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什么是電能的傳輸?

已解決 懸賞分:0 - 解決時間 2011-09-11 13:14
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Electrical Energy Transmission ⑴Consider the situation shown in Figure1-1. The rate of electrical energy flow (power ) from network A to network B is p=vi Network A ---i, v---Network B ( Figure 1-1 Power transmission between two networks.) ⑵Lowercase letters are used to indicate instantaneous values, 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 on allowable currents and voltages. ⑶What 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= Jmax A ⑷The maximum current density Jmax is determined by the maximum conductor temperature that will not damage the conductor&nbsp;or&nbsp;its insulation system. ⑸What are the limiting factors on voltage? The fundamental 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 was 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 either temporarily&nbsp;or&nbsp;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. ⑹The meaning of "ground" is important I we quote from the IEEE standard Dictionary of Electrical and Electronic Terms: ground (earth ) (electric system). A conducting connection, whether Intentional&nbsp;or&nbsp;accidental, by which an electric circuit&nbsp;or&nbsp;equipment is connected to the earth,&nbsp;or&nbsp;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 constituted an equipotential surface when perfectly grounded. Insulation of conductors&#39; from ground is a basic problem. ⑺Let us consider some different schemes for implementing the transmission line indicated in Figure 3-1. For a fair comparison we select&nbsp;constraints 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 lines 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 Jo. ⑻We require that at least one conductor be grounded and shall refer to such a conductor as the neutral, designated as "n. H 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 1 therefore we allow all schemes to make this assumption. We require that for all schemes, no voltage to ground exceed VO. It is assumed that the reader has a background in basic circuit theory. The adjective "dc" essentially means time invariant&nbsp;or&nbsp;constant with time. Recall that the term n ace, 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. 電能的傳輸 ⑴考慮 圖 1-1 所顯示出的狀況。從網絡A一聯網B,電能流動的功率是: p=vi Network A ---i, v---Network B (圖 1-1:2 個網絡之間的電能傳播) ⑵其中,小寫字母用來顯示瞬時值, 即p、v ,并且i隨時間變化。高功率要求高電壓 和電流值。對于給定的電流值,高功率的能量流動可以通過增加電壓獲得, 反之也如此。 但是, 就目前存在的技術而言,實際中許可的電流和電壓成了最高的限制。 ⑶電流的限制因素是什么呢?我們通過使用具有高導電性、合適的機械特性的材料來制造輸電導體,而且這些材料要求最經濟:鋁是最普通的選擇, 在一些應用中銅也被使用了。導體的輸電能力與它的最大電流密度和載面積有關: Imax= Jmax A ⑷最大電流密度 Jmax 取決于不破壞導體(或者說不破壞導體的絕緣系統)的最大導體溫度。 ⑸電壓的限制的因素是什么呢?首先要考慮的是相鄰的傳導電流的部分之間所能提供的隔離 (或屏蔽)作用,即, 限制電流傳輸到它預期流動的路徑上。當電壓超過確定絕緣系統的擊穿值時,將出現預料之外的電流路徑而且這個絕緣系統將暫時或永久地停止作用了。流體的屏蔽趨于是“ 自我修復“ ( 如果短時間內釋放能量,接著充滿能量,系統將從故障中恢復過來), 但是固體的屏蔽卻在故障中永久地損壞了。 ⑹“ground”的內在含義是重要的,我們從 IEEE電氣與電子校準術語詞典中摘抄了如下內容: 地 ( 大地 )( 電力系統術語 ) 。一個傳導性的連接, 無論是故意還是意外, 通過一個電路或設備被連結到地球上,或連結到可以代替地球的相對較大的導體上。注意:它是用來建立和維持地球的電勢 ( 或導體 ) 或近似的。大地電勢是指,當有導體與之聯接時,傳導大地的電流到或者離開地球 ( 或取代地球的導電體 ) 。*譯者注:任何一點的的電位通常都取作為零的大地導電體。 我們明白這意味著在電能系統中一個給定的位置, 當時,電能裝置和大地的接近的部分組成了等勢面如果是理想地面。導體與地面的屏蔽是一個基本問題。 ⑺讓我們為實現圖 3-1 所示的電能傳輸考慮一些不同的方案。為了客觀的比較,我們制定了各個方案必須滿足的條件。我們允許任意數量的導體被使用,只要每個方案使用相同數量的導體材料。假設網絡A和 B 被隔離為固定的物理長度, 這意味著在觀察導線的載面時,我們一定能觀察到相同的傳導載面積(A)對于所有的方案。另外, 我們也認為沒有導體能傳導的電流大于由電流密度 Jo 所決定的限定值。 ⑻我們要求至少有一個導體與地相接而且將其作為參考導體, 并將這個導體表示為“ n ”。 如果它沒被要求傳導任何預計的電流,我們在A中將不包括它的載面積。這個條件在一定的對稱負荷條件下是可以實現的, 這涉及到“平衡”負荷,并且在一些實際狀況中是能被維持的; 因此我們允許所有的方案做這個假設。我們要求所有的方案中沒有一個接地電壓能超過 VO 。在此假定讀者對基本的電路理論已有一定的背景知識。形容詞“ dc "在實質上表示時間是不變的或者隨著時間變化而呈常態。記得術語“ac”, 它在歷史上曾代表“交流電”, 而在現代意味著“穩定的正弦狀態”。在不變的( 經常 ) 穩定狀態和正弦穩定狀態模式中,這些術語被用來描述電壓和電流。 New Words 1. network n.網絡;電路 2. lowercase a. 小寫的;n.小寫字體 3. instantaneous a. 瞬間的;即刻的 4. allowable a. 可容許的;可承認的 5. fabricate vt. 制作;裝配;組合 6. conductivity n. 傳導;傳導性;導電率 7. sectional a. 截面的;部分的;章節的 8.Insulation n.隔離;絕緣;孤立 9.consideration n. 考慮;思考 10. Isolation n.隔離;絕緣 11.Adjacent adj.鄰近的;毗連的 12.Breakdown n.擊穿;機器的損壞 13.Undesirable adj.不合需要的;不受歡迎的 14.Temporarily adj. 暫時性的 15.disable vt.使失去能力;損壞 16.heal vt.治愈;恢復 17.De-energize v.去激勵;釋放;斷開;斷電 18.Re-energize v.重新激勵,重新通電‘ 19.Intentional a.有意識的;故意的, 20. accessible a.易接近的;能進去的 21. equipotential a.多勢的;恒勢的;等位勢 22.Ground vt.使接地;使落地 23. implement vt.貫徹;完成;補充;n.工具 24.Constraint n.強制;抑制 25. Designate vt.表示;表明 26.appreciable a.可估計的;可看到的 27.Symmetrical a.對稱的;勻稱的 28.Assumption n.假定;設想 29.Invariant a.不變的,恒定的 30. sinusoidal a.正弦的。 Phrases and Expressions; l. Instantaneous values 瞬時值 2. current density 電流密度 3. cross-sectional area截面積 4. Insulation system 絕緣系統 5. designated as 以......表示 6. IEEE Standard Dictionary of Electrical and Electronic Terms IEEE 電氣與電子標準術語詞典<br> 7. Equipotentlal surface等勢面 8. Transmission line 輸電線路. 9. symmetrical loading condition 對稱負荷條件。 10.Circuit theory 電路理論、 11.steady state穩定狀態
[我來評論]   回答者: whsfnhs   2011-09-11 11:12   
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