Unusual stopping of the generator 2

[Karmicpower]

Dear folks,
I am a newbie here, so please don't mind if i am not that descriptive of the subject.
Well, we have a 15 MVA synchronous (hydro) generator with 14 poles, rated speed of 428.6 rpm and moment of inertia of 12,000 kgm2. Recently, the machine tripped on some temperature fault and the machine came to a complete standstill within less than 3 minutes against a normal time of 35-40 minutes. We don't have any type of braking mechanism put in place. We were completely shocked by its unusual behaviour and inspected the runner, the NDE and DE bearings, air gap, IR of the stator windings, XLPE cable, etc. Everything was normal but we were skeptic to run it. However, we did run the machine and everything was just normal.
I just want to know whether anybody has ever experienced such incidence or have knowledge about what could be the reason.

 

[edison123]

Yours is a vertical or horizontal machine ?

I have not seen vertical hydro machines without any brakes/jacks, which are applied to low speeds to protect the thrust bearings.

Horizontal machines usually have a braking jet for the same purpose.

What sort of temp alarm that tripped the machine in the first place ? Winding ? Bearings ? Cooling Water ?

 

[Karmicpower]

Oh..i forget to mention...it is a horizontal shaft machine having two nozzle arrangement. We dont have a braking mechanism. And it tripped on bearing oil temp faulty signal. The tripping signal was activated while changing the sensors.The concern here is the unusual braking force which brought down the machine to stanstill in less than 3 mins.

 

[Kiribanda]

Karmicpower,

As far as I can remember, a sync generator can be stopped very fast if the 3-phases of its stator are short circuited till it is stopped, but at a reduced field current.

Could you please think of such a situation - such as unexpected gen breaker re-closing during its shutting down period?

I am sure that one of our learned members will throw some light on this braking method.

 

[Karmicpower]

Kiribanda,
There was no sign of stator winding terminals getting shorted. Moreover, the chances of the unit breaker re-closing is not possible as the syncheck relay will not allow that to happen and the events in the SCADA also doesnt show of any such reclosing.

 

[Viceng]

You could check to see if the deflector has come out of proper alignment. Perhaps, when it came into the stream it forced water to bounce off the turbine walls and back onto the unit...acting like a braking jet.

 

[davidbeach]

A three phase fault on the terminals of the generator would unload the generator and cause acceleration, not braking.

 

[waross]

If he generator is at speed, there is a very large amount of inertial energy to dissipate. I can think of three possible explanations for this event that may not leave physical signs of damage.
1> For some reason such as a misaligned nozzle or a partially flooded scroll case, the water acted to brake the generator to a rapid stop.
2> For some reason the generator was running below rated speed with much less inertial energy and so came to a stop much quicker than normal.
3> Instrumentation error. The stop time was normal but for some reason was reported as much less than normal.
On a completely different tack, is it possible that your control gear shuts down the water flow very slowly for a normal stop but on a lube fault, closes the control valve much faster?


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[wolf39]

Karmicpower:

A hydro generator disconnected from the grid, excitation then reduced to zero and with the Pelton nozzles closed is idling until coming to rest. If, however, the excitation is reduced to zero, the stator winding then short-circuited and the excitation thereafter increased to a point that rated current or even 120% rated current is generated, the resulting short-circuit losses decelerate the unit to standstill. The stator current remains constant until the speed is almost zero. Reason: Because the generated voltage decreases with speed (frequency) and the resulting inductive resistance decreases with speed also, the current remains constant. This is a very effective way to brake the unit. You confirmed that this scenario can be ruled out.

If the generator is disconnected from the grid and the stator remains excited, the iron losses would brake the unit as well but not as effective as in the short-circuit case as the voltage and iron losses drop with decreasing speed. Still, with the excitation on the unit comes to rest earlier than unexcited. Was the excitation still on in your case?

Your generator has not only two pedestal bearings. There must be an additional bearing which is limiting the axial shaft movement. Check this one. Also, is there some extra friction inside the Pelton wheel housing? Check for displacement of built-in components.

You may have had another occasion in the meantime where the unit idled until standstill. With the same result as mentioned in your original post?

