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Reverse power operation
Continuous process industries, DCS questions. topic
1 out of 1 members thought this post was helpful...
Posted by sd on 3 February, 2009 - 5:51 am
I have a very interesting experience to share with all here. Last month we had a small short circuit at our interposing panel which cause the Mark V to unload and shutdown the GT. However the short circuit has also caused the 125Vdc supply for 52G operation to trip.

When this happened, 52G remained closed even after the GT when to shutdown mode. When the guys ring me up on the matter (i was about to have my dinner tho.. ;)), I asked them to check out the breaker position and once they normalize the breaker, 52G opened.

Due to this, the generator was on sync motoring for about 10 minutes. Although there were some bad burnt smell throughout the switchgear room, our inspection shows everything is in good condition (god bless those ABB SACE switchgear engineers ;) ). I also monitored the generator winding temperature throughout the motoring period and there were no significant changes. The vibration level were also normal.

Throughout the motoring period, the speed and terminal voltage remained the same although the load has gone down to zero. We started back the unit and there were no problem.

What I'm still wondering is that from what I have read on books and articles before, motoring of the generator would cause some serious damage but in my case everything is 'seem' to be normal.

Any one have experienced similar situation before hope can share here.

Lastly I had one question by my guys that when the generator is motoring, should it be on the opposite rotation? I'm very sure that as long as the generator is sycnh to the grid, the phase sequence would be the same and hence the rotation should remain the same. Isn't it?



Posted by CSA on 4 February, 2009 - 12:17 am
You said the unit was operating in reverse power, and you say the load was zero. 0 MW is not reverse power; it's zero power. Reverse power is negative MW, like -1.0 MW or -3.0 W, or -5.0 MW.

The only difference between a synchronous motor and a synchronous generator is the direction of current flow/torque flow. If current is flowing into the synchronous machine and torque is flowing out of the machine, it's a motor. If torque is flowing into the synchronous machine and current is flowing out of it, it's a generator. (Same is true for induction machines and DC machines!)

When you put more torque into a machine that is connected to a grid than is required to keep the machine spinning at a speed that is proportional to the grid frequency, it's a generator, and current is said to be flowing out of the machine (positive power flow from a generator). If the torque decreases below that required to keep the machine spinning at a speed that is proportional to the grid frequency, it's a motor (negative, or reverse, power flow for a generator).

Dynamic braking of an AC motor is really turning it into a generator.

The direction of rotation is not affected by the direction of current flow or torque flow. You said the speed remained the same; that's because it was still connected to the grid and the speed of a generator is directly proportional to the frequency, and vice versa.

It's good that you say the generator terminal voltage didn't change, which means the excitation was probably stable. The lack of excitation is probably the worst condition for a motoring machine; it causes a great deal of heating of the generator rotor (induction heating). And because you say the speed was also stable, that's another indication that the excitation was probably on and stable.

I would submit that the smell you experienced was more related to the short circuit than to the reverse power flow, especially if the unit just "sat" at zero MW, which is really zero power flow, not reverse power flow.

There are two really damaging effects of motorizing a generator or reverse power flow (probably more, and I'm sure we'll briefly and nebulously hear about them all). The first is that when the machine is in reverse power it is trying to turn the prime mover that is supposed to be providing torque to the generator. In other words, the synchronous machine is providing torque to the prime mover instead of the prime mover providing torque to the synchronous machine.

This can be very damaging to the prime mover, depending on the type of prime mover. This is really not good for most steam turbines. They are trying to pull steam into the turbine. Many reciprocating engines don't like to be rotated by the synchronous machine they are supposed to be rotating.

Gas turbines, on the other hand, don't really dislike this condition. Because the air flow through the turbine and compressor doesn't change directions the unit can withstand "low" levels of reverse power. At some point, the flame would be snuffed out by the air flow. At that point, the synchronous machine would just be a big air compressor drawing LOTS of power and most synchronous machines are not designed for that much reverse power flow.

The other really damaging effect is the heating of the generator rotor and stator that can occur with excessive amounts of reverse power. This usually occurs when there is very low excitation or no excitation, and from I've been told, the heat rise can occur very quickly and be very damaging. From what you say, the stator, at least, didn't experience any appreciable temperature rise.

So, it doesn't really appear like the unit really suffered a reverse power condition--from the zero load condition you describe to the lack of stator heating to the stable speed and generator terminal voltage.

Are you sure the load transducer can actually indicate negative power flow--less than 0 MW?



