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1、電力系統(tǒng)電力系統(tǒng)介紹隨著電力工業(yè)的增長(zhǎng)，與用于生成和處理當(dāng)今大規(guī)模電能消費(fèi)的電力生產(chǎn)、傳輸、分配系統(tǒng)相關(guān)的經(jīng)濟(jì)、工程問題也隨之增多。這些系統(tǒng)構(gòu)成了一個(gè)完整的電力系統(tǒng)。應(yīng)該著重提到的是生成電能的工業(yè)，它與眾不同之處在于其產(chǎn)品應(yīng)按顧客要求即需即用。生成電的能源以煤、石油，或水庫和湖泊中水的形式儲(chǔ)存起來，以備將來所有需。但這并不會(huì)降低用戶對(duì)發(fā)電機(jī)容量的需求。顯然，對(duì)電力系統(tǒng)而言服務(wù)的連續(xù)性至關(guān)重要。沒有哪種服務(wù)能完全避免可能出現(xiàn)的失誤，而系統(tǒng)的成本明顯依賴于其穩(wěn)定性。因此，必須在穩(wěn)定性與成本之間找到平衡點(diǎn)，而最終的選擇應(yīng)是負(fù)載大小、特點(diǎn)、可能出現(xiàn)中斷的原因、用戶要求等的綜合體現(xiàn)。然而，網(wǎng)絡(luò)可靠性的增

2、加是通過應(yīng)用一定數(shù)量的生成單元和在發(fā)電站港灣各分區(qū)間以及在國(guó)內(nèi)、國(guó)際電網(wǎng)傳輸線路中使用自動(dòng)斷路器得以實(shí)現(xiàn)的。事實(shí)上大型系統(tǒng)包括眾多的發(fā)電站和由高容量傳輸線路連接的負(fù)載。這樣，在不中斷總體服務(wù)的前提下可以停止單個(gè)發(fā)電單元或一套輸電線路的運(yùn)作。當(dāng)今生成和傳輸電力最普遍的系統(tǒng)是三相系統(tǒng)。相對(duì)于其他交流系統(tǒng)而言，它具有簡(jiǎn)便、節(jié)能的優(yōu)點(diǎn)。尤其是在特定導(dǎo)體間電壓、傳輸功率、傳輸距離和線耗的情況下，三相系統(tǒng)所需銅或鋁僅為單相系統(tǒng)的75%。三相系統(tǒng)另一個(gè)重要優(yōu)點(diǎn)是三相電機(jī)比單相電機(jī)效率更高。大規(guī)模電力生產(chǎn)的能源有：1. 從常規(guī)燃料（煤、石油或天然氣）、城市廢料燃燒或核燃料應(yīng)用中得到的蒸汽；2. 水；3. 石油

3、中的柴油動(dòng)力。其他可能的能源有太陽能、風(fēng)能、潮汐能等，但沒有一種超越了試點(diǎn)發(fā)電站階段。在大型蒸汽發(fā)電站中，蒸汽中的熱能通過渦輪輪轉(zhuǎn)換為功。渦輪必須包括安裝在軸承上并封閉于汽缸中的軸或轉(zhuǎn)子。轉(zhuǎn)子由汽缸四周噴嘴噴射出的蒸汽流帶動(dòng)而平衡地轉(zhuǎn)動(dòng)。蒸汽流撞擊軸上的葉片。中央電站采用冷凝渦輪，即蒸汽在離開渦輪后會(huì)通過一冷凝器。冷凝器通過其導(dǎo)管中大量冷水的循環(huán)來達(dá)到冷凝的效果，從而提高蒸汽的膨脹率、后繼效率及渦輪的輸出功率。而渦輪則直接與大型發(fā)電機(jī)相連。渦輪中的蒸汽具有能動(dòng)性。蒸汽進(jìn)入渦輪時(shí)壓力較高、體積較小，而離開時(shí)卻壓力較低、體積較大。蒸汽是由鍋爐中的熱水生成的。普通的鍋爐有燃燒燃料的爐膛燃燒時(shí)產(chǎn)生的熱

