毕业论文（设计）发电机新型交流灭磁41186.doc

发电机新型交流灭磁 陈贤明 王伟 吕宏水 朱晓东 ( 国电自动化研究院 电气控制研究所 南京323信箱 , 210003)摘要：大型发电机已普遍采用非线性电阻并接在磁场绕组两端的来灭磁，然而随着巨型的，如三峡700Mw的发电机投运，磁场电压、电流都十分大，为使非线性电阻能在灭磁时投入，对直流磁场开关的开断弧压要求愈来愈高，大电流、高弧压，导致磁场开关十分巨大，结构复杂，价格昂贵，运行维护工作量大。为此近年来出现用交流灭磁，即从向转子供电的整流桥进线的交流侧采取措施来灭磁，至今交流灭磁并不十分可靠，在三峡发电机采用的交、直流灭磁中，交流灭磁仅起辅助直流灭磁的作用，主要仍依靠直流磁场开关，本文提出的新型交流灭磁，利用和非线性电阻并联的电容器，在灭磁时和磁场绕组电感、电阻发生RLC串联谐振时1,2产生的高电压，将非线性电阻投入，达到快速灭磁。并完全取消了直流磁场开关。仿真表明，这种方法已完全克服了原有交流灭磁的缺点，可靠性相当高，本文给出了对它进行仿真的结果。关键词：发电机，新型交流灭磁，非线性电阻，电容器，RLC谐振New Alternating current De-excitation for GeneratorsChen,Xianming Wang,Wei Lu,Hongshui Zhu,Xiaodong ( Nanjing Automation Research Institute, P.O.Box 323 Nanjing 210003)Abstract：De-excitation with varistor parallel connected to field winding for large generators has been widely used. However, field current and voltage become quite large for Larger Generators, such as 700Mw generator for Three-Gorge power plant. In order to put varistor into operationduring de-excitation requirement for opening arc-voltage of dc field breaker is high and high. This causes big size, structure complexity, high cost and difficult maintenance of field breaker. That is why an alternating current de-excitation (ACDE) is adopted, which acts on ac side of thyristor rectifier-bridge for de-excitation. But up to now ACDE is not reliable enough, so ACDE in Three-Gorge power plant is used as an auxiliary for De-excitation.New ACDE proposed in the paper uses capacitor parallel connected to varistor, the former with inductance and resistance of field winding is able to cause serial resonance of RLC1,2 and produce high voltage during de-excitation, which may force varistor to operate and fast de-excitation can be implemented. The field breaker on dc side of field may be omitted at all. Simulation proved that the new ACDE overcomes shortcoming of origin ACDE, and has high reliability.Keywords：Generator, New alternating current de-excitation, varistor, capacitor ,RLC resonance0 .概述 近年来为了加快发电机故障灭磁过程，灭磁电阻由线性电阻改用非线性电阻，后者直接并接於磁场电路2端，图1为当今广泛采用移能型磁场开关加非线性电阻的灭磁原理图。由于非线性电阻动作阀值电压(残压)高，正常时只有少量漏电流流过，故障灭磁时要求开关FB能快速断开，并有足够高的断弧电压，以便非线性电阻投入运行，使磁场电路和励磁电源断开，把大部分磁能消耗在非线性电阻上。通常将这类开关称 图1 用移能型磁场开关和非线性电阻R灭磁为移能型磁场开关，由于它在灭磁过程吸收能量不大，因此在理论上，并不需要很大的尺寸，但是其开断弧压和非线性电阻的残压配合，十分重要。事实上开关FB断开时，磁场电流i减小，磁场绕组产生与电流变化率正比的感应电势e，电流变化率可按回路电势合成为零确定，即 UUk+e=0， 得出Uk=U+e， (1)U是励磁电源电压，而e是加于非线性电阻上，要使其导通，应不小于非线性电阻的残压Ux。也就是说灭磁开关的开断弧压要大於电源电压(正向最大幅值)加非线性电阻的残压。否则如移能失败，必将导致磁场开关烧毁。为此三峡700Mw发电机用双断口磁场开关，每个断口由4亇开关串联，十分厖大。因而又出现了从晶闸管整流桥进线处的交流灭磁的想法。1).交流灭磁 图2为交流灭磁的原理图，在磁场图2 交流灭磁原理图绕组直流侧接入非线性电阻R，而在晶闸管整流桥进线侧加装三相交流磁场开关，其本意是希望切除交流开关或切除晶闸管整流桥脉冲，或整流桥逆变导致非线性电阻R投入。