Posted by sd on 5 February, 2009 - 3:23 am
Yes the excitation to the generators was still intact during the motoring period. As for the MW display, although my historian records zero MW, my operator until today insists that he saw a 12MW load on the HMI. The more confusing part is that none of the protection relays (esp reverse power) operated.

The reverse power relays were calibrated recently and I believe that they should be in working condition. So can we consider this event as full speed operation of the generator by the grid or as motoring? (this is getting more exciting ;))

As for the smell incident, I dont think that its caused by the short circuit as the distance from the place of short circuit is very far and its only noticeable in the relay room.

I dont think that the MW transducer could display negative flow of load as normally during unit shutdown, the value would be negative (maybe now is the right time to calibrate those nasty transducers ;) )
Have any one experienced an over speed condition or problems with the load coupling in an event of motoring?

I would conduct my partial discharge monitoring on the generator this week to assess any damage that might have occurred on the stator coils. Lets keep our fingers crossed ;)







Posted by CSA on 5 February, 2009 - 3:09 pm
Ah, yes. Lovely Historians. Configuration of Historians to collect meaningful data can take years and even then, the data is subject to all kinds of conditions and situations.

I would estimate that if the unit were drawing 12 MW (-12MW should have been the indication on the HMI) from the grid that that would have sufficient to blow out the flame for the minimum fuel flow that was probably being experienced.

The reverse power protection usually operates to open the breaker using the same coil as any other protective circuit; it's usually in parallel with several other contacts to energize the breaker open coil. So, if power was lost to the generator breaker open (trip) coil, then the reverse power contact can't open the breaker. Right?

The speed of a synchronous AC machine is dictated by the frequency of the AC "source" with which it is connected. The formula is:

N = (120 * F) / P

where N is speed, in RPM
F is frequency, in Hz
P is the number of poles of the generator (usually the rotor)

So, if a machine is connected to a grid that is operating at 50.0Hz, and it has a two-pole generator rotor, the generator will spin at 3000 RPM. No faster, and no slower.

When the prime mover is providing torque to the generator, it is trying to spin the generator rotor faster. But, the frequency of the AC grid won't let the generator rotor spin any faster. The extra torque, over and above what's required to keep the generator rotor spinning at the same speed as the dictated by the grid frequency (which would be zero power flow) gets turned into amperes by the generator and "powers" some of the grid load.

If the torque being provided by the prime mover drops below that required to keep the generator rotor spinning at a speed equal to that dictated by the grid frequency, then the synchronous generator starts to draw power from the grid, becoming a motor, and providing torque to the prime mover.

If one were to keep the generator breaker closed with the excitation running and block the reverse power relays from operating to open the generator breaker, and shut off the fuel to a combustion turbine, then the generator rotor would continue to spin at the same speed as before the fuel was shut off. That's because the frequency of the grid dictates the speed of the generator rotor as long as it's connected to the grid.

Because of the power required to drive the axial compressor of a combustion turbine (we're talking about a single-shaft combustion turbine, not a multiple shaft turbine such as a aero-derivative turbine but a heavy duty single shaft combustion turbine), the power drawn off the grid might be huge! Remember: Roughly two of every three HP produced by a combustion turbine is used to drive the axial compressor. That's why they're roughly 33% efficient (in simple cycle mode).

So, to keep the compressor spinning would require a lot of power under the above conditions.

If the fuel were gradually reduced to the point that the power output of the generator was zero MW, and then reduced slowly even further so that power flow went negative (as is typically done when shutting down a GE heavy duty gas turbine) and then suddenly held constant, there would still be a small amount of power being provided by the fuel but the generator would be drawing power from the grid (motoring). If the fuel were reduced further and the generator breaker didn't open then it's likely that at some point the air flow would blow out the flame, and at that point the generator would really be drawing some serious power off the grid.

The unit would still be rotating in the same direction at the speed dictated by the grid frequency, and it would just be a big compressor, blowing air through the unit and out of the exhaust using the generator as the motor to drive the compressor.

So, there can't be an overspeed condition. Nor an overload on the coupling, unless the load coupling is sensitive to the direction of torque or there is a serious loss of flame and prolonged operation while motoring under some pretty unusual conditions.


0 out of 1 members thought this post was helpful...
Posted by Namatimangan08 on 27 March, 2012 - 2:22 pm
I know one my client's hydro unit. His generator went to reverse power. Protection relay operated. Breaker opened but one pole had failed to open. Secondary protection relay failed to pick up. So one pole remained in closed position. To make the thing worse, the breaker was remotely located. Probably about 15 to 20km from control center.