4、被傳導(dǎo)至金屬爐壁來生成與爐體內(nèi)壓力相等的蒸汽。在核電站中，蒸汽的生成是在反應(yīng)堆的幫助下完成的。反應(yīng)堆中受控制的鈾或盥的裂變可提供使水激化所必需的熱量，即反應(yīng)堆代替了常規(guī)電站的蒸汽機(jī)。水電站是利用蘊(yùn)藏在消遣的能來發(fā)電的。為了將這種能轉(zhuǎn)換為功，我們使用了水輪機(jī)?，F(xiàn)代水輪機(jī)可分為兩類：脈沖式和壓力式（又稱反應(yīng)式）。前者用于重要設(shè)備，佩爾頓輪是唯一的類型；對(duì)于后者而言，弗朗西斯渦輪或其改進(jìn)型被廣泛采用。在脈沖式渦輪中，整個(gè)水頭在到達(dá)葉輪前都被轉(zhuǎn)化為動(dòng)能，因?yàn)樗峭ㄟ^噴嘴提供給葉輪的；而在壓力式或反應(yīng)式渦輪中，水通過其四周一系列引導(dǎo)葉版先直接導(dǎo)入葉片再提供給葉輪（或轉(zhuǎn)子）。離開引導(dǎo)葉片的水有壓力，并且以

5、一部分動(dòng)能、一部分壓力的形式來提供能量。對(duì)于低于10,000千伏安的發(fā)電站而言柴油機(jī)是出色的原動(dòng)機(jī)。其優(yōu)點(diǎn)是燃料成本低、預(yù)熱時(shí)間短以及標(biāo)準(zhǔn)損耗低。此外，其所需冷卻水量極少。柴油發(fā)電通常選擇用于滿足少量電力需求，如市政當(dāng)局、賓館及工廠等；醫(yī)院通常備有獨(dú)立的柴油發(fā)電機(jī)，以備緊急情況時(shí)使用。通過電線來傳輸電能是電力系統(tǒng)中的一個(gè)重大問題。而從下面研修目的目的架設(shè)輸電線路又是必要的：1. 將電力從水電站輸送到可能很遠(yuǎn)的負(fù)載中心；2. 從蒸汽站到相對(duì)較近負(fù)載中心電力的批量供應(yīng)；3. 出于內(nèi)部連接目的將電能在緊急情況下從一系統(tǒng)轉(zhuǎn)換至另一系統(tǒng)。傳輸電壓主要由經(jīng)濟(jì)因素決定。實(shí)際上，當(dāng)距離、功率、功耗固定時(shí)，輸電

6、線路中導(dǎo)體的重量與傳輸電壓成反比。因此，出于經(jīng)濟(jì)方面的考慮，長(zhǎng)距離傳輸時(shí)電壓一定要高。當(dāng)然，電壓超高絕緣成本也就超高，要找到最佳電壓必須通過減小導(dǎo)體橫截面積來取得絕緣成本與經(jīng)濟(jì)節(jié)省之間的平衡。高壓傳輸通常使用配以懸掛式絕緣設(shè)備的高架結(jié)構(gòu)。稱為路標(biāo)鐵塔用于負(fù)載絕緣設(shè)備，各導(dǎo)體懸掛于一組或一串絕緣體的底部。最常用的導(dǎo)體類型如下：多芯銅導(dǎo)線、空心銅導(dǎo)線和acsr（用鋼加強(qiáng)的鋁線）導(dǎo)線。電力分配系統(tǒng)包括由高奪線供能的子電站與用戶開關(guān)之間的所有部件。電力從子電站接收，并以電壓標(biāo)準(zhǔn)按不同類型用戶均能接受的持續(xù)程度分送到各用戶。在大城市中，高架與地下布線方式均被采用。盡管地下布線方式比高架方式昂貴，但在高度

