外文翻譯--基于單片機的電動保護控制器系統(tǒng)的設(shè)計

1、<p><b>　　中文1880字</b></p><p><b>　　電子信息科學(xué)系</b></p><p><b>　　外文翻譯</b></p><p><b>　?。?012屆）</b></p><p>　　基于單片機的電動保護控制器系統(tǒng)的

2、設(shè)計</p><p>　　學(xué)生姓名 </p><p>　　學(xué) 號 </p><p>　　專 業(yè) </p><p>　　班 級 </p><p>　　指導(dǎo)教師 </p><p>　　完成日期

3、 </p><p><b>　　外文原文：</b></p><p>　　A.C. Motor Protection</p><p><b>　　AREVA</b></p><p>　　INTRODUCTION</p><p>　　There are a wide ran

4、ge of a.c. motors and motor characteristics in existence, because of the numerous duties for which they are used.All motors need protection, but fortunately, the more fundamental problems affecting the choice of protecti

5、on are independent of the type of motor and the type of load to which it is connected. There are some important differences between the protection of induction motors and synchronous motors. </p><p>　　Motor

6、characteristics must be carefully considered when applying protection; while this may be regarded as stating the obvious, it is emphasised because it applies more to motors than to other items of power system plant. For

7、 example, the starting and stalling currents/times must be known when applying overload protection, and furthermore the thermal withstand of the machine under balanced and unbalanced loading must be clearly defined.&l

8、t;/p><p>　　The conditions for which motor protection is required can be divided into two broad categories: imposed external conditions and internal faults.</p><p>　　THERMAL(OVERLOAD) PROTECTION<

9、/p><p>　　The majority of winding failures are either indirectly or directly caused by overloading (either prolonged or cyclic), operation on unbalanced supply voltage, or single phasing, which all lead through

10、 excessive heating to the deterioration of the winding insulation until an electrical fault occurs. The generally accepted rule is that insulation life is halved for each 10° C rise in temperature above the rated va

11、lue, modified by the length of time spent at the higher temperature. As an electrica</p><p>　　Furthermore, the thermal withstand capability of the motor is affected by heating in the winding prior to a fault

12、. It is therefore important that the relay characteristic takes account of the extremes of zero and full-load pre-fault current known respectively as the 'Cold' and 'Hot' conditions.</p><p>　

13、　The variety of motor designs, diverse applications, variety of possible abnormal operating conditions and resulting modes of failure result in a complex thermal relationship. A generic mathematical model that is accurat

14、e is therefore impossible to create. However, it is possible to develop an approximate model if it is assumed that the motor is a homogeneous body, creating and dissipating heat at a rate proportional to temperature rise

15、. This is the principle behind the ‘thermal replica’ model of</p><p>　　The temperature T at any instant is given by:</p><p><b>　　where:</b></p><p>　　=final steady state

16、temperature</p><p>　　=hrating time constant</p><p>　　Temperature rise is proportional to the current squared:</p><p><b>　　where:</b></p><p>　　=current which

17、,if flowing continuously,produces temperature in the motor.</p><p>　　Therefore, it can be shown that, for any overload current I , the permissible time t for this current to flow is:</p><p>　　I

18、n general, the supply to which a motor is connected may contain both positive and negative sequence components, and both components of current give rise to heating in the motor. Therefore, the thermal replica should t

19、ake into account both of these components, a typical equation for the equivalent current being:</p><p><b>　　Where:</b></p><p>　　=positive sequence current</p><p>　　=nega

20、tive sequence current</p><p><b>　　And:</b></p><p>　　=negative sequence rotor resistance/positive sequence rotor resistance </p><p>　　At rated speed. A typical value of K

21、 is 3.</p><p>　　Finally, the thermal replica model needs to take into account the fact that the motor will tend to cool down during periods of light load, and the initial state of the motor. The motor will h

22、ave a cooling time constantthat defines the rate of cooling. Hence, the final thermal model can be expressed as:</p><p>　　…Equation 19.1</p><p><b>　　Where:</b></p><p>　　

23、=heating time constant</p><p>　　= initial state of motor (cold or hot)</p><p>　　=thermal setting current</p><p>　　Equation 19.1 takes into account the ‘cold’ and ‘hot’ characteristi

24、cs defined in IEC 60255, part 8.</p><p>　　Some relays may use a dual slope characteristic for the heating time constant, and hence two values of the heating time constant are required. Switching between the

25、two values takes place at a pre-defined motor current. This may be used to obtain better tripping performance during starting on motors that use a star- delta starter. During starting, the motor windings carry full line

26、current, while in the ‘run’ condition, they carry only 57% of the current seen by the relay. Similarly, when the mo</p><p>　　Since the relay should ideally be matched to the protected motor and be of capable

