loss of excitation

LOSS OF EXCITATION

AT THE TIME OF LOSS OF EXCITATION IF GCB NOT OPENED WHAT ARE THE DAMAGE WILL HAPPEN TO MACHINE (IN COMBINED CYCLE OPERATION)

25 days ago

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VENKIDU PERIASAMY, Gloria Javier and 6 others like this

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JohnUnfollow Follow John

John Fry •I'm assuming that this occurred during operation with the turbine driving the generator and generator is on-line. Generator essentially motorizes, that is becomes a large inductive load to the distribution system. When excitation is suddenly lost the generator may initially accelerate to above synchronous speed, slip poles, and then become a motor at synchronous speed. Damages to the generator are the mechanical forces on the rotor/field and overheating of the field core.

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VENKIDUUnfollow Follow VENKIDU

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VENKIDU PERIASAMY •Thanks for your comment sir... Its affect turbine also? ??



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VENKIDU PERIASAMY •Thanks for your comment sir... Its affect turbine also? ??



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John Fry •I'm not sure on turbine. If the governor valves stayed open steam flow would continue and aid in keeping the turbine cool. On the other hand if they went closed the turbine could overheat due to trapped steam, but condenser would still be in service which would draw the steam in the turbine out in which case turbine pressure would equalize with condenser pressure.I think we need some help on this one.



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VENKIDU PERIASAMY •Thank you sir..



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hannesUnfollow Follow hannes

hannes hedquist •If DCS system correctly designed and generator- steam turbine interface too, this should NOT effect ST negatively.



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WernerUnfollow Follow Werner

Werner Villatoro •When generator come to motorizer for long time, you will have acummulative damage on the blades of the last stage on turbine, the blades could be bented and close the steam path to the condenser.



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hannesUnfollow Follow hannes

hannes hedquist •I think if your DCS system is properly set-up you will trip on reverse power in generator or high temp of turbine exhaust before there is any mechanical failure. Please check your DCS patameters to ensure this is the case.



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Emerson B.Unfollow Follow Emerson B.

Emerson B. James •Hay algunas detalles en la secuencia lógica de disparo de una central que implica una reacción del sistema de protección de la planta. Por ejp. Si el GCB no abre al recibir una orden de apertura ( la razón de NO apertura es por baja presión del GAS), hay una consecuencia lógica de protección como EL DISPARO DE EL STOP VALV., disparo del sistema de excitación, Ahora bien, la consecuencia inmediata es la Motorización del generador. NO HAY UN CAMBIO DEL SENTIDO DE GIRO, por lo que se tendrá la lubricante del turbo-generador ( que no cunda el PÁNICO!!!) , No ocurrirá ningún daño inmediatamente, El asunto es que hacer para desconectar el generador de la red. La apertura de los disyuntores del GCB es la medida mas atinada. El problema es que el Turbo-generador esta bajo el control de la RED. La carga de la Red es el turbo-generador propiamente. Las toneladas del peso de los rotores acoplados y accesorios que están girando. !!!!Tenga cuidado en romper las condiciones de operación de la turbina. Hay vacío??? El ciclo combinado es de un SOLO EJE? Si es un solo eje también esta incluido el rotor de TG . Ahora bien , mucha atención!!!! TIENEN QUE VIGILAR SERIAMENTE LA EXPANSIÓN DIFERENCIAL. Vigilar las respuestas de las tensiones térmicas de la turbina.