7、城市化地區(qū)卻必不可少。而在小城鎮(zhèn)和較疏散的大城市區(qū)域，整個(gè)分配系統(tǒng)通常都是高架式的。電力系統(tǒng)監(jiān)控現(xiàn)代電力系統(tǒng)都是由許多部分組成的統(tǒng)一體，而每個(gè)部分又都影響著其他部分的特性。為了整個(gè)系統(tǒng)能正常運(yùn)行，就必須在系統(tǒng)的許多不同地點(diǎn)監(jiān)控其工作情況以確保系統(tǒng)處于最佳工作狀態(tài)。用戶方根關(guān)心的是供電頻率和電壓的變化范圍要小。既然系統(tǒng)內(nèi)的頻率在任何地方都是相同的，因而只要在便于觀測(cè)之處安裝一個(gè)頻率表就能進(jìn)行監(jiān)控與此相反，系統(tǒng)的電壓在不同的測(cè)試點(diǎn)上會(huì)有很大的差別。因此，有必要在系統(tǒng)的某些關(guān)鍵測(cè)點(diǎn)上對(duì)電壓進(jìn)行不斷的觀測(cè)，以便提供滿意的服務(wù)。給系統(tǒng)內(nèi)每臺(tái)發(fā)電機(jī)規(guī)定適當(dāng)?shù)呢?fù)荷時(shí)間訪問演出能使系統(tǒng)有效地運(yùn)行?？v然一些新的

8、電廠各自都有較高的效率，但是由于設(shè)置地點(diǎn)不當(dāng)，可能產(chǎn)生巨大的系統(tǒng)電能損失。發(fā)電機(jī)之間的負(fù)荷分配應(yīng)使總的燃料費(fèi)用最小。要在任何意相不到的情況下保證正常供電，所有發(fā)電機(jī)的總額定功率就必須略大于總負(fù)荷加上損耗值。這種富余電量通稱為旋轉(zhuǎn)備用容量，它可以彌補(bǔ)用戶突然增加的負(fù)荷或某臺(tái)發(fā)電機(jī)因需要檢修而突然甩掉的負(fù)荷。為了向用戶征收電費(fèi)，就必須安裝電表。在不同的電力系統(tǒng)之間有許多聯(lián)結(jié)線。儀表必須安裝在聯(lián)接點(diǎn)上，以便記錄從一個(gè)系統(tǒng)傳輸?shù)搅硪粋€(gè)系統(tǒng)的電能。這種監(jiān)測(cè)要連續(xù)進(jìn)行，以確保相互輸送的電力在合同允許的限度之內(nèi)。為防止過截造成的損壞，有必要對(duì)各個(gè)主要設(shè)備的工作狀態(tài)進(jìn)行不斷的監(jiān)測(cè)。由于負(fù)載逐月增加，需要增加設(shè)

9、備容量的地點(diǎn)是可以查明的，附加設(shè)備的安裝就有可能得到保障。因此，在日益擴(kuò)大的電力系統(tǒng)的未來建設(shè)中，檢測(cè)儀表起著指南的作用。有時(shí)，在緊急情況下，系統(tǒng)操作人員觀測(cè)到時(shí)他的系統(tǒng)負(fù)荷超過了發(fā)電能力和輸電能力。那么他就面臨著甩掉部分負(fù)荷或者確切地說保持負(fù)荷不變這樣的問題因而就有必要在中斷供電造成損失最小的地區(qū)有選擇地甩掉負(fù)荷。在這種情況下，系統(tǒng)操作人員依靠許許多多儀表進(jìn)行工作。這些儀表顯示著系統(tǒng)運(yùn)行情況的信息。當(dāng)設(shè)備的運(yùn)行走出其設(shè)計(jì)極限時(shí)，儀表可以發(fā)出警報(bào)聲，作為要求采取避免設(shè)備損壞的行動(dòng)的預(yù)告。在出現(xiàn)像電力系統(tǒng)發(fā)生故障這種極端不利的情況時(shí)，有毛病的設(shè)備就會(huì)自動(dòng)停止運(yùn)行。監(jiān)控電流、電壓和其他指標(biāo)的儀表必