现分三种情况分析：A).逆变 图3 逆变时整流桥输出电压灭磁时，令可控整流桥逆变(前提是励磁调节装置和供电的交流电源工作正常)，图3是三相可控整流桥在触发控制角75度到150度逆变时，输出整流电压的波形，只有后者的负向幅值大於非线性电阻的残压Ux。非线性电阻投入，移能成功。B) 封锁或切除晶闸管整流桥全部脉冲通常脉冲切除前，整流桥上、下桥臂各有一臂导通例如是+A、C臂导通，灭磁时由于脉冲己切除整流桥无法换相，而磁场电流因转子电感很大，已导通的+A.C晶闸管无法关闭，交流侧线电压会直接加到直流侧如图4所示图4 切脉沖，交流电压加至直流侧的等效电路这时如交流侧线电压幅值大於非线性电阻的残压Ux，当交流电压到负值后，非线性电阻将投入。注意这里交流开关没分断，50周波的交流线电压因大的磁场电感也不会产生多少交流电流。C) 切断三相交流开关假定不切脉冲。图5表示了触发控制角75°在t时图5切整流桥进线开关时，输出电压波形切交流开关的情况，假定这时+A,C相导通，因有大电感磁场电流不可能立刻变零，开关A、C相触头有电弧维持导通，B相触头因无电流而立即断开，下一个+B相脉冲来也不会导通，再过60度A相脉冲来(图6)，使A导通，这样导致电流流过+A、A臂导通，使开关A、C相触头息弧断开，同时加到直流侧整流电压为零，如图6所示。注意：正常情况下电压Uac(1000v)导通60°，这里由於下1个B相脉冲来没法导通，使Uac继续导通到120°。考虑到角是75°而角是从线电压60°处算起则，可知Ud的最大负值应为：图6 三相整流桥各桥臂脉冲序列Um×Sin(120+75+60)°=0.966Um 其中Um是进线交流线电压的幅值。 注意：切交流开关能得到的负电压峯值，正好等于交流侧线电压幅值和触发控制角正弦值的乘积。如果拉交流进线开关产生负电压值小于非线性电阻残压，则后者无法投入，磁场电流只能按自已的时间常数缓慢衰减至零。由此看出交流灭磁有其局限性。因为：A)逆变灭磁。必须励磁调节器和交流供电电源要正常，和最大负电压高于非线性电阻残压。B)封锁脉冲灭磁，要求整流桥进线交流线电压正常，且其幅值要大於非线性电阻残压。目前常用的自并励发电机如因机端故障，整流桥进线三相电压无法保证正常，所以不能有绝对把握。至於C)开关断开过程产生的负电压和触发控制角有关，非线性电阻投入的可能不确定。在我国長江三峡水轮发电机上为保证发电机安全，采用了交流灭磁加直流灭磁开关和非线性电阻的交、直流综合灭磁的方案3，虽然比较可靠，但设备较多，尤其所用直流磁场双断口开关，每亇断口有4亇开关串联，以滿足开断弧压要求。2新型交流灭磁本文提出的发电机新型交流灭磁，利用了和非线性电阻并联的电容器和磁场绕组的电感产生谐振。依图7 新型交流灭磁原理图靠谐振时电容上的高电压将非线性电阻投入。文献1,2较祥细探讨了这种灭磁新方法。图7是它的原理图，这里非线性电阻R并联上了电容器C，再和二极管D串联，跨接到磁场绕组两端当发电机正常运行时，二极管D防止正向励磁电压加到电容器C上，当触发控制角大于60°时，300周波的整流励磁电压负值对电容C有充电电流。但无法流过晶闸管整流桥，基本不起作用。由於非线性电阻R投入前，只流过小於毫安级的漏电流，所以可考虑其阻值为无限大，即不计其影响，当触发控制角60°整流电压为正值，电容器支路不通，电容器C不充电。在上述三种灭磁情况发生时，都会产生负电压，它们的大小虽不足使非线性电阻R投入，但为了克服二极管D的管压降，使电容器C支路导通，都足足有余，电容器C在导通瞬间类似于短路，全部电流流经它，(使交流开关触头息弧断开)同时发生了RLC电路的串联谐振，随着磁场能量向电场能量转换，电容器电压迅速上升，直到使非线性电阻R投入，实现移能灭磁，R的投入限制了电容器C电压进一步上升，限制了加于磁场绕组的灭磁过电压。当触发控制角60°整流电压300赫芝波形中有负值，这时电容器C会充负电压，但在切除整流桥全部脉冲后，或整流桥逆变后产生的负电压，远大于电容器C原先充的负电压，保证电容器支路导通。现在分析切除整流桥进线开关的灭磁情况。参见图9C例，当触发控制角在75°时，电容器C上最高充电电压Uc约380v，而切除开关时，由于整流桥原先导通的两亇晶闸管延长导通60°，使整流电压的最大负电压近1400V，所以也足够使电容器支路导通。应该指出电容器C支路串联二极管D当触发控制角>60°时，会使电容器C不断地充放电，长期运行，会使电容器老化加速，影响其寿命，为此可以考虑使用晶闸管代替，其缺点是在灭磁时，要使用控制脉冲使其开通，增加了复杂性。 （A） (B) (C) (D) (E) (F) (G) (H) (I) 图9 图8的仿真效果图(A)(B)(C)切开关，(D)(E)(F)封脉冲，(G)(H)(I)整流桥逆变3).仿真结果图8为新型交流灭磁图7的仿真结构图。仿真利用了Simulink的SimPowerSystems库4,5的元件进行，图8中电源通过三相开关接至三相晶闸管全控整流桥，其触发脉冲由同步6脉冲发生器提供，后者的同步信号来自三只同步变压器Vab,Vbc,Vca，触发脉冲控制角是外给的(这里=75°)整流桥直流侧接感性负载L, r，与其并联的有电容器C，非线性电阻R和二极管D。注意SimPowerSystems库中并没有非线性电阻元件，因此需利用Simulink中元件和可控电流源实现，非线性电阻的伏安特性可用下式表示U=kia (1) 即图8中 In1-out1长方模块，并和可控电流源组成R。此外还有负载电流id，电压Vdc和电容器电流ic、电压Vc，非线性电阻电流i1及ic+i1的测量单元及示波器等。定时器Timer1用来控制三相开关的切、合的定时。Timer2用来按时给出或封锁晶闸管的触发脉冲的。逆变灭磁可通过定时器来控制角的变化。图8中未表示。为了简化分析：1) 先只考虑电容器C支路的作用假定非线性电阻R断开，只观察电容器接入的结果。