電能質(zhì)量翻譯

1、四川大學(xué)電氣信息學(xué)院電能質(zhì)量報(bào)告第二章王飛鵬1143031228108班2014/6/1在此處鍵入文檔摘要。摘要通常為文檔內(nèi)容的簡(jiǎn)短概括。在此處鍵入文檔摘要。摘要通常為文檔內(nèi)容的簡(jiǎn)短概括。ORIGIN OF POWER QUALITY VARIATIONS電能質(zhì)量變化的來源This chapter describes the origin and some of the basic analysis tools of power quality variationsORIGIN電能質(zhì)量變化這章介紹了電能質(zhì)量變化的基本分析工具的起源The consecutive sections of the

2、chapter discuss (voltage) frequency variations, voltage (magnitude) variations, voltage unbalance, voltage fluctuations (and the resulting light flicker), and waveform distortion. 本章的連續(xù)的章節(jié)討論（電壓）頻率變化，電壓（幅度）的變化，電壓不平衡度，電壓波動(dòng)（和由此產(chǎn)生的光線閃爍），和波形失真。A summary and conclusions for each of the sections will be gi

3、ven at the end of this chapter.一種用于每個(gè)部分的摘要和結(jié)論將在本章的最后給出。2.1 VOLTAGE FREQUENCY VARIATIONS2.1電壓頻率變化Variations in the frequency of the voltage are the first power quality disturbance to be discussed here. After a discussion on the origin of frequency variations (the power balance) the method for limitin

4、g the frequency variations (power frequency control) is discussed. The section closes with an overview of consequences of frequency variations and measurements of frequency variations in a number of interconnected systems.在該電壓的頻率變化是第一個(gè)電源質(zhì)量擾動(dòng)在這里討論。后討論在頻率變化（功率平衡）的來源用于限制頻率的變化（功率 - 頻率控制）。本節(jié)用的頻率變化，和許多相互連

5、接的系統(tǒng)的頻率變化的測(cè)量結(jié)果相近。2.1.1 Power Balance2.1.1功率平衡Storage of electrical energy in large amounts for long periods of time is not possible, therefore the generation and consumption of electrical energy should be in balance. Any unbalance in generation and production results in a change in the amount of ene

6、rgy present in the system. The energy in the system is dominated by the rotating電能的大量用于長(zhǎng)時(shí)間儲(chǔ)存是不可能的，因此，電能的產(chǎn)生和消耗應(yīng)該是平衡的。任何不平衡的產(chǎn)生和消耗在能量的系統(tǒng)中存在的量的變化。在系統(tǒng)中的能量以轉(zhuǎn)動(dòng)為主energy Erot of all generators and motors:所有的發(fā)電機(jī)和電動(dòng)機(jī)的能量EROT： With J the total moment of inertia of all rotating machines and v the angular velocity

7、 at which these machines are rotating. An unbalance between generated power Pg and the total consumption and losses Pc causes a change in the amount of rotational energy and thus in angular velocity:令J為所有旋轉(zhuǎn)電機(jī)的轉(zhuǎn)動(dòng)慣量和v為角速度在這些機(jī)器都旋轉(zhuǎn)的總力矩。發(fā)電電力Pg和總消耗量和損失的PC之間的不平衡導(dǎo)致的旋轉(zhuǎn)能量的量，從而在角速度的變化 ： The amount of inertia i

8、s normally quantified through the inertia constant H, which is defined as the ratio of the rotational energy at nominal speed v0 and a base power Sb:轉(zhuǎn)動(dòng)慣量的值會(huì)通過慣性常數(shù)H，其定義為旋轉(zhuǎn)能量的額定速度v 0和基電源的Sb的比值通常量化 The base power is normally taken as the sum of the (apparent) rated powers of all generators connected to

9、 the system, but the mathematics that will follow is independent of the choice of base power. Typical values for the inertia constant of large systems are between 4 and 6 s.該基本功率通常取為連接到系統(tǒng)的所有發(fā)電機(jī)的（明顯的）額定功率的總和，但這個(gè)數(shù)字是獨(dú)立于基本功率的。對(duì)于大型系統(tǒng)的慣性時(shí)間常數(shù)典型值是4和6秒之間。Inserting (2.3) in (2.2), assuming that the frequency

10、remains close to the nominal frequency, and replacing angular velocity by frequency give the following expression:代入（2.3）在（2.2）中，假設(shè)頻率保持接近標(biāo)稱頻率，并置換角速度由頻率給下面的表達(dá)式： Where Pg and Pc are per-unit (up) values on the same base as the inertia constant H.其中PG和PC為每單位（最多）在同一基地為慣性常數(shù)H。值Consider a 0.01-pu unbalance