27、 close sustained overload protection, a wide range of relay adjustment is desirable together with good accuracy and low thermal overshoot.</p><p>　　SHORT-CIRCUIT PROTECTION</p><p>　　Motor short-

28、circuit protection is often provided to cater for major stator winding faults and terminal flashovers. Because of the relatively greater amount of insulation between phase windings, faults between phases seldom occur. As

29、 the stator windings are completely enclosed in grounded metal, the fault would very quickly involve earth, which would then operate the instantaneous earth fault protection. A single definite time over current relay ele

30、ment is all that is required for this purpose, s</p><p>　　Differential (unit) protection may be provided on larger HV motors fed via circuit breakers to protect against phase- phase and phase-earth faults, p

31、articularly where the power system is resistance-earthed. Damage to the motor in case of a fault is occurring minimized, as the differential protection can be made quite sensitive and hence detects faults in their early

32、stages. The normal definite time over current protection would not be sufficiently sensitive, and sensitive earth fault protection </p><p>　　UNDER VOLTAGE PROTECTION</p><p>　　Motors may stall wh

33、en subjected to prolong under voltage conditions. Transient under voltages will generally allow a motor to recover when the voltage is restored, unless the supply is weak.</p><p>　　Motors fed by contactors h

34、ave inherent under voltage protection, unless a latched contactor is used. Where a specific under voltage trip is required, a definite time under voltage element is used. If two elements are provided, alarm and trip sett

35、ings can be used. An interlock with the motor starter is required to block relay operation when the starting device is open, otherwise a start will never be permitted. The voltage and time delay settings will be system

36、and motor dependent. They must all</p><p>　　LOSS-OF-LOAD PROTECTION</p><p>　　Loss-of-load protection has a number of possible functions. It can be used to protect a pump against becoming unprime

37、d, or to stop a motor in case of a failure in a mechanical transmission (e.g. conveyor belt), or it can be used with synchronous motors to protect against loss-of-supply conditions. Implementation of the function is by a

38、 low forward power relay element, interlocked with the motor starting device to prevent operation when the motor is tripped and thus preventing a motor start. Whe</p><p>　　The setting will be influenced by

39、the function to be performed by the relay. A time delay may be required after pickup of the element to prevent operation during system transients. This is especially important for synchronous motor loss-of supply prot

40、ection.</p><p><b>　　翻譯內(nèi)容：</b></p><p><b>　　交流電機保護</b></p><p><b>　　阿海琺集團</b></p><p><b>　　引言</b></p><p>　　交流電動

41、機因為應(yīng)用的范圍很廣，并且擁有較好的電動機特性，所以被用來完成許多任務(wù)。所有電動機都需要保護，但幸運的是，更多影響保護選擇的基本問題與電動機類型和電動機帶的負荷沒有關(guān)系。感應(yīng)電動機和同步電動機的保護有一些重要區(qū)別。</p><p>　　應(yīng)用電機保護時必須認真考慮電動機的特性。這也許會被認為是很明顯的，但是在這里強調(diào)它，是因為將特性應(yīng)用在電動機上遠比應(yīng)用在電力系統(tǒng)其他設(shè)備上多。例如，應(yīng)用過負荷保護時必須知道電動機的

42、啟動和停轉(zhuǎn)電流/時間，此外還必須知道電動機在平衡負荷和不平衡負荷下的熱承受能力。</p><p>　　電動機保護所需的條件可分為兩大類：強加的外部條件和內(nèi)部故障。</p><p><b>　　熱（過載）保護</b></p><p>　　大部分的繞組故障都是直接或間接地由過載引起（無論是長期的還是周期性的）的，不平衡供電電壓運行或是單相運行，都會

43、導(dǎo)致過熱，然后使繞組絕緣惡化，直到電氣故障發(fā)生。一般認為，溫度每升高10℃，絕緣壽命就會減少一半，具體還要由其處在高溫的時間長短來修正。電動機一般都具有相對較大的熱存儲容量，因此很少發(fā)生的短期過載對電動機的影響并不大。然而，持續(xù)性過載哪怕只超過幾百個百分點也可能導(dǎo)致過早老化和絕緣故障。</p><p>　　此外，電動機的熱承受能力受到故障前繞組加熱的影響。因此，繼電器的特性要考慮電動機的極端情況：零電流和故障前滿