10、須能識(shí)別有故障的設(shè)備，并使斷路器開始工作，讓有故障的設(shè)備停止運(yùn)行，而讓運(yùn)行系統(tǒng)中的其余設(shè)備仍繼續(xù)工作。電力系統(tǒng)上的那些眾多而不同的電氣設(shè)備和用戶擁有的那些電氣設(shè)備都是按照在特定范圍內(nèi)運(yùn)行而設(shè)計(jì)的。凡超出這些設(shè)計(jì)限度的運(yùn)行都是不合適的，因?yàn)檫@可能導(dǎo)致運(yùn)行效率很低、能量損耗很多或者（在極端的情況下）使設(shè)備毀壞。仔細(xì)觀察設(shè)備的運(yùn)行情況就可以發(fā)現(xiàn)需要采取的修正措施。各種電氣設(shè)備都不應(yīng)該出現(xiàn)過量電流，因?yàn)樗鼤?huì)產(chǎn)生過高的溫度使運(yùn)行效率降低而縮短設(shè)備的使用壽命。居民區(qū)電路上出現(xiàn)過量電流時(shí)，由于保險(xiǎn)絲和電閘的作用會(huì)造成電路中斷。馬達(dá)內(nèi)的過量電流有可能損壞絕緣而過早出現(xiàn)絕緣故障。電壓不足會(huì)極大地降低白熾燈泡的亮

11、度，并可能使熒光燈不發(fā)光。供電電壓過低會(huì)在馬達(dá)內(nèi)造成過量電流而有可能損壞馬達(dá)的線圈。電壓過高提高了燈泡的發(fā)光亮度，但是在許多情況下會(huì)嚴(yán)重縮短燈泡的使用壽命。給馬達(dá)和變壓器超奪供電能在繞組鐵芯里產(chǎn)生過多的能量損失，有可能使保持鐵芯或繞組線圈的絕緣遭到損壞。機(jī)器的超速旋轉(zhuǎn)有可能對(duì)機(jī)器的轉(zhuǎn)動(dòng)部件造成結(jié)構(gòu)上的損壞。用戶生產(chǎn)設(shè)備的超速運(yùn)行可能造成劣質(zhì)產(chǎn)品。在兩臺(tái)發(fā)電機(jī)或一臺(tái)發(fā)電機(jī)與一臺(tái)同步馬達(dá)之間存在的不協(xié)調(diào)狀態(tài)會(huì)導(dǎo)致這兩臺(tái)機(jī)器有效功率傳輸?shù)闹袛?。不協(xié)調(diào)狀態(tài)應(yīng)立即查明，兩臺(tái)機(jī)器也應(yīng)立即分開。然后，才可以使它們恢復(fù)同步繼續(xù)運(yùn)行。在電力系統(tǒng)的許多工作位置上和許多用戶的家中都要安裝許多各種型號(hào)的儀表。有了這些

12、儀表，就能連續(xù)監(jiān)控供電的工作狀態(tài)。electric power systemsintroduction of electric power systemsas the power industry grows, so do the economic and engineering problems connected with the generating, transmission and distribution systems used to produce and handle the vast quantities of electrical energy consumed toda

13、y. these systems together form an electrical power system.it is important to note that the industry that produces electrical energy is unique in that it manufactures its product at the very instant that it is required by the customer. energy for the generation of electricity can be stored in the for

14、m of coal and oil, and of water in reservoirs and lakes, to meet future requirements, but this does not decrease the need for generator capacity to meet the customers demands.it is obvious that the problem of the continuity of service is very important for an electrical power system. no service can