11、 between generation and production in a system with an inertia constant of 5 s. This leads to a change in frequency equal to 0.05 Hz/s. If there would be a 0.01-pu surplus of generation, the frequency would rise to 51 Hz in 20 s; for a 0.01-pu deficit in generation the frequency would drop to 49 Hz

12、in 20 s. It is very difficult to predict the load with a 1% accuracy. To keep the frequency constant some kind of control is needed.考慮發(fā)電和生產(chǎn)之間的0.01pu不平衡在一個(gè)系統(tǒng)中的5秒的慣性時(shí)間常數(shù)。這導(dǎo)致了在頻率0.05赫茲/秒的變化。如果將有0.01普盈余，頻率將上升到51赫茲在20秒中;對(duì)于0.01-PU缺額產(chǎn)生的頻率將降低到49赫茲在20秒中。這是用1的精度非常難于預(yù)測(cè)的負(fù)載。為了保持頻率恒定的某種控制是必要的。The sudden loss of a

13、 large power station of 0.15 pu will result in a frequency drop of 1 Hz/s. In 1 s the frequency has dropped to 49 Hz. As the sudden unexpected loss of a large generator unit cannot be ruled out, there is obviously the need for an auto-matic control of the frequency and of the balance between generat

14、ion and consumption.大電站突然喪失0.15 PU將導(dǎo)致1赫茲/秒的頻率下降。在1 s中頻率已經(jīng)下降到49赫茲。作為一家大型發(fā)電機(jī)組的突然意外損失無法排除，顯然需要對(duì)生產(chǎn)和消費(fèi)之間的平衡的頻率和自動(dòng)控制。For comparison we calculate the amount of electrical and magnetic energy present in 500 km of a 400-kV three-phase overhead line when transporting 1000 MW of active power at unity power fac

15、tor. Assuming 1 mH/km and12 nF/km as inductance and capacitance, respectively, gives for theelectrical energy 12 CU2 =320 kJ and for the magnetic energy 12Li2=1040kJ.For unity power factor the peaks in magnetic and electrical energy(current and voltage) occur at the same time, so that the maximum to

16、tal electromagnetic energy equals 1360 kJ. As before we can express this as a time constant by dividing with the rated power. For a 1000-MVA base, we find a time constant of 1.4 ms. This is significantly less than the 4- to 6-s time constant for the rotational energy. This example confirms the state

17、ment at the start of this section that the energy present in a power system is dominated by the rotational energy of generators and motors.為了便于比較，我們計(jì)算傳輸時(shí)的電和磁的存在于400千伏三相架空線路500公里的架空線中單位功率因數(shù)中1000兆瓦的有功功率在。假設(shè)使用1 mH/公里 12 NF /公里的電感和電容，分別給出了電能320千焦和磁能1040千焦。為了統(tǒng)一， 功率因數(shù)在磁性能和電能的峰值（電流和電壓） 發(fā)生在同一時(shí)間，以使最大總電磁能量等于1

18、360千焦。和以前一樣，我們可以通過與額定功率相除表示這是一個(gè)時(shí)間常數(shù)。對(duì)于一個(gè)1000兆伏安的 基站，我們發(fā)現(xiàn)有1.4毫秒的時(shí)間常數(shù)。這是小于4 - 至6 - S的時(shí)間常數(shù)對(duì)于旋轉(zhuǎn)能量。這個(gè)例子證實(shí)了在本節(jié)中的陳述即電力系統(tǒng)中的能量由發(fā)電機(jī)和電動(dòng)機(jī)的轉(zhuǎn)動(dòng)能量占主導(dǎo)地位。Power Frequency Control電能頻率的控制To maintain the balance between generation and consumption of electrical energy most large generator units are equipped with power fre

19、quency control. Maintaining the frequency close to its nominal value is a natural consequence of maintaining the balance between generation and consumption. The principle of power frequency control is rather simple. The measured frequency is compared with a frequency setting (the nominal frequency,

20、50 or 60 Hz, in almost all cases). When the measured frequency is higher than the nominal frequency, this indicates a surplus of rotational energy in the system. To mitigate this, the generator reduces its active power output. More correctly, the mechanical input to the turbine generator is reduced.

