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1、发电机灭磁失败原因分析及改进措施陈小明（葛洲坝水力发电厂 443002 湖北宜昌市）摘要：针对近期某发电厂发生的灭磁开关烧毁事故，本文介绍了由双断口灭磁开关（FMK）和ZnO非线性电阻构成的灭磁系统的工作原理，分析了其灭磁失败的主要原因，并提出了减轻灭磁开关灭弧负担，保证灭磁成功的改进措施：(1)降低ZnO非线性电阻的残压；(2)在ZnO非线性电阻两端并联线性电阻，以得到一条近似SiC非线性电阻的伏安特性，现场工业试验已验证其有效性。关键词：励磁装置 灭磁开关 非线性电阻0 引言1999年4月3日，某一大型水轮发电机组的励磁装置因强励失控，励磁电压和电流最高达到1200V和3800A，造成励磁
2、变压器过流保护动作，机组停机灭磁。而在这一灭磁过程中，却发生了灭磁开关严重烧毁的事故。本文分析了其主要原因，并提出了减轻灭磁开关负担，保证灭磁成功的改进措施。1 灭磁原理介绍该机组的励磁装置灭磁系统由DM4-1600双断口灭磁开关（FMK）和ZnO非线性电阻（Rf）组成，其灭磁原理图如图所示。 Uk1 Id + - SCR + - DG FMK Ud Uf Rf R - - + + If Uk2 虚线连接的电阻R是本文的改进措施之一 图 灭磁原理图 Fig.1 principle of de-excitation在图1中，二极管D保证Rf只在发电机励磁电压Uf反压时(即下正上负)时投入，Rf两
3、端的工作电压即阀片残压为1500V。正常运行时，FMK合上，可控硅整流桥SCR输出整流电压Ud和整流电流Id，Uf正压（即上正下负）。FMK跳闸灭磁时，其双断口同时断开，触头拉弧并将电弧吹入灭弧罩，电弧在FMK双断口形成电弧电压Uk1和 Uk2，极性如图1所示。以此同时，Uf由正压变负压，当Uf大于Rf的残压1500V时，励磁电流If流经Rf，FMK双断口电弧熄灭，磁场电流由FMK转到Rf上来，Uf仍被限制在1500V，If按直线衰减，直到Uf和If均为零，转子磁能变为热能，发电机灭磁成功。2 灭磁失败原因分析灭磁成功的关键是磁场电流由FMK转移到Rf上来，其转移时间约为40毫秒。若转移不成功
4、，整个磁场电流将由FMK单独消耗，此时若磁场电流超过FMK的灭弧能力，则必然造成灭磁开关烧毁。由于DM4双断口灭磁开关，在灭磁回路中主要是作为一个移能开关来使用的，不具备单独吸收强励失控下的转子磁能。在该机组FMK跳闸灭磁中，因能量太大，灭弧栅的电弧向外大量喷射导电的物质，大大的降低了灭弧室及触头间的绝缘，并使其发生短路烧毁，从而导致灭磁失败。为什么该机组的磁场电流不能有效的转移呢？首先来分析磁场电流的转移条件，从上图可见，Uk1+Uk2-Ud2-Uf=0，又Uf=1500V，则得到该灭磁系统的磁场电流转移条件：Uk1+Uk2-Ud1500V。按照直流电弧的理论，电弧的伏安特性是非线性的，电弧
5、产生的过程中，电弧电压值是变化的。跳闸前Uk1和Uk2的值为零，跳闸瞬间Uk1和Uk2的值急剧上升，拉弧越快其值越高，但随着电弧被吹进灭弧罩的灭弧栅，电弧被拉长，电弧电压也就随电弧长短而变化。倘若电弧被灭弧栅分割成短电弧，电弧电压值就为灭弧栅的片数乘以短电弧电压值，该灭弧栅为28片，短电弧电压值取30V，则Uk1=Uk2=2830=840V。考虑上述因数并结合DM4灭磁开关的试验资料，一般来说，在FMK跳闸过程中，当电弧被电动力和热气流吹进灭弧栅燃烧时，断口两端将产生900V-1200V电弧电压。将该范围内的电弧电压值代入Uk1+Uk2-Ud1500V，就得到一组磁场电流转移成功，整流输出电压
6、必须满足的条件：条件1：当Uk1+Uk2=900 +900 = 1800V时，Ud300V；条件2：当Uk1+Uk2=1000+1000=2000V时，Ud500V；条件3：当Uk1+Uk2=1100+1100=2200V时，Ud700V；条件4：当Uk1+Uk2=1200+1200=2400V时，Ud900V。将上述条件同该励磁装置典型运行工况的整流输出电压进行比较，就得到不同条件下，是否满足磁场电流转移的结论，如表1所示。 表1 改进前计算转移条件的结果 Table 1 Transfer characteristic without improvement运行工况电压Ud/电流Id条件1条
7、件2条件3条件4空载额定180V/900A满足满足满足满足负载额定480V/1650A不满足满足满足满足强励限制720V/2450A不满足不满足不满足满足强励失控1200V/3800A不满足不满足不满足不满足从表1我们看出，在灭磁开关每一断口只能建立900V1200V电弧电压的条件下，该励磁装置灭磁系统完全满足空载额定状态下的灭磁任务，部分满足负载和强励限制状态下的灭磁任务，完全不满足强励失控状态下的灭磁任务。正是由于该机组强励失控，整流输出电压Ud已达到1200V，此时要满足Uk1+Uk2-Ud1500V的条件，则要Uk1+Uk22700V ，即每一断口的电弧电压要达到1350V才行。正是由
8、于FMK的断口不能建立1350V的电弧电压，才使得该机组的磁场电流不能有效的转移。3 改进措施3.1降低ZnO非线性电阻的残压若Rf的残压由原来的1500V降为1000V，则磁场电流的转移条件变为：Uk1+Uk2-Ud1000V。按照上述分析方法，又得到一组新的磁场电流转移条件和结论，如表2所示。条件5：当Uk1+Uk2= 900+ 900= 1800V时，Ud800V；条件6：当Uk1+Uk2=1000+1000=2000V时，Ud1000V；条件7：当Uk1+Uk2=1100+1100=2200V时，Ud1200V；条件8：当Uk1+Uk2=1200+1200=2400V时，Ud1400V
9、。 表2 改进后计算转移条件的结果 Table 2 Transfer characteristic after improvement运行工况电压/电流条件5条件6条件7条件8空载额定180V/900A满足满足满足满足负载额定480V/1650A满足满足满足满足强励限制720V/2450A满足满足满足满足强励失控1200V/3800A不满足不满足满足满足由此可见，同样在灭磁开关每一断口只能建立900V1200V电弧电压的条件下， 该改进方法可使灭磁系统完全满足空载、负载和强励限制状态下的灭磁任务，但只能部分满足强励失控状态下的灭磁任务，除非Rf的残压降低至600V或提高FMK的技术指标。可见，