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Emerson B. James •Hay algunas detalles en la secuencia lógica de disparo de una central que implica una reacción del sistema de protección de la planta. Por ejp. Si el GCB no abre al recibir una orden de apertura ( la razón de NO apertura es por baja presión del GAS), hay una consecuencia lógica de protección como EL DISPARO DE EL STOP VALV., disparo del sistema de excitación, Ahora bien, la consecuencia inmediata es la Motorización del generador. NO HAY UN CAMBIO DEL SENTIDO DE GIRO, por lo que se tendrá la lubricante del turbo-generador ( que no cunda el PÁNICO!!!) , No ocurrirá ningún daño inmediatamente, El asunto es que hacer para desconectar el generador de la red. La apertura de los disyuntores del GCB es la medida mas atinada. El problema es que el Turbo-generador esta bajo el control de la RED. La carga de la Red es el turbo-generador propiamente. Las toneladas del peso de los rotores acoplados y accesorios que están girando. !!!!Tenga cuidado en romper las condiciones de operación de la turbina. Hay vacío??? El ciclo combinado es de un SOLO EJE? Si es un solo eje también esta incluido el rotor de TG . Ahora bien , mucha atención!!!! TIENEN QUE VIGILAR SERIAMENTE LA EXPANSIÓN DIFERENCIAL. Vigilar las respuestas de las tensiones térmicas de la turbina.



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SamuelUnfollow Follow Samuel

Samuel Santana •In the event that a generator loses its excitation, works as an induction generator, rotating at a speed greater than synchronous. The cylindrical rotor generators are not designed for these operations can overheat to dangerous levels in two or three minutes



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kalpeshUnfollow Follow kalpesh

kalpesh Shah •I agree with Samuel Santana and John Fry.When Excitation loses, generator work as induction generator, take reactive power from grid. In such case the speed of generator increase more than synchronous speed. Ideally speaking it should sustain operation,



but practically there are number of other parameters to be monitored. If Grid frequency is higher then it may trip on overspeed as steam flow is maintain to turbine.



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John Fry •I just want everyone to know that I have enjoyed and learned a few things in this conversation.



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JosephUnfollow Follow Joseph

Joseph Clappis •When the excitation of a synchronous generator is lost while the generator is synchronized to a grid with other generators the Loss of Excitation relay should open the generator breaker (even before excitation is completely lost).

When there is no excitation on a running synchronous generator the field collapses and the stator fields will induce voltage on the rotating field--particularly on the generator rotor retaining rings. This causes a GREAT deal of heat on the retaining rings in a very short period of time.

Another thing that happens when the excitation is lost is that the unit speed will increase above rated--this is what causes the induced voltage and circulating currents on the retaining rings that causes the heat to increase quickly.

Usually, synchronous generators don't run for very long (a few minutes) in this condition. The air gap between the rotor and the stator is not very large and as the retaining rings expand from the heat they will cause increased vibration and eventually will cause "interference" between the rotor and stator--which leads to catastrophic results.

This is one reason why there are usually two or more protective functions monitoring for low excitation and



loss of excitation. Low excitation can cause pole slippage which can be just as catastrophic as loss of excitation. So, a generator breaker is usually tripped (opened) by under-excitation detection long before excitation is ever fully lost. And if that didn't occur then the loss of excitation relay should trip the breaker to prevent operation without any excitation.

As for damage to the prime mover, when the generator rotor tries to stop the prime mover the load coupling will usually break, which will usually cause damage to the prime mover.

All in all, continued operation for any appreciable time with a true loss of excitation will usually cause some pretty serious mechanical damage, and collaterally, electrical damage as well. To both the generator and the prime mover.

It's been my experience that most Loss of Excitation relays are set to operate before excitation is completely lost to protect the equipment (both generator and turbine). Further, most excitation control systems have minimum setpoints (limits) and alarms when the minimum excitation limit is reached to protect against pole slippage caused by low excitation which may occur before excitation is completely lost.

Low-, and loss of, excitation conditions while a synchronous generator is synchronized to a grid are both to be avoided. And there are generally protective relays to do so.



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Emerson B. James •Se estan suponiendo varios escenarios para manejar el asunto y me gustaria que los escenarios se justifiquen porque hay verdades y media verdades. Quien que un turbogenerador. Como puede un generador electrico motorizado y con la valvula pare (STOP Valv. closed) puede alcanzar una frecuencia de giro mayor que la de la red. Como puede suceder eso???? Como puede un generador sin corriente de campo ( por lo que disminuye la fuente de calor) y enfriado por aire o hidrogeno destruirse en minutos.