The plant operators managed to open the pole that failed to break after more than 1hour. What will happen next? I think happy ending. That unit got new windings. After all the generator was about 25 years old during that incident.


Posted by sumeet on 4 July, 2013 - 2:07 pm
could anyone explain me why situation would be more worst when we lose excitation while reverse power operation of generator?


1 out of 2 members thought this post was helpful...
Posted by Phil Corso on 5 July, 2013 - 11:08 am
Sumeet... My interpretation of your question is, "A Loss-of-Excitation (LoE) event occurring while the machine is already responding to a Prime-Mover failure (LoPM) is worse than the LoPM, itself!"

If my interpretation is correct, then it is not worse because "typical" protective relaying practice blocks the LoE-trip until the Reverse-Power relay trips!

Regards, Phil Corso


Posted by A. OZTAS on 4 February, 2009 - 4:23 am
Well, in case a short in your interposing panel, hard to predict what will happen because you don't know which relays are tripped during the incident. It depends also whether your system is black start with dead bus etc. Without having the trip history, will be guess work.

In any case, if you loose the DC power to your Gen. Breaker the breaker will indeed not open (fail safe to remain closed). You will have to disengage the mechanical locking system manually from the breaker cubical.
As regard your inquiry for motoring of the generator, indeed the Gen. will not spin in reverse direction because the prime mover (GT) is rotating the load shaft CW. Motoring means actually reversing of the Gen. current. The Gen. needs minimum amount of current to maintain it's magnetic flux in order to work as Gen.So if the Gen. dropped the load VAR will increase. How can you see all this, well again your VAR will increase and the COS PHI will be low maybe go to negative. The major problem in the Gen. will start if you also loose the excitation, you will heat up excessively the rotor windings.

Good Luck...TEMPUS FUGIT



Posted by surameen on 9 February, 2009 - 9:43 am
About Gen Breaker, if all the protection use DC supply. When DC supply fail, and there is no protection for Gen Breaker and Generator.in this situation will the breaker remain close or trip. If consider of fail safe condition, should the breaker trip is't.?



Posted by CSA on 9 February, 2009 - 1:34 pm
Methinks you are making an assumption.

Would you recommend using an AC supply for protection? (I wouldn't.)

Most generator breakers (at least the ones I've worked on) all have coils that are energized to close the breaker and a separate coil that is energized to open (trip) the breaker.

If loss of DC protection was considered during the initial design of the plant and considered to be a potential problem, then appropriate measures would have been taken as seen fit to mitigate the potential problem. I have seen a couple of installations that used multiple breaker trip coils, though I believe both were supplied by the same 125 VDC battery (through separate fuses). On one installation, an interposing relay was used to provide multiple contacts to drive the two breaker trip coils, and the coil of the interposing relay failed and it was necessary to open the breaker manually.

I can't even count on one hand the number of times I've heard of a DC failure resulting in an inability to energize a breaker open (trip) coil. Usually the fuses are sized such that they are the last things to fail in a breaker trip circuit.

In fact, the only two times I've ever heard of such conditions are here on control.com on this very post.

I believe another "back-up" protection scheme which would become active in the event of such a condition would be some transformer protection relays acting to isolate the transformer in the event of "excessive" reverse current flow (flowing into the plant instead of out of the plant).

Everyone is free to have a power system study of their installation to see if the safety and reliability can be improved.

There's a *lot* of confusion about what constitutes 'fail-safe'. And most every manufacturer sees fit to bandy the term about and use it in a manner that makes their systems or schemes seem superior to others. For some systems and processes, there are very concrete 'fail-safe' elements of design and operation; for others, it's quite open to interpretation. In fact, it could be rightly said that many times the term is used when it is unnecessary.


Posted by sd on 10 February, 2009 - 1:51 am
As for the DC supply to open and close the breaker, I did try 2 open and close the breaker at local with the 125Vdc supply isolated and it can be done. Its just that the breaker could not be operated from the GCP and by Mark V. This is because the supply to the breaker at local is not 125Vdc but 220Vdc. Do anyone have similar configuration?

I think the reverse flow of the current would not trip the transformers as current always flows in reverse when the plant is on shutdown and the auxiliaries are powered from the grid. The only possible condition that would need a transformer trip would be differential voltage and also over current.

Is there any method that can be done to mitigate this failure mode? I mean to have a fail safe tripping whereby when ever the GT trips, the breaker is to open even with or without it's 125Vdc supply. The only possible method that I could see is to tap a secondary 125Vdc from the neighboring panel so that in an event of either 125Vdc supply is interrupted, the supply from the other panel could be used to open the breaker.