10、在允许的范围内降低Rf的残压，可大大提高磁场电流由FMK向Rf成功转移的可靠性，而这对于此类灭磁系统的灭磁成功是极其重要的。3.2在ZnO非线性电阻Rf两端并联线性电阻R正常运行时，因二极管D的隔离作用，电阻R不吸收正向电压下的磁能。当FMK跳闸灭磁时，双断口拉弧建压，一旦Uk1+Uk2-Ud0即Uf由正变负，电阻R就开始续流灭磁，磁场电流首先由FMK开始向电阻R转移，直到FMK双断口电弧熄灭。在电流转移的过程中，当R两端的电压超过Rf的残压即1500V时，Rf就参于灭磁吸能，并限制Uf1500V。这种分段转移磁场电流，Rf和R共同灭磁吸能的方法，最先由葛洲坝电厂的黄大可工程师提出，并成功的进
11、行了两次灭磁试验。ZnO非线性电阻和线性电阻并联，也就是二者伏安特性的叠加，选择合适的R值，就能得到一条近似SiC非线性电阻的伏安特性，如图2所示。在国外的励磁装置上，大量采用的灭磁电阻就是SiC非线性电阻。 i i i + = u u u ZnO 电阻 SiC 图2 伏安特性的叠加过程 Fig.2 Superposition of VA curve4 结束语发电机灭磁失败对励磁装置的安全运行构成了较大的危害，必须从各个方面采取预防措施。首先要对励磁调节器的过无功限制和强励限制进行认真整定校核，加强脉冲变的绝缘，杜绝励磁装置强励失控的发生。在强励失控的条件下灭磁，过高的整流输出电压会影响磁场电
12、流的转移，采取降低非线性电阻的残压或并联线性电阻，有助于这一问题的解决。ANALYSIS OF CAUSE OF FAILURE AND IMPROVEMENT OF GENERATOR DE-EXCITATIONChen Xiaoming, (Gezhouba Hydropower plant, 443002, Yichang, China）Abstract: Due to the accident a de-excitation breaker was burned in a power plant lately. This paper introduces the principle o
13、f de-excitation which consists of a double-break de-excitation breaker and ZnO non-Ohmic resistor. The cause of the de-excitation breaker to ensure the successful de-excitation, i.e., decreasing the residual voltage of ZnO non-Ohmic resistance. Field test was done to verify the validity of proposed
14、measures.Keywords: excitation equipment de-excitation breaker non-Ohmic resistor 作者简介：陈小明，男，高级工程师，毕业于西安电力专科学校，长期从事水轮发电机励磁装置的检修维护工作，曾从事励磁产品的开发和设计制造。作者照片：陈小明同万森海师傅在三峡电厂开关站留影纪念。Editors note: Judson Jones is a meteorologist, journalist and photographer. He has freelanced with CNN for four years, coverin
15、g 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 worlds first satellite - Sputnik. I also missed watching Neil Armstro
16、ng 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 dr
17、eams to reach for the stars.As a meteorologist, Ive still seen many important weather and space events, but right now, if you were sitting next to me, youd hear my foot tapping rapidly under my desk. Im anxious for the next one: a space capsule hanging from a crane in the New Mexico desert.Its like
18、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
19、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 abo
20、ve 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 kicke
21、d 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 s
22、ee 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 weathe
23、r 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 d
24、ay-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 exp
25、ect 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 b
26、ecomes the concrete bottom of a swimming pool that he wants to land on, but not too hard. Still, hell 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
27、 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 h
28、as a stabilization parachute that can be deployed to slow him down. His team hopes its 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 m
29、ph (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 wont. 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
30、,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 Id miss this.