仿真分3种情况进行即：合交流开关后经一定时间a.交流开关分闸。b.封锁脉冲c.整流桥逆变。每种情况都包括电容器接入和断开情况。本例内选用三相线电压为50Hz,1000V，电感负载L=0.1H, r=0.098，C=1000f，触发控制角=75°。图9为其仿真结果。图9(A)为电容器未接时，合闸后在t=0.2”分闸时，负载电流id和电压Udc的波形。合闸后因L较大，id逐步上升，到t=0.2”分闸时由于电感，电流不能立即降至零，三相开关开断后原先导通的两个晶闸管延长导通60°，使整流电压变负，直到当同一相的上、下晶闸管都导通时变零，电源断开。图9(B),(C)是接入电容噐C后，三相开关分闸的情况，可看出合闸初因C存在而反向充电，Udc的负电压逐渐变大，电容器电压Uc也反向增大，充满后不再变化。到t=0.2”分闸时，当整流的负载电压迅速变负，导致二极管D导通，使C快速反向充电，到Udc的最大负值。图9(D),(E)为合上三相交流侧开关后，t=0.2”封锁整流桥脉冲的不接和接入电容器C的仿真结果。图9(G),(H)为合上三相交流侧开关后，t=0.2”整流桥逆变(角75°->150°)的不接和接入电容器C的仿真结果。可看出仅在合闸初瞬两者有不同，这两种情况下电容器C上的电压Vc都和图9C相似。 注意图9(G),(H)因逆变，负载电流id很快衰减。由上可知，电容器的接入，只是合闸时因反向充电对电路有些影响，充满电后，相当于开路不起作用，如触发控制角小于60度，整流电压中无负值，电容器C和二极管D接入，完全没作用。2）非线性电阻R和电容器C同时接入这便是新型交流灭磁的情况，为了更接近大型水轮发电机的实际情况，现将上述仿真实例部分参数更改如下, L=0.99H, C=1000F。假定非线性电阻的参数(见式1)为k=500，=0.046，三相可控整流桥的触发控制角为75°，和前面一样分三种情况仿真。可发现这三种情况下，由于电容器C在灭磁时的充电电压，都足以顺利使非线性电阻投入运行，因此这三种仿真结果相似，图10表示了整流桥三相进线开关在t=0.3”分闸时的各电气量的波形。图10(A)是整流器输出或负载电压Vdc和电源电流id。合闸后id由零上升到100安，灭磁时立即到零(图10C)。灭磁时产生的负电压使二极营D导通，导致电容器C和负载电感电路L,r产生RLC谐振，负载电流向电容器充电，当电容器电压到达Ux=500×1000.046=618 V，的非线性电阻残压时，非线性电阻投入运行，约经0.175”负载电流ic+i1=0而灭磁。图10(B),(C),(E),(D)分别为电容器电流ic、整流桥输出电流id，非线性电阻电流i1及它和ic之和ic+i1。可看出Vdc有负值分量，所以合闸后电容器C就有充电电流，充电电压逐渐稳定，从图10(F)可看出因电容器C充电，在t=0.3”灭磁开始电容电压Vc猛增到非线性电阻的残压值，从而导致电容器电流迅速向非线性电阻R转移(见图10(E)，随着非线性电阻电流i1下降，Ux=Vc也下降，但当i1+ic=0，灭磁完成，但因电容器电压未放电完，仍有电压需慢慢经非线性电阻放完，注意i1=0后的Vc已不再加到负载上。而是加在二极管D上。注意在t=0.3”时图10(A)中Vdc突变为负值，它的大小此时正好与电容器电压Vc相等、反向，这显然是因C和L,r 构成了闭合回路。图10(F)为非线性电阻在灭磁时吸收的磁能 (A) (B) (C) (D) (E) (F) (G)图10. 三相交流开关分闸时新型交流灭磁的仿真结果应该指出，在上述负载参数下其时间常数T=L/r=0.99/0.098=10.1” 加上电压，要使电流稳定至id=1.35*×1000×cos75/0.098=3565安，需30秒以上时间，仿真有困难，所以用了小电流短时间仿真代替以上仿真是针对t=0.3”时三相交流开关分闸所得仿真结果，对t=0.3”时只封锁脉冲或只逆变(角从75°到150°)进行仿真，得到了和图10完全相同的结果。4).结论本文提出的新型交流灭磁方案，是利用了和非线性电阻并联的电容器，及二极管，在灭磁动作瞬间由电容器C的电容和磁场绕组电感、电阻L,r 发生串联谐振，使电容器电压快速上升，导致磁场电流转移至非线性电阻，克服了现有的交流灭磁的致命缺点。由于产生非线性电阻残压是靠电路谐振，因此不对整流桥进线侧交流电压幅值有任何要求，所以本方案灭磁的可靠性已大大提高，为今后取消庞大的、昂贵的直流磁场开关成为可能。本文中讨论的交流灭磁三种方法即，三相交流开关分闸，封锁脉冲和逆变。实际中可互相结合使用，比如同时切开关、封锁脉冲或同时逆变、切开关等。本文的仿真结果说明了新型交流灭磁实现的巨大可能性和应用前景，但只仅是开始，由于篇幅所限，尚有许多内容待进一步阐述或研究。比如：1) 自并励发电机三相短路，或空载误强励下，发生晶闸管整流桥进线电压不正常或为零时，本方案是否仍能可靠工作。又如进线电压正常，但可控硅触发控制角=0°交流开关分闸后，整流桥输出电压中没有负值电压，能否使电容器支路导通。2) 电容器的电容值C的正确选择。选择过大，当负载电流小时，充电电压不大于非线性电阻残压。无法移能。这种情况一般不会发生。除非电容C选得太大。反之电容C取得小，电容充电电流脉宽窄，电容器电压升得快和高，对向非线性电阻移能有利，但如充电电流脉宽小于亳秒级是否能转移成功，恐怕需试验确定。3) 当触发控制角>60°，整流器输出电压波形中有负电压，这时在负电压下电容器C仍有反向充电电流(图10B)，如果电容器如此长期充电易于老化，有一个寿命问题，也许还应考虑用晶闸管代替二极管。4)在有条件可能情况下，宜进行试验验证。参考文献1. 陈贤明等<水轮发电机新型灭磁方案>发表于 2007年8月 “电工装备创新与发展论坛” 北京2. 陈贤明等<新型灭磁在自并励发电机上应用研究>发表于2007年8月”第三届电工技术前沿问题学术论坛”哈尔滨3. 陈小明，胡先洪<励磁系统交直流灭磁原理分析> 水电站自动化 2006 N0.4 第113116页 4.