21、 This leads after a transient to a new steady state with a lower amount of electrical energy supplied to the system.為了發(fā)電和消耗之間的平衡，大型發(fā)電機(jī)組都配備了功率 - 頻率控制。維護(hù)頻率接近其標(biāo)稱值是保持生產(chǎn)和消費(fèi)之間的平衡的結(jié)果。電源-變頻調(diào)速的原理是相當(dāng)單的。所測(cè)量的頻率與設(shè)置的（標(biāo)稱頻率，50或60赫茲，在幾乎所有的情況下）相比較。當(dāng)所測(cè)量的頻率比標(biāo)稱頻率更高，這表明在系統(tǒng)中的旋轉(zhuǎn)能量過剩。為了減輕這種情況，發(fā)電機(jī)降低其有功的功率輸出。更準(zhǔn)確地渦輪機(jī)發(fā)電機(jī)，機(jī)械輸入被降

22、低。此導(dǎo)致了新的供給系統(tǒng)較少能量的穩(wěn)定狀態(tài)。The principle of power frequency control is shown in Figure 2.1. The input to the speed governor is a corrected power setting (corrected for the deviation of the frequency from its setting). We will come back to the choice of the power setting below. The speed governor is a con

23、trol system that delivers a signal to the steam valves with a thermal power station to regulate the amount of steam that reaches the turbine. The turbine reacts to this, with a certain delay, by changing the amount of mechanical power. Much more detailed models can be found in the literature e.g, 6,

24、 Chapter 10; 26, Chapter 3. For the purpose of this chapter, it is sufficient to know that there is a time delay of several seconds (10 s and more for large units) between a change in the power signal at the input of the governor and an increase in the mechanical power produced by the turbine.電源的原理

25、- 頻率控制如圖2.1所示。輸入到調(diào)速器的是一個(gè)校正后的功率（是頻率對(duì)于設(shè)定值的修正）。我們回來到下面的電源的選擇設(shè)置。限速器是一個(gè)控制系統(tǒng)，它提供了一個(gè)信號(hào)，蒸汽閥與熱電站來調(diào)節(jié)蒸汽到達(dá)渦輪機(jī)的量。渦輪對(duì)這個(gè)結(jié)果給予反應(yīng)，具有一定的延遲，通過改變機(jī)械動(dòng)力的量。更詳細(xì)的模型可以在文獻(xiàn) 例如，6，第10章找到; 26，第3章。本章的目的是必須要知道，有幾秒鐘的延遲時(shí)間（10秒多為大戶型）在速度調(diào)節(jié)器的輸入功率信號(hào)的變化和由渦輪機(jī)產(chǎn)生的機(jī)械功率增加了之間。 If we consider the system in steady state, where the production of the

26、generator equals the input signal to the speed governor, the mechanical power is as follows:如果我們考慮到系統(tǒng)的穩(wěn)定狀態(tài)中由發(fā)電機(jī)產(chǎn)生的量等于輸入調(diào)速信號(hào)需要的量，機(jī)械功率為： where R is referred to as the drop setting. This relation is shown in Figure 2.2. When the system frequency drops, the power production increases. This compensates f

27、or the cause of the frequency drop (a deficit of generation). The frequency setting is equal to the nominal frequency of the system and the same for all generators connected to the system. In the Nordic system the frequency should not only be within a certain band but also on average be equal to 50

28、Hz to ensure that clocks indicate a correct time. When the integrated frequency error (the time difference for a clock) exceeds a certain value, the frequency setting of the generators is slightly changed. But the setting remains the same for all generators.其中R被稱為下降設(shè)置。此關(guān)系示于圖2.2。當(dāng)系統(tǒng)頻率下降，電力生產(chǎn)的增加。這可以補(bǔ)償

29、的頻率下降（代赤字）。頻率設(shè)置為系統(tǒng)中最重要的頻率并連接到系統(tǒng)的所有發(fā)電機(jī)的額定頻率。在北歐的系統(tǒng)中頻率不應(yīng)該只在一定的范圍內(nèi)，而且平均等于50赫茲，確保鐘表指示正確的時(shí)間。當(dāng)集成的頻率誤差（為一個(gè)時(shí)鐘的時(shí)間差）超過一定值時(shí)，發(fā)電機(jī)的頻率設(shè)定稍有改變。但所有發(fā)電機(jī)的設(shè)置保持相同。Spinning Reserve旋轉(zhuǎn)備用To allow for an increase in generated power when there is a deficit of generation, for example, because a large unit has been disconnected f