Un Turbogenerador a vapor se destruye en pocos minutos si este cae en sobrevelocidad y la stop valv no cierra como tampoco las valvulas de admision de vapor en violacion a todas las logicas de proteccion. Esta destruccion tambien puede sobrevenir si hay un disparo del GCB del turbogenerador y con suficiente de MW y la valvula de Pare no cierre. El flujo de vapor equivalente a la carga en MW llevara el turbogenerador a una condicion de sobrevelocidad destructiva, SOLO ASI SOLO es posible una destruccion inmenente y rapida.

Mi experiencia en OPERACION de planta me ha permitido vivir esta situaciones.

hora y no paso nada!!!!! Se resolvio abriendo manualmente el OCB del generador y LISTO. Se le dio mantenimiento al OCB y se hallo la bobina de mando de cierre abierta y las tres horas el generador estaba en


linea.

2 ocasion.... Generador de 100MVA al disparar no abrio el interruptor GCB y , cerro la stop valv.,disparo el siste ma de excitacion y dos horas despues de la Motorizacion hubo que abrir manualmente los seccionadores del GCB. A las 6 horas el Turbogenerador estaba eb linea. NO SON CONSIDERACIONES TEORICAS , SON REALIDADES.

La otra experiencia de mis 30 años en OPERACION DE PLANTA que observe yme relataron fue la destruccion del turbogenerador de HAINAV en 2005. El evento fue el siguiente: La unidad con una carga de 40 MW se disparo por ALTO NIVEL DEL DOMO PROVOCANDO LA APERTURA DEL GCB . La valvula de PARogenerE no abrio y el turb



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Emerson B. James •La valvula de Pare ( stop valv. ) no cerro y consecuecia el flujo de vapor equivalente a 40 MW guio el turbogenerador a SOBREVELOCIDAD Y SU DESTRUCCION TOTAL.



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Emerson B. James •Si el breaker del circuito DC de excitacion esta abierto no circula corriente pero se indu ce un voltage oscilante bajo el campo variable del estator del generador. !!!!!!!





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MarceloUnfollow Follow Marcelo

Marcelo Malinsky •Alguien Bien Menciono sobre la SobreVelocidad, en este caso tanto en los colectores de la excitatriz como en los anillos rozantes las escobillas tienen un tope maximo de soporte de velocidad periferica, en las de excitacion "generalmente" suele ser de 30 Mts/seg. Si se supera esta velicidad por tiempos prolongados las patinas de las escobillas tienden a dejar de conmutar y/o dañarse generando un desbalanceo en el conjunto o juego de escobillas. Con una pinza amperometrica pueden chekear los consumos que deben ser Iguales o con poca diferencia. Cualquier duda, me consultan. Abrazo.



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Emerson B. James •Si el sistema de enfriamiento( Hidrogeno o Aire) del generador electrico extrae el calor con la operacion de los ventiladores colocados en los extremos del rotor, los enfriadores de hidrogeno o aire, la circulacion del agua de enfriamiento en marcha, La lubricacion recogiendo el calor en los cojinetes frutos del deslizamiento, El operador de cuadro abriendo un poco mas la valvula de salida de los enfriadores les permitira al staff de operacion controlar la situacion. hasta romper la motorizacion del generador, abriendo los disyuntores del GCB o ENFRIANDO LA SUBESTACION....

La ultima palabra de este asunto la tiene el EQUIPO DE OPERACIONES DE LA PLANTA. Operacion een este asunto supera las responsabilidades del area de mantenimiento...



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Emerson B. James •Que no cunda el Panico, por favor...!!!!! NO ABANDONEN EL CUARTO DE CONTROL SI HAY UNA MOTORIZACION. NO HABRA FALLA INMEDIATA. El staff de Operaciones puede hacerse carga. No lo duden. Creanme.!!!!