Regards

Wolf

http://WWW.HYDROPOWER-CONSULT.COM

 

[rasevskii]

Hello Karmicpower.

The unusual stopping time can be as mentioned, an error in observation.
Is this a remote unattended unit or is the station manned?

The next time the unit was stopped did you get the same result?

Do you trust the SCADA system actually, or is there an operator present?

It is unusual to be working on a unit when it is running, usually not allowed. If a wire was lifted on a RTD of course it will trip as the system sees high temperature.
Is there a thermometer on the bearing. What is the usual reading?

It can be leakage at the nozzles gives a protracted stopping time assuming the deflector is not in deflected position in a normal stop.

Is this a modern unit controlled by a PLC or an old unit that was automated?

In any case 40 minutes normal stopping time is very bad for the bearings unless there is a HP jacking oil pump which has to run during stopping and starting sequence. How about this?

As said was the excitation on or not? Is this a brushless,static, or classic DC rotating exciter? In some very old machines the exciter is always connected to the field /a DC shaft exciter that is/ and the excitation builds up as the unit comes up to speed.

What part of the world are you in? It has to be in the mountains somewhere...

It can be of course a bearing problem, was there any change in operating temperature?

regards, rasevskii

 

[rmw]

I am a ME reading this thread and with the scant information given, based on (1) the drastically reduced coastdown time and (2) the fact that you tripped on a bearing or lubrication temperature trip, my mind goes to the possibility that you may have wiped the (a) bearing and that has caused the sudden stop, either because of the change in bearing friction itself, or due to potential rubbing somewhere in the train due to a bearing failure. But, I am like the rest, just guessing.

rmw

 

[dpc]

But the OP said inspection of bearings was negative, I think.

 

[rmw]

He did, but he didn't stipulate to what extent they were "inspected". Did they just pull the caps and take a look, or did they roll the bottom half out? It can be ruled out quickly, but not by a cursory glance.

rmw

 

[waross]

The OP indicated that the trip was a false trip caused by an accidental open in a monitoring circuit during maintenance.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Karmicpower]

I am happy that many intellectuals here are trying to give their best analytical answers to this problem. Some of the respondents have raised many queries and i would like to give specific answers to them.

1. The nozzle wasnt misaligned as we did inspect it and also can be infered from the post incidence operation.

2.The machine was running on its rated capacity and speed prior to tripping. The droop characteristics would not allow for such variation of speed.

3.It isnt an instrumentation error either, I was personally present near the machine witnessing the event. I was dumbstruck by its awry behaviour.

4.We have a brushless excitation system. It got de-excited instantenously with the tripping of the unit breaker.

5.There are only two hydro-static bearings; DE and NDE holding the whole turbine-generator mass.

6. It is a manned power plant with the modern tech equips. We have RTU-SCADA system for the control and monitoring part and a microprocessor based digital governor for the hydraulic part.Both the systems have been reliable as of date.

7.35 minutes of coastdown time is normal. We have lube oil circulating until the machine comes to a complete standstill. The pump was in operation prior to reaching the standstill.

8. We inspected the bearing shell on both the halves. The white metal( babetted) was just as fine with no signs of stress or shearing.

9. We test ran the machine on unloaded condition. No unusual noise, smell or vibration was observed. The machine also did not come to a sudden halt unlike before. This very incidence have been the first of its kind.

 

[edison123]

May be God made it do it. or Bill Gates...

 

[wolf39]

The only explanation left: There must have been a temporary backsurge of tailwater in the Pelton turbine housing, thus creating a strong braking torque.

Regards

Wolf

http://WWW.HYDROPOWER-CONSULT.COM

 

[rasevskii]

I agree with Wolf39. Since you had a trip at full output, there had to be some disturbance in the tailwater, which would not happen on a normal shutdown from no-load or running under test off line.

How far above TWL are the pelton wheels? It can be the design is not the best or that the TWL was high due to other units at high load.

Not a problem. All power plants have their oddities....

rasevskii

 

[edison123]

Wolf - That could explain it. But isn't the pelton runner well above the TR level ?

 

[edison123]

And wouldn't a full load trip result in overspeed first ?