15、be completely protected from the possibility of failure and clearly the cost of the system will depend on its requirements. however a net reliability gain is obtained by employing a certain number of generating units and by using automatic breakers for the separation into sections of the bus bars in

16、 generating stations and of the transmission lines in a national or international grid system. in fact a large system comprises numerous generating stations and loads interconnected by high-capacity transmission lines. an individual unit of generation or set of transmission lines can usually cease t

17、o function without interrupting the general service.the most usual system today for generation and for the general transmission of power is the three-phase system. in favor of this system are its simplicity and its simplicity and its saving with respect to other a.c. system. in particular, for a giv

18、en voltage between conductors, with a given power transmitted, with a given distance, and with a give line loss, the three-phase system requires only 75 per cent of the copper or aluminium needed in the single-phase system. another important advantage of the three-phase system is that three-phase mo

19、tors are more efficient than single-phase ones. the sources of energy ofr large-scale electricity generation are:1. steam obtained by means of a conventional fuel (coal, oil or natural gas), the combustion of city refuse or the employment of nuclear fuel;2. water;3. diesel power from oil.there are o

20、ther possible sources of energy such as direct solar heat, wind power, tidal power, etc., but none of these has yet gone beyond the pilot-plant stage.in large steam power plants, the thermal energy stored in steam is converted into work by means of turbines. a turbine consists essentially of a shaft

21、 or rotor fixed in bearings and enclosed in a cylindrical casing. the rotor is made to turn smoothly by means of jets of steam from nozzles around the periphery of the turbine cylinder. these steam jets strike blades attached to the shaft. central power stations employ condensing turbines in which t

22、he steam passes into a condenser after leaving the turbine. condensation is effected by the circulation of large quantities of cold water through the tubes of the condenser,thus increasing the expansion ratio of the steam and the consequent efficiency and work output of the turbine. the turbines are

23、 connected directly to large electricity generators.in turbines the action of the steam is kinetic. there is progressive expansion of the steam from the high pressure and relatively small volume at which it leaves.steam is made by heating water in a boiler. the usual boiler has a furnace in which fu

24、el is burned, and the heat given off during combustion is conducted through the metal walls of the boiler to generate steam at a pressure within the boiler vessel. in nuclear plants, steam is generated with the aid of a reactor in which the controlled fission of uranium or plutonium supplies the nec

25、essary heat for the vaporization of water. thus the reactor rep; aces the steam generator of conventional plants.use is made of the energy possessed by water in hydroelectric stations. in order to transform this energy into work, hydraulic turbines are used. modern hydraulic turbines may be divided

26、into two classes: impulse turbines and pressure or reaction turbines. of the former, the pelton wheel is the only type used in important installations; of the latter, the francis turbine or one of its modifications is universally employed.in an impulse turbine, the whole head of water is converted i

27、nto kinetic energy before the wheel is reached, as the water is supplied to the wheel through a nozzle. in the pressure or reaction turbine the wheel (or runner) is provided with vanes into which water is directed by means of a series of guide vanes around the whole periphery. the water leaving thes

28、e guide vanes is under pressure and supplies energy partly in the kinetic form and partly in the pressure form.the diesel engine is an excellent prime mover for electricity generation in plant below about 10,000 kva. it has the advantage of low fuel cost, a brief warming-up period and low standing l

29、osses. moreover it requires little cooling water. diesel generation is generally chosen for small power requirements by municipalities, hotels and factories; hospitals often keep and independent diesel generator for emergency supply.the transmission of electrical energy by means of lines is a great

30、problem in electrical power systems. transmission lines are essential for three purposes:1. to transmit power from a hydroelectric site to a load center perhaps a considerable distance away;2. for the bulk supply of power from steam stations to load centers a relatively short distance away;3. for in