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John Fry •Joseph Clappis. Great explanation and you are correct on low excitation protection and generator damage. I was a little fuzzy on the turbine aspects. Thank you for your contribution to this discussion.



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WojciechUnfollow Follow Wojciech

Wojciech Betlej •Extend of the damage on the generator depends on the design of the rotor and on how long the machine was working as a motor. In general you should expect high voltages to be induced in the rotor surface, wedges and damper circuit (this might include retaining rings). If the design is good the rotor should easily sustain several minutes of the motoring. At 3000/3600 rpm due to CF all components of the damper circuit should be fully engaged and working properly.

Having said that I would recommend inspecting the rotor at the next possible opportunity including removal of the retaining rings.


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NareshUnfollow Follow Naresh

Naresh Bansal •@JohnOverheating of LP rotor last blades will take place with in few minutes.



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JorgeUnfollow Follow Jorge

Jorge Marcano •My knowledge are more related to hydroelectric machines, however I would like to participate in this discussion. I am agree with Mr. Betlej. This issue is linked with the design stage, for example in Macagua Dam (Venezuela) our old machines, a small machines of 60MW, are design to cope with 100% of motorization for long period of time. It is true that machines get overheating, but this robust design let them to work in this abnormal condition without damages.

It is always true that this machines work with low speed, less than 100rpm, and their prime mover are as robust as the machines itself.

In contrast, we had a catastrophic event in a big machine, 750MW, which design didn't contemplate this condition for so long, as a result we lost the entire generator. On the other hand, neither the turbine, neither the bearing were affected.



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VENKIDU PERIASAMY •Really i m very happy for start this discussion i got many clear points and i understand as a board operator what will do at time and after that . i thank to all for giving their valuable points in this discussion.



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AramUnfollow Follow Aram

Aram Sarkisyan •In case of speed up due to loss of excitation with closed Circuit breaker your turbine should tripped by mechanical over-speed protection and hydraulic governor will immediately close the Steam Check Valves



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ikramUnfollow Follow ikram

ikram ulhaq •As per GE control standared features; at GCB (52CB/52XCB) closed and lost exciatation then 1st control will trip the prime mover (GT/ST). Following that speed of machine will rapidly coastdown because of falmeout.

2nd station electrical interlocking will open feeding to generator breaker (s) of power transformer, So motoring action as said Mr Josph will not be possible and deshaping of rataining ring + field high vibration as well.




As per my field experience, it is common fault of electrical system which I have faced many time but never faced such concequences. Hence no worries for this common considered fault.

Regards to all.



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SandipUnfollow Follow Sandip

Sandip Bhatt •In the turbine, there is over speed protection is available & if speed increase above it, it will trip the unit. Hence, due to motoring for some time, it will not affect turbine.



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YunusUnfollow Follow Yunus

Yunus Latupapua •GETTING LOST EXITATION ON GENERATOR,THE AUTO REVERSE RELAY WILL ENERGIZE AND OPEN THE MAIN CRIRCUIT BREAKER TO THE GRID



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NareshUnfollow Follow Naresh

Naresh Bansal •Just to add to all the valuable comments already made:Till the unit remains connected to the grid, speed will be governed by the grid frequency.However with loss of excitation,slip will change.This change will be sensed by the governor and all control valves will tend to close.



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Emerson B. James •Correctamente!!!!!!!!!!!!



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AssifUnfollow Follow Assif

Assif Salman •Good dicussion guys, Please keep it up.

The point I would like to highlight here is that some commenters are confused between Loss of Excitation and Revers Power.

LOE means that the prime mover is working normally while the field current to generator rotor is lost. therortically, this causes the generator to work as an induction generator and draw reactive power from the grid.




Reverse power means that the michanical power from the prime mover is lost. This causes the unit to draw active power from the grid and work as synchronous motor.



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UdhayakumarUnfollow Follow Udhayakumar

Udhayakumar Venkataraman •In such case, Generator will become motoring, severe damage including fire will happen. Generally Inadvertent energisation relay take care in case gen is off line. If generator is on line, Reverse Power protection should take care. In ST lP stage blades may overheat and damages may happen.