30、rom the system, the power produced by a generator should be less than its maximum capacity. The amount of extra power which can be produced in a matter of seconds is called spinning reserve. 為了在發(fā)電能力在缺額時(shí)能增加發(fā)電，例如，因?yàn)橐粋€(gè)大的單元已經(jīng)斷開從系統(tǒng)連接，由一臺(tái)發(fā)電機(jī)產(chǎn)生的功率應(yīng)小于其最大容量。額外的電能，可以在幾秒鐘內(nèi)就可以產(chǎn)生的量稱為旋轉(zhuǎn)備用。 Figure 2.3 Daily load c

31、urve (thick solid curve) for a power system, with the sum of generator power settings (thin solid lines) and the spinning reserve (dashed lines).圖2.3日負(fù)荷曲線（粗實(shí)線）的動(dòng)力系統(tǒng)，與發(fā)電機(jī)電源設(shè)置（細(xì)實(shí)線）之和為旋轉(zhuǎn)備用（虛線）。interconnected system should at least be equal to the largest unit connected to the system. For smaller system

32、s during low load, the spinning reserve should be relatively high. However, in large interconnected systems like the Nordic system or the European interconnected system, a spinning reserve of a few percent is acceptable.互聯(lián)系統(tǒng)應(yīng)該至少等于連接到系統(tǒng)的最大單元。對(duì)于低負(fù)載時(shí)更小的系統(tǒng)，旋轉(zhuǎn)備用應(yīng)該是相對(duì)較高的。然而，在大型互聯(lián)系統(tǒng)，如北歐系統(tǒng)或歐洲的互聯(lián)系統(tǒng)，百分之幾的一個(gè)旋轉(zhuǎn)

33、備用是可以接受的。Choice of Power Set Point Figure 2.3 shows a hypothetical daily load curve for a system. Such a curve is used to schedule the amount of generation capacity needed. The day is divided into a number of time intervals, typically of 15 to 30 min duration. For each interval the expected load is

34、determined. This required generation is then spread over a number of generator stations. For each time interval the sum of the power settings is chosen equal to the expected load. The actual scheduling is in most countries done by a free-market principle where each generator can place bids. When suc

35、h a bid is accepted for a certain time block, it will become the power setting of the generator for that block. Even the load is in principle market based, but the distribution companies typically place bids based on the expected consumption of their customers. For example, see 25 for a description

36、of the various market principles.2.32.1.2.2選擇顯示了一個(gè)系統(tǒng)假想的日負(fù)荷曲線。這樣的曲線被用來安排必要的發(fā)電容量的量。一天分成多個(gè)時(shí)間間隔一般為15至30分鐘的持續(xù)時(shí)間。對(duì)于每個(gè)時(shí)間間隔的預(yù)期由負(fù)載來確定。這些分散至被需要的發(fā)電機(jī)數(shù)。每個(gè)時(shí)間間隔的功率設(shè)置的總和等于預(yù)期的負(fù)載。在實(shí)際用電計(jì)劃中，大多數(shù)國(guó)家是有一個(gè)自由市場(chǎng)的原則，每個(gè)發(fā)電機(jī)可以出價(jià)來完成發(fā)電計(jì)劃。當(dāng)這樣的出價(jià)在一定時(shí)間段可接受，它將成為產(chǎn)生該塊的電能設(shè)置。即使負(fù)載在原則上是市場(chǎng)化，但發(fā)行公司通常根據(jù)其客戶預(yù)期消耗來出價(jià)。例如，見25對(duì)各種市場(chǎng)原則的描述。 Also for each t

37、ime interval a spinning reserve can be decided, but this is typically kept at a fixed percentage of the total power. Even for the spinning reserve and the droop setting market principles can be applied (as discussed, e.g., in 335).另外每次間隔的旋轉(zhuǎn)備用是可以確定的，而這通常保持在總功率的一個(gè)固定百分比。即使對(duì)旋轉(zhuǎn)備用和固定偏差設(shè)置，市場(chǎng)原則仍然可以應(yīng)用（如討論，例如

38、，在335）Sharing of Load A change in load, or a change in generation setting, results in a change in generated power for all generator units equipped with power frequency control. Consider a system with n generators with power setting = 1, . ; n; droop setting Ri; and frequency setting fset Note that t