邱晓林等<基于Matlab的动态模型与系统仿真工具Simulink 3.0/4.X> 西安交通大学出版社 2003年10月5.吴天明等 <MATLAB电力系统设计与分析> 国防工业出版社2004年Editor's note: Judson Jones is a meteorologist, journalist and photographer. He has freelanced with CNN for four years, covering severe weather from tornadoes to typhoons. Follow him on Twitter: jnjonesjr (CNN) - I will always wonder what it was like to huddle around a shortwave radio and through the crackling static from space hear the faint beeps of the world's first satellite - Sputnik. I also missed watching Neil Armstrong step foot on the moon and the first space shuttle take off for the stars. Those events were way before my time.As a kid, I was fascinated with what goes on in the sky, and when NASA pulled the plug on the shuttle program I was heartbroken. Yet the privatized space race has renewed my childhood dreams to reach for the stars.As a meteorologist, I've still seen many important weather and space events, but right now, if you were sitting next to me, you'd hear my foot tapping rapidly under my desk. I'm anxious for the next one: a space capsule hanging from a crane in the New Mexico desert.It's like the set for a George Lucas movie floating to the edge of space.You and I will have the chance to watch a man take a leap into an unimaginable free fall from the edge of space - live.The (lack of) air up there Watch man jump from 96,000 feet Tuesday, I sat at work glued to the live stream of the Red Bull Stratos Mission. I watched the balloons positioned at different altitudes in the sky to test the winds, knowing that if they would just line up in a vertical straight line "we" would be go for launch.I feel this mission was created for me because I am also a journalist and a photographer, but above all I live for taking a leap of faith - the feeling of pushing the envelope into uncharted territory.The guy who is going to do this, Felix Baumgartner, must have that same feeling, at a level I will never reach. However, it did not stop me from feeling his pain when a gust of swirling wind kicked up and twisted the partially filled balloon that would take him to the upper end of our atmosphere. As soon as the 40-acre balloon, with skin no thicker than a dry cleaning bag, scraped the ground I knew it was over.How claustrophobia almost grounded supersonic skydiverWith each twist, you could see the wrinkles of disappointment on the face of the current record holder and "capcom" (capsule communications), Col. Joe Kittinger. He hung his head low in mission control as he told Baumgartner the disappointing news: Mission aborted.The supersonic descent could happen as early as Sunday.The weather plays an important role in this mission. Starting at the ground, conditions have to be very calm - winds less than 2 mph, with no precipitation or humidity and limited cloud cover. The balloon, with capsule attached, will move through the lower level of the atmosphere (the troposphere) where our day-to-day weather lives. It will climb higher than the tip of Mount Everest (5.5 miles/8.85 kilometers), drifting even higher than the cruising altitude of commercial airliners (5.6 miles/9.17 kilometers) and into the stratosphere. As he crosses the boundary layer (called the tropopause), he can expect a lot of turbulence.The balloon will slowly drift to the edge of space at 120,000 feet (22.7 miles/36.53 kilometers). Here, "Fearless Felix" will unclip. He will roll back the door.Then, I would assume, he will slowly step out onto something resembling an Olympic diving platform.Below, the Earth becomes the concrete bottom of a swimming pool that he wants to land on, but not too hard. Still, he'll be traveling fast, so despite the distance, it will not be like diving into the deep end of a pool. It will be like he is diving into the shallow end.Skydiver preps for the big jumpWhen he jumps, he is expected to reach the speed of sound - 690 mph (1,110 kph) - in less than 40 seconds. Like hitting the top of the water, he will begin to slow as he approaches the more dense air closer to Earth. But this will not be enough to stop him completely.If he goes too fast or spins out of control, he has a stabilization parachute that can be deployed to slow him down. His team hopes it's not needed. Instead, he plans to deploy his 270-square-foot (25-square-meter) main chute at an altitude of around 5,000 feet (1,524 meters).In order to deploy this chute successfully, he will have to slow to 172 mph (277 kph). He will have a reserve parachute that will open automatically if he loses consciousness at mach speeds.Even if everything goes as planned, it won't. Baumgartner still will free fall at a speed that would cause you and me to pass out, and no parachute is guaranteed to work higher than 25,000 feet (7,620 meters).It might not be the moon, but Kittinger free fell from 102,800 feet in 1960 - at the dawn of an infamous space race that captured the hearts of many. Baumgartner will attempt to break that record, a feat that boggles the mind. This is one of those monumental moments I will always remember, because there is no way I'd miss this.