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.

 

[wolf39]

There is an overspeed after the generator is tripped. Question is whether is was a full-load or part-load rejection. Nevertheless, because of the overspeed situation the coasting period is extended compared with a normal no-load shut-down.

The Pelton runner should be installed well above tail water level. However, if at the time of load-rejection some other units were operational under full load, the tailwater level may have been high enough to disturb the unit in question. Also, the tailwater channel may have been partially blocked or the tailwater could not have escaped properly because the tailwater outlet led into a stream or river with above normal water level. Was such a temporary flood scenario present at that time?

Regards

Wolf

http://WWW.HYDROPOWER-CONSULT.COM

 

[Karmicpower]

Well wolf39, there is enough clearance of the runner from the TWL say about 2 mtrs from the surface.However, the tail water ends up in a perpendicular channel before it enter into a pond.

The turbine is housed inside a casing/hood and we have had problem with the turbine efficiency attributed to the rise in tail water level. To cope up this issue, an aeration duct was later on put in place which almost solved the problem.

My doubt here is, due to the sudden throwing off of the load, a turbulence might have been created inside the housing which blocked the aereation pipe and resulted into momentary creation of vacuum. But will the vacuum have that amount of braking force to nullify the high inertial momentum of the runner?
Please highlight.

 

[wolf39]

A sketch or photograph of the tailwater section and the aeration duct would be helpful.

Vaccuum would even increase duration of coasting. However, if vaccuum is resulting in increasing the tailwater level, this would be an answer. I can't imagine, however, that vaccuum was created and kept for so long. There must have been an unusual and temporary situation on the tailwater side.

You are mentioning that you were near the unit at the time of this incident. Was there anything noticable? Unusual noise or vibration?

Regards

Wolf

http://WWW.HYDROPOWER-CONSULT.COM

 

[rasevskii]

Sounds like a really bad tailrace design. Under no conditions should a vacuum exist in the pelton housing. Adding an air vent will do little. The wheel should never flood out. There is no effect on efficiency until the wheel gets flooded out, then you can forget all your calculations.

It can in fact be dangerous resulting in a failure of the wheel or the housing. See what happened in Siberia...

What every good pelton turbine wants to see is a tailrace channel the cross section of a garage door straight out to the river. The water leaves and never bothers the turbine again.

Forget about the noise. Most engineers find the sound from a pelton to be absolutely beautiful...

Why not send us some photos of your plant.

regards, rasevskii

 

[wolf39]

Karmicpower:

Don't forget the noise. I didn't mean the humming sound comming from a Pelton unit in proper operation. I meant noise originating from possible contact with tail water.

Regards

Wolf

http://WWW.HYDROPOWER-CONSULT.COM

 

[PHovnanian]

Regarding the vacuum hypothesis: Many yeads ago, I had an opportunity to work on a small Pelton wheel hydro plant. The owner (and builder) related a problem he had experienced with a vacuum building up in the casing (due to poor tailstock design). The solution was to add ventilation to break the vaccum.

The symptom of the problem was a 10 to 20% reduction in power output. Not what one would expect of the buckets were actually hitting high water. But the turbine builder had stated that this was due to the increased drag created by spray or mist above the water. Breaking the vacuum dropped the water level as well as the layer of mist above it.