39、he power setting and the droop setting are different for each unit whereas the frequency setting is the same. The produced power for each generator at a system2.1.2.3負(fù)載分擔(dān)負(fù)載的變化，或產(chǎn)生背景的變化，將導(dǎo)致所有配帶功率 - 頻率控制機(jī)組的出力變化。考慮有n個(gè)發(fā)電機(jī)， Pi，SET, i = 1，.; N;暫降和頻率設(shè)定FSET。需要注意的是電源設(shè)置和偏差的設(shè)置是不同的每個(gè)單位，而頻率設(shè)置是一樣的。每個(gè)發(fā)電機(jī)在系統(tǒng)中產(chǎn)生的電能是

40、 The sum of all power settings is equal to the total predicted load: 所有的電源設(shè)置的總和等于總預(yù)測(cè)負(fù)荷： Assume that the actual load deviates from the predicted load by an amount DPc, so that in steady state假定實(shí)際負(fù)載從預(yù)測(cè)的負(fù)載偏離Pc，因此，在穩(wěn)定狀態(tài) Combining (2.6), (2.7), and (2.8) gives結(jié)合（2.6），（2.7）和（2.8）給出 which gives for the ste

41、ady-state frequency這給出了穩(wěn)態(tài)頻率 The increase in consumption causes the system frequency to drop by an amount determined by the power frequency control settings of all the generators that contribute. Each generator contributes to the increase in consumption by ratio of the inverse of its droop setting:消費(fèi)

42、的增加導(dǎo)致系統(tǒng)頻率下降到由所有的電能的頻率控制發(fā)電機(jī)所決定的一個(gè)值。每臺(tái)發(fā)電機(jī)有助于通過其偏差設(shè)置的增加： The droop setting is normally a constant value in per unit with the generator rating as a base. For a generator of rated power S and per-unit droop setting Rpu, the droop setting in hertz per megawatt is偏差的設(shè)置通常是在每單元與發(fā)電機(jī)的評(píng)估作為基礎(chǔ)的恒定值。對(duì)于額定功率S和單位下降的發(fā)電

43、機(jī)設(shè)置Rpu上，在每兆瓦赫茲偏差設(shè)置 with typically fSET = f0, the nominal frequency. The new steady-state frequency is obtained from inserting (2.12) in (2.10) under the assumption that the per-unit droop setting is the same for all units: 與通常FSET=F0，標(biāo)稱頻率。新的穩(wěn)態(tài)頻率是從假設(shè)每單位偏差的設(shè)置是相同的對(duì)于所有單位代入（2.12）在（2.10）得到： The relative d

44、rop in frequency is equal to the relative deficit in generation times the per-unit droop setting:在頻率的相對(duì)降等于在產(chǎn)生時(shí)間每單位設(shè)置的相對(duì)缺額： Each generator contributes by the ratio of its rated power to any deficit in generation:每個(gè)發(fā)電機(jī)有助于其額定功率在任何生產(chǎn)電能中的比例： Thus large generators contribute more than small generators. Th

45、is calls for a spin- ning reserve which is a fixed percentage of the rated power of the generator unit.因此，大型發(fā)電機(jī)貢獻(xiàn)超過小型發(fā)電機(jī)。這需要一個(gè)旋轉(zhuǎn)儲(chǔ)備，是發(fā)電機(jī)組的額定功率的固定百分比。2.1.3 Consequences of Frequency VariationsAs far as the authors are aware, no equipment problems are being reported due to frequency variations. Still s

46、ome of the consequences and potential problems are mentioned below.2.1.3頻率的變化后果 據(jù)筆者所知，沒有任何設(shè)備問題被報(bào)道出來是由于頻率變化。還是一些后果和潛在的問題如下2.1.3.1 Time Deviation of Clocks 2.1.3.1時(shí)間偏差Clocks still often synchronize to the voltage frequency (typically by counting zero crossings). A consequence of frequency variations i

47、s therefore that clocks will show an erroneous time. The size of the error depends on the deviation of the frequency from the nominal frequency.Consider a system with nominal frequency f0and actual frequency f (t). The number of zero crossings in a small time t is Note that there are two zero crossi

48、ngs per cycle. In a long period T (e.g., one day), the number of zero crossings is時(shí)間偏差還是經(jīng)常同步到電壓頻率（通常通過計(jì)算過零點(diǎn)）。因此，頻率變化的后果是，時(shí)鐘將顯示一個(gè)錯(cuò)誤的時(shí)間。該誤差的大小依賴于頻率的標(biāo)稱頻率的偏差?？紤]標(biāo)稱頻率f0和實(shí)際頻率f（t）的一個(gè)系統(tǒng)。過零點(diǎn)在一個(gè)小時(shí)間t是多少需要注意的，每個(gè)周期有兩個(gè)零交叉點(diǎn)。在很長(zhǎng)的周期T（例如，一天），過零點(diǎn)的數(shù)目是Because the clock assumes a frequency f0, the apparent elapsed time is