31、terconnection purposes to transfer energy from one system to another in case of emergency.the transmission voltage is determined largely by economic factors. in fact, in a transmission line, if the distance, the power and the power loss are fixed, the total weight of the conductor varies inversely a

32、s the square of the transmission voltage. for the economic transmission of power over considerable distances the voltage must therefore be high. naturally with higher voltages the insulation cost also rises and to find the optimum voltage we must strike a balance between this cost and the saving thr

33、ough the reduction of the cross-section of the conductors.for high voltages, overhead-line construction is generally used with suspension-type insulators. steel tower, called pylons serve to carry the insulators, with each conductor suspended from the bottom of a group or string of insulator units.

34、the following types of conductor are those most commonly used: stranded copper conductors, hollow copper conductors and acsr (aluminum cable, steel reinforced) conductors.distribution includes all the parts of the electricity system between the power substations supplied from high-voltage transmissi

35、on lines and the consumers switch. electric power is received from substations and distributed to the consumers at the voltage levels and with the degree of continuity that are acceptable to the various types of consumer. in large metropolitan systems both over head and underground distribution meth

36、ods are used. although underground distribution is more expensive than an overhead system, it is virtually a necessary in heavily urbanized areas. in smaller towns and in the less congested districts of large cities, the entire distribution system is usually overhead.electric system monitoringa mode

37、rn electric power system is an assembly of many components each of which influences the behavior of every other part. proper functioning of the system as a whole makes it necessary to monitor conditions existion at many different points on the system in order to assure optimum operation.the concern

38、of the customers is primarily that the frequency and voltage of the supply are held within certain rather narrow limits. since frequency of the system is the same everywhere, it may be monitored by a single frequency meter located at any convenient point. in contrast the voltage of the system may be

39、 quite different at different points. consequently, it is necessary to make continuous observation of the voltage at certain key points on the system in order to provide acceptable service.efficient operation of the system is obtained by assigning proper load schedules to each of the generators on t

40、he system. newer plants, although individually more efficient, may be located at points on the system where their loading occasions large system losses. it is desirable to operate with a division of the load between generators so that the total cost of fuel consumed is minimized. to provide reliabil

41、ity of the power supply in tile event of unexpected conditions, it is desirable to have the total kilowatt rating of all machines in operation somewhat greater than the total load plus losses. this excess of generation, known as spinning reserve, is then available for picking up suddenly applied cus

42、tomer loads or to pick up the load dropped by a generator that must be removed from service for emergency maintenance.instrumentation is necessary to permit billing of customers for energy used. many interconnections exist between different power systems. instruments must be provided at interchange

43、points to permit billing for energy transferred from one system to another. the continuous monitoring of energy transfer is necessary to assure that interchanged power is within the limits of contract agreements.the continuous measurement of conditions on major pieces of equipment is necessary to av

44、oid damage due to overload. as load increases from month to month, points at which additional capacity of equipment is required may be recognized and provision made for the installation of additional equipment. thus instrumentation serves as a guide for future construction in a growing power system.

45、occasionally, under emergency conditions, a system operator observes that his system load exceeds the ability of the available generating and transmission equipment. he is then faced with the problem of load shedding or ,more properly, load conservation. it is then necessary to drop selected loads w

46、here service interruption is least objectionable. in such an event, he relies on the many instruments which provide information relative to system-operation conditions.instruments may sound alarms as advance warnings of conditions requiring action to avoid damage to equipment operating beyond its de

47、sign limitation. in the event of extreme conditions such as power-system faults, defective equipment is switched out of service automatically. instruments that continuously monitor current voltage, and other quantities must be able to identify the faulted equipment and to bring about operation of th

48、e circuit breakers which remove it from service, while leaving in service all other equipment on the operating system.the many different electrical devices on a power system and those owned by the customers are designed for operation within certain specified ranges. operation, excessive deterioration, or (in extreme cases ) the destruction of the device. careful attention to the conditions under which equipment is operating may indicate corrective action