

变压器效率很高，损耗通常低于3%。因此这使我们假定它在计算时是无损耗的。

无损耗意味着线圈不没有电阻，并且铁芯中没有能量损耗。进一步可以假定没有漏磁，即所有的磁力线穿过线圈的每一匝。

励磁电流

欲知损耗的大小，可以看一下励磁电流。假定二次绕组空载，在一次绕组施加额定电压后会产生一较小电流。典型情况是该励磁电流小于额定电流的3%。

励磁电流由两部分组成。一部分与电压同相，该电流供给铁芯中的能量损耗。铁芯损耗的原因是涡流和磁滞。

涡流电流在铁芯中旋转，产生感应现象，因为铁芯毕竟是变化磁场的导体。 磁滞损耗是由于用来排列铁芯中的磁畴需要能量。这种排列调整是连续不断的，开始是一个方向，然后是另一个方向。

另一部分励磁电流磁化铁芯。磁化电流供给“往复能量”，往复能量就是储存在磁场中的能量，并在每个周期内返回电源两次。磁化电流与所加电压正交（相位差为90度）。