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JorgeUnfollow Follow Jorge

Jorge Marcano •I think that both sides are ok. Due to, If we have a fault which lead to excitation´s lost, when machine is on line, surely, excitation´s protection is going to open the GCB. However, If there is a problem in the GCB that inhibit it to operate, this machine will start to consume reactive energy from the grid. After that, any other protection isn´t going to operate the GCB because it is faulted.Aditionally if prime mover start to reduce machine´s speed, surely, the generator is going to have a catastrophic fault. In our case, It was the fault that happened to our 750MW unit.To conclude, we have to test periodicaly both all protection systems and GCB, in order to be sure that they are ready to operate properly in case of a emergency.Mainly to units which are on line long time.Regards.


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ASHRAFUnfollow Follow ASHRAF

ASHRAF ISMIEL •A loss of excitation will cause the generator to start drawing more and more reactive power from grid and voltage levels will drop. Eventually, the magnetic field between the stator and rotor degrades too much, pole slipping and loss of synchronism will occur. At this point, catastrophic damage will likely have been done to the generator.



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BhalchandraUnfollow Follow Bhalchandra

Bhalchandra Tamhankar •If generator on load looses its excitation and GCB does not trip, (reason may be failure of loss of excitation relay or breaker fail to open). For good designed protection system this is hypothetical question. But I will try to put my views for two scenarios separately

1) If relay has failed (to detect and operate)-

a) GCB and turbine both does not trip:- Generator will enter in to asynchronous operation above synchronous speed(as a Induction generator) as steam flow will also be there, till turbine trips on other independent protection like over speed etc. How ever great extent of overheating will damage the rotor. (retaining ring in cylindrical rotor or damper winding in the salient pole machine).

b) GCB does not trip but turbine trips: Generator will run as large induction motor drawing reactive and active power from grid to keep shaft in to rotation. Reverse power protection will operate and trip the set. If it does not trip in few cycles, damage to generator rotor is sure. I can not comment on what will happen to turbine blades

2) If relay has detected loss of excitation and impart tripping signals to GCB and turbine:Turbine will trip by loss of excitation relay and Local breaker backup protection (Stuck breaker protection ) will save the set by tripping connected units and lines on the same bus.





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JUMUnfollow Follow JUM

JUM ARA •IF GCB NOT OPENED when exitation loss, I thing your generator system of safety devise is not working properly , not will damage to the machine if the generator runing alone but if your generator runing pararel the generator will runing as motor so that the alternator will be damaged engine is likely to



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Ejaz AhmedUnfollow Follow Ejaz Ahmed

Ejaz Ahmed Raja •do check if you have a reverse power protection and also the breaker failure protection . discuss with electrical engineer to know the action of protections in case of GCB failure.



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BradleyUnfollow Follow Bradley

Bradley Piatt •To answer your question directly if you lose excitation with the generator connected to load you will instantly turn the machine from a synchronous generator to an induction generator. The first damage will likely be ot the rotor but in a few moments the stator would be damaged too and you will make a repair




shop quite a large amount of money. DO NOT ATTEMPT THIS DO NOT BYPASS THE PROTECTION!!!!



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VENKIDU PERIASAMY •we aren't bypass protection we want to know the real factor.. thanks Mr.bradley piatt



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DaveUnfollow Follow Dave

Dave Sawicki •Earlier posts were good senarios of IF the fail safe protection devices were to be inoperative. The first device to activate should be the REVERSE POWER RELAY (I liken this to an electrical check valve) it prevents motorizing of the generator. The second device to activate would be the govenor overspeed trip which will shut off the steam to the turbine. BTW, the condition where a turbine spins at a high rate of speed without cooling flow (steam) is called WINDAGE.



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Bradley Piatt •The machine should trip on loss of excitation and or field breaker open



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Dave Sawicki •My apologies, my statement should have been a bit more concise -

- Upon loss of exitation the Reverse Power Relay should trip preventing motorizing.