44、負荷電流，將它們分別作為“冷”、“熱”條件。</p><p>　　不同的電動機設(shè)計、多樣化的應(yīng)用、各種可能的異常運行狀況以及故障模式都使電機發(fā)熱關(guān)系十分復(fù)雜，因此無法建立一個通用的精確的數(shù)學(xué)模型。然而，假設(shè)電動機歸屬于同一類別，并且熱量的產(chǎn)生和消散與溫度增長成比例，那么就可以建立一個近似的數(shù)學(xué)模型，這就是基于電動機熱模擬模型用于過載保護的原理。</p><p>　　任意時刻的溫度T由下式給

45、定：</p><p>　　式中，為最終穩(wěn)定溫度；為熱時間常數(shù)。</p><p>　　溫度升高與電流平方成比例：</p><p>　　式中，為在電動機中產(chǎn)生溫度的持續(xù)電流。</p><p>　　因此，可以知道對于任何過載電流，這個電流流動的允許時間t為</p><p>　　一般說來，電動機連接的電源包括正序和負序元件，兩

46、種電流元件都會使電動機升溫。因此，熱模擬應(yīng)該考慮這些元件，一個典型的等效電流的方程如下：</p><p>　　式中，為正序電流； 為負序電流。</p><p><b>　　在額定轉(zhuǎn)速時，</b></p><p>　　K=負序轉(zhuǎn)子電阻/正序轉(zhuǎn)子電阻</p><p><b>　　K的典型值為3。</b>

47、</p><p>　　最后，熱模擬模型要考慮到電動機在輕載和最初始狀態(tài)下會逐漸冷卻下來。電動機有一個冷卻時間常數(shù)，定義了冷卻的速度。因此，最終的熱模型可描述為</p><p><b>　?。?9.1）</b></p><p><b>　　式中，</b></p><p>　　為加熱時間常數(shù)；為電動機的

48、初始狀態(tài)（冷和熱）；為熱定值電流。</p><p>　　式（19.1）考慮了IEC 60255第8部分定義的“冷”和“熱”特性。</p><p>　　一些繼電器可能使用了雙斜率特性的熱時間常數(shù)，因此熱時間常數(shù)需要兩個值，在預(yù)先定義好的電動機電流處進行兩個值之間的切換。對于使用Y/△啟動器的電動機，這可以在啟動階段獲得更好的跳閘性能。啟動期間，電動機繞組承載滿負荷的線電流，而在運轉(zhuǎn)狀態(tài)下，承

49、載57%的油繼電器觀測到的電流，相似地，當電動機與電源斷開連接時，熱時間常數(shù)設(shè)定的與冷卻時間常數(shù)相等。</p><p>　　因為理想情況下，繼電器應(yīng)該與被保護的電動機相匹配，并且能夠接近持續(xù)的過載保護，所以期望其能夠在一個范圍內(nèi)進行調(diào)節(jié)，而同時具有良好的精度和低的熱超調(diào)。</p><p><b>　　欠壓保護</b></p><p>　　電動機

50、長時間運行在欠電壓狀態(tài)下可能導(dǎo)致停轉(zhuǎn)。一般情況下，除非是弱電源供電，在暫時性低電壓的情況下，允許電動機在電壓恢復(fù)后恢復(fù)正常運行。</p><p>　　除非使用的是鎖銷接觸器，一般通過接觸器供電的電動機都具有固有的欠壓保護。凡需要特定的低壓跳閘的場合就需要用到定時限欠電壓元件，如果有兩個元件，就可以設(shè)置告警和跳閘。使用帶有電動機啟動器的互鎖功能，當啟動裝置打開時，對繼電動作進行閉鎖，否則啟動將無法進行。電壓和延時整

51、定將依賴于系統(tǒng)和電動機。它們必須允許在瞬時性故障、電動機啟動等事件期間可能出現(xiàn)的系統(tǒng)電壓跌落現(xiàn)象的存在，以避免錯誤動作。由于電動機啟動可導(dǎo)致電壓下降的正常值的80%，電壓整定可能要低于這個值。對于電壓跌落持續(xù)0.5-2s的情況，通?？梢栽偌铀?，視系統(tǒng)、電動機和傳動特性而定。因此，延時整定應(yīng)將這些因素考慮在內(nèi)。</p><p><b>　　甩負荷保護</b></p><p&

52、gt;　　甩負荷保護可能擁有許多功能，它可以用來在泵未被注滿時對泵提供保護，或者是當機械傳輸（如輸送帶）發(fā)生故障時停止電動機工作，或是可以使同步電機在沒有電源條件下提供保護。它的功能通過低正向功率繼電器元件實現(xiàn)，與電動機啟動裝置互鎖，以防止切除電動機時保護動作，從而阻止電動機啟動。當帶有一個非常低的負荷（如壓縮機）啟動時，也需要在啟動持續(xù)階段對這一功能進行閉鎖，以防止錯誤動作。</p><p>　　整定將受到繼電