49、因?yàn)闀r(shí)鐘假定一個(gè)頻率為f0，經(jīng)過的時(shí)間是From (2.17) and (2.18) the time error T is obtained as從（2.17）和（2.18）的時(shí)間誤差T可得A frequency of 50.05 Hz instead of 50 Hz will cause clocks to run 0.1% faster. This may not appear much, but after one day the deviation in clocks is0.001360024¼86.4 s. Thus 0.05 Hz frequency deviati

50、on causes clocks to have an error of over 1 min per day. A frequency equal to 50.5 Hz (1% deviation)would cause clocks to be 15 min ahead after one day50.05赫茲，而不是50赫茲的頻率將導(dǎo)致時(shí)鐘運(yùn)行速度更快的0.1。這可能不會(huì)出現(xiàn)太大，但有一天，在后時(shí)鐘偏差 0.001*3600*24=86.4秒。因此，0.05赫茲的頻率偏差導(dǎo)致時(shí)鐘每天有超過1分鐘的誤差。一個(gè)頻率等于50.5赫茲（1的偏差）會(huì)導(dǎo)致時(shí)鐘一天后超前15分鐘。2.1.3.2 Va

51、riations in Motor Speed2.1.3.2 Variations in Motor Speed Also the speed of induction motors and synchronous motors is affected when the voltage frequency changes. But as the frequency does not vary more than a few percent, these speed variations are hardly ever a problem. Very fast fluctuations in f

52、requency could cause mechanical problems, but in large interconnected systems the rate of change of frequency remains moderate even for large disturbances. Variations in voltage magnitude will have a bigger influence.異步電機(jī)速度變化也反應(yīng)電機(jī)和同步電機(jī)的轉(zhuǎn)速，電壓頻率變化時(shí)的影響。但是，由于頻率變化不超過百分之幾，這些速度變化幾乎不是問題。在頻率非?？斓牟▌?dòng)可能會(huì)造成機(jī)械故障，但

53、在大型互聯(lián)系統(tǒng)頻率的變化率保持適中甚至對(duì)大擾動(dòng)。電壓幅度的變化會(huì)有更大的影響。 2.1.3.3 Variations in Flux Lower 2.1.3.3磁通下限的變化frequency implies a higher flux for rotating machines and transformers. This leads to higher magnetizing currents. The design of rotating machines and transformers is such that the transition from the linear to th

54、e nonlinear behavior (the “knee” in the magnetic flux-field, B-H, curve) is near the maximum normal operating flux. An increase of a few percent in flux may lead to10% or more increase in magnetizing current. But the frequency variation is very rarely more than 1%, whereas several percent variations

55、 in voltage magnitude occur daily at almost any location in the system. One percent drop in frequency will give the same increase in flux as 1% rise in voltage magnitude. Where saturation due to flux increase is a concern, voltage variations are more likely to be the limiting factor than frequency v

56、ariations. 頻率意味著對(duì)旋轉(zhuǎn)較高的機(jī)器和變壓器的通量。這導(dǎo)致較高的磁化電流。旋轉(zhuǎn)電機(jī)和變壓器設(shè)計(jì)是從直線的過渡到非線性行為（在磁通量場(chǎng)的“拐點(diǎn)”，BH曲線）靠近最大正常工作磁通。百分之幾的磁通的增加可能會(huì)導(dǎo)致10以上的磁化電流增加。但頻率的變化是很少超過1，而幾個(gè)百分點(diǎn)的電壓變化每天在系統(tǒng)幾乎任何地方都會(huì)發(fā)生。在頻率的百分之一降將給予1電壓上升。飽和度導(dǎo)致磁通量增加是值得考慮，電壓的變化更可能成為比頻率變化更重要的限制因素。2.1.3.4 Risk of Underfrequency Tripping 2.1.3.4低頻跳閘的風(fēng)險(xiǎn)Larger frequency deviations

57、 increase the risk of load or generator tripping on over frequency or on under frequency. Over frequency and under frequency relays are set at a fixed frequency to save the power system during very severe disturbances like the loss of a number of large generating units. The most sensitive settings a

58、re normally for the under frequency relays. In some systems the first level of under frequency load shedding occurs already for 49.5 Hz, although 49 Hz is a more common setting. The loss of a large generator unit causes a fast drop in frequency due to the sudden deficit in generation followed by a recovery when the power frequency control increases the product