- The main breaker should then trip/open thereby removing the generator from the bus.

- When this occurs, the generator which was acting as a brake to the turbine, when released, will cause the turbine to rapidly increase in speed to the point it reaches the limit set on the govenor overspeed trip thereby protecting the prime mover (turbine) from further possible damage.

Dave



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Bradley Piatt •That is incorrect. The generator can put out positive power as an inductive generator and may not reach over speed but the rotor will over heat



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Bradley Piatt •During normal operation of the Synchronous Generator or Alternator excitation to the rotor is provided by the excitation system a small DC generator or an inverter system generator. When synchronous generator delivering power to the grid or load it delivers both real power and reactive power to the system. Reactive power is the magnetizing power which helps to deliver the real power from source to the load and also reactive power helps to improve the voltage of the system. Almost all the loads (except capacitance loads) in the power system draws reactive power from system. Real power delivered by the generator (MW output power) is governed by the prime mover and the Reactive power delivered (MVAR output ) is governed by the field excitation. When field excitation to the synchronous generator is lost then synchronous generator operates as induction generator and instead of delivering reactive power it absorbs the reactive power from the system as much as 2 to 4 times the generator's rated load. However the real power (MW output) delivered by the induction generator will remain almost the same as this is controlled by the prime mover.But loss of generator's reactive power in the system may cause instability to the system. Also, as alternator is operating as induction generator, high currents are induced in to the rotor teeth and wedges and may damage the rotor of the generator. You could try to run it a while but the field will likely be damaged which is it wise to trip the turbine on loss of excitation>



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Dave Sawicki •Brad, Thanks for your supplemental input. Judging from the info provided in your profile you are by far more the subject matter expert than I. My only experiance with synchronous steam generators was



with the Navy. Always interested in learning more. Please feel free to email me, it would be nice to hear from you.



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SantoshUnfollow Follow Santosh

Santosh Mondal •your question is not clear. you didn't mention the status of TLR(turbine lock out relay) or MFR(master fuel relay) when GCB didn't operate. There are several consequences depending on the position of the TLR. to say in brief, if TLR do not operate along with GCB in the case of loss of exicitation, the alternator will still then be able to deliver power to the grid but of much less amount. It will then run asynchronusly as a induction generator. Forged steel rotor body will then act just like the rotor of a induction motor. huge rotor current will induce in this case and can damage the rotor. And if the TLR trips but GCB didn't open, then the alternator will start running as a induction motor at no load taking huge inductive load from the grid.Stator winding will be damaged due to overheating. details may be discussed later on, if needed.



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aurangzaibUnfollow Follow aurangzaib

aurangzaib chishti •no matter that you r running in combined cycle or any other cycle of plant operations, The effect would be only Generator Because when loss of excitation occured and Generator acts as an induction generator and reactive power will draw from system as many partispents said, fist of all heating of rotor will occured and due to large reactive power instablity in the system, cause the breaker trip station transformer as well, GCB failure is not final protection, is it primary protection and complex is design for keeping in view all posible situations so practicaly very difficaut to experianced these type of problem,When always exciataion breaker tripped during operation first action tripped GCB to protect generator, means if fail GCB take considration fist Generator after effects, and in this snario most important perameter which would be disturbed is reactive power, so its means 2 to 4 time ractive power cause increased Amps on the system will also increased and overcurrent protection will be aditional monitoring for rest of system.





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thakorUnfollow Follow thakor

thakor patel •machine should tripped on reverse power if not tripped Lp/hp stean turbine blade damaged due to steam condensation on hp blade



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AbdulUnfollow Follow Abdul

Abdul Majeed •in short generator will run as Induction generator slightly at higher speed if steam valves are closed. If generator has not damping coils damage will also occur to generator winding.If there is no vacuum in condenser trapped air will heat the blades. If this situation remains for long time blades will be damaged.

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