About the overspeed condition that might happen, would it be more appropriate for steam turbines?




Posted by CSA on 10 February, 2009 - 2:24 pm
So, what you're saying is that there is an interposing relay between the Mark V and the generator breaker trip circuit and the coil of the interposing relay requires 125 VDC and that the trip coil of the generator breaker uses 220 VDC. Further, the 125 VDC supply for the interposing relay coil failed but the 220 VDC supply for the breaker didn't? (That's how the operators "normalized" the breaker during the event?)

But, you didn't say this in the original post.

The reason that an interposing relay was probably chosen was because the current requirement for the *generator breaker trip coil* circuit exceeded the capability of the Speedtronic relay contacts at 220 VDC.

I imagine that if the current required by an interposing relay coil operating at 220 VDC did not exceed the contact ratings of the Mark V relay, you could use a properly fused 220 VDC source for an interposing relay with a 220 VDC coil through the Mark V to drive the interposing relay.

In other words, determine the current required by an interposing relay with a 220 VDC coil with contacts sufficiently rated to switch the generator breaker's trip coil. If the current required by the 220 VDC interposing relay coil is less than the Mark V relay contact rating's maximum, then you could replace the 125 VDC interposing relay with the 220 VDC interposing relay. Size a fuse to protect the 220 VDC interposing relay circuit (including the Mark V relay output circuit), and use 220 VDC as the supply through the Mark V relay contacts for the interposing relay instead of the 125 VDC supply that "failed".

I believe the nominal Mark V relay contact ratings can be found in either the Mark V Maintenance Manual (GEH-5980) or the Mark V Application Manual (GEH-6195). Or, look at the relay itself; many times the ratings are printed (in very tiny, hard-to-read print) on the case of the relay.

Or, if you can find the manufacturer's name and part number, you could look up the contact ratings at various voltages, and possibly contact the manufacturer or a manufacturer's representative to ask the current rating at 220 VDC if that information isn't on the relay or available on the manufacturer's data sheet/catalog.


Posted by sd on 11 February, 2009 - 5:22 am
Sorry I left out those crucial information in my first post. I will look into my drawings and relay ratings. Hope I can come up with something. I will surely update you guys ;)




Posted by Clifton D on 4 February, 2009 - 12:07 pm
We just had 2 Siemsns V type generators motored for about 20 minutes. During the event they were pulling in 25MW and 80MVARS, the unit was running as an induction motor with less than 1% slip, therefore the current induced into the rotor forging was very low. We pulled a retaining ring and some end wedges and found no damage what soever. The fact that the breaker never opened kept the generator close to syn speed which saved us. By the way the rotor will not spin backwards when motored from the grid you normally supply.


Posted by John H on 27 March, 2012 - 2:03 am
Motoring a generator without generator damage is an interesting thing. I have seen this same set of circumstances a couple of times in my 30 year generation career. Once with a 3 MW EMD generator generating at 4160 and once with a GE Frame 5 Generating at 13,800. The EMD had a brush less excitation system and the Frame 5 had an old static excitation system. Both times the machines lost the DC trip power at the same time the machine lost the ability to receive fuel.... Each time causing the generator to have to become a motor and pull the prime mover.

In the case of the EMD a slightly low coolant temp triggered an engine shut down - the fuel shut off to the engine but the Breaker did not trip. The Frame 5 lost a battery charger and the 125 DC system finally depleted until there was not enough power to hold open the fuel solenoid - or trip the breaker.

Each of these generators were not damaged in any way. I checked every thing there is to check... It is amazing how much noise and EMD engine will make when being driven.... and the compressor load on a frame 5 that is being driven is about 10MW.

The saving grace to both of these machines is that when the machines cannot trip the breaker and the excitation system stays on there won't be much heating in the rotor... If the controls are such that the excitation system does shut down the slip between the rotor and the stator will become very large and create wayyyy too much heat really fast.... odds are things will get liquid before anybody can do anything about it.

In both of my circumstances the generators were motoring for around 1/2 hour and I tripped the main breaker with the manual trip on the breaker. So as you can see... if the excitation system stays on you never loose the magnetic lockup with the system... Thereby negating the creation of a lot of heat.... So see.... if you had a good generator controls guy design your system it would not shut down the excitation system in reverse power..... I know this is an old post and just found this website... but had to throw my 2 cents in...

John

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