Generator breakdown

[Denan]

Hi all,

A generator was damaged few days ago, and I received some pictures showing few curves at the moment of the failure. I am not sure what the cause of the failure is/could be, so I wonder if someone out there might have seen similar before. There are few more curves I have but I think these are best visible (I got a scanned copy)

The generator voltage is 10,5kV, and it was running parallel with grid.

Stator windings are damaged, rotor seems to be ok.

We are planning to have an inspection to try to fing the cause but I don't know when that might be.

 

[wolf39]

It would help if your handwriting was legible.

Hydro generator or turbo or diesel?

Wolf

 

[Bramb]

Hi Denan,
more data are needed in order to try to suggest you something....
- Which protection relay(s) is(are) tripped?
- Which are the primary scales of the fault record attached to your post?
- Where is the fault recorder connected? (sld showing CT's & VT's will be helpful)
- Which is the generator rated current?

Ciao
Erminio

The difference between overload and short circuit lies in the nature of the fault, not in the value of the current.

 

[Denan]

I am sorry but that is all I have, the text is in Dutch, and I don't understand what it means. I have asked to get translation and also color copy, but I am still waiting for that.

Generator is salient pole, gas engine driven, 1000RPM.

I have also asked for log from the protection units, hope to have them very soon.

 

[Bramb]

My first impression is that the generator have fed an external 3ph fault (duration around 70 ms).

Then I suppose they've got a disconnection of the grid and the plant started to run in island condition: your generator seem to start to regulate voltage and frequency in the network, if I'm not wrong the increase of the excitation current is in accordance to this idea.

Here a further question arise: is your generator the only installed in the plant? Yes should not be the right answer as the voltage, the first trace in the fault record, anywhere it was measured, does not go to zero when the generator is stopped.

In the following 2 seconds I don't see any other fault current.....
Are you sure the stator is damaged?

This is only a preliminary idea... further investigation are needed.

Ciao
Erminio

The difference between overload and short circuit lies in the nature of the fault, not in the value of the current.

 

[Bramb]

...An overcurrent trip???? It could be compatible with the time delay (about 2 s) and it could be caused by malfunction (or absence) of load shedding function in the plant...

We are back to previous questions: which is the generator rated current? And which are the primary values of the traces?

Sorry for going on with this brainstorming....

Thanks
Ciao
Erminio

The difference between overload and short circuit lies in the nature of the fault, not in the value of the current.

 

[Denan]

Erminio, thanks for your comments so far.

I am not sure what is In of the generator but I believe someting around 300A. I will check tomorrow though.

Regarding your questions from your first post: so far I don't have anything more than these trends, but I have asked for more data. Fault record is done by the grid company (or someone else on their side).
If you look at the voltage curve (on top of the page), it shows 15 in the begining. Suppose it is in kV, since the RMS value of that would be 10,5kV which is Un of the generator.
For current, I think is shows values in Amperes, and same for the excitation current - nominal excit. current is somewhere below 4A.
I have no idea how they could have this measurement of the excitation current since it is not connected to any external unit, it goes directly between the AVR and the generator.

The handwriting at time 14:58:53,2 (where current goes to zero) says Breaker out. It can be grid breaker or generator breaker, I am not sure which one it is.

And you are right, there is another machine at the plant, same as the broken one, but they are not connected together. Or, they don't share the same busbar, and are connected with their own grid connections. The machines are placed in the same building.

Voltage that you pointed is not going to zero, is probably measured at grid side of the breaker, while the currents are measured at generator mounted CTs. We use the same CTs and VTs for our control cabinet, so I am hoping to get log from our protection relays to see what happened there, which breaker tripped etc.

I will keep you updated as soon as I know more.

 

[eeprom]

This may be stating the obvious, but that's usually how I figure things out.

There is a large change in current which affects all three loads. I would guess this is probably not a fault because it is on all three phases, and it is not very high compared to operating current, not like a fault would be. The change in load was big enough to cause a transient, and probably a dip in frequency. Considering the number of cycles in the transient, this could be a 50 Hp motor starting across the line.

The excitation curve shows that the exciter is working pretty hard to keep the voltage up, but it does appear to be stabilizing prior to the trip.

Whatever caused the generator to fail does not seem to be apparent in this information.

How do you know the stator windings are damaged? Megger results? Hi pot? Perhaps you need to look further back in time. Is there a record of the winding temperatures?

EE

 

[Denan]

Hi,

Thanks for the input, I will ask for winding temp. record, not sure if it is available.

I got information from site about the damage, I have not been there myself yet, it is in another country.

I attach SLD for now..

 

[Bramb]

Hi Denan,
I'm still putting ideas on the table...
According to sld generator CT's are 150 A primary: machine rated current should be less.
The currents measured from 14:58:52,250 is 250 A (peak) = 175 A (rms): overcurrent protection trip is becoming a more real hypothesis.... But I agree with Eeprom that it should not damage the stator and to look at the past could be an explanation of the internal fault.

Due to the shape of the currents (which in my opinion shows a dc component) I'm still convinced that the first event (14:58:52,040) was a short circuit in the network. Another idea related to that could be out of step conditions: but in such a case I would expect rather a rotor mechanical damage than stator damage.

Finally I'm not able to find any explanation for the line currents and for the excitation's shape recorded at 14:58:53,500....... except an internal 3 phase fault which appear while the generator is stopping....

Please Denan go ahead collecting data an keep us updated!!
Ciao
Erminio

The difference between overload and short circuit lies in the nature of the fault, not in the value of the current.

 

[Denan]

Thanks Erminio for the ides. We were also thinking of some kind of short circuit.

When I first heard of the event, I also got info that it might have been that the breaker opened but got a flashover which may have caused this.

I am still waiting for more info..

 

[Denan]

Generator Sn=3021kVA, PF=0,8. CTs inside generator are 250/1A (apparently there is revised SLD which I did not find earlier).
CTs outside of generator are 200/1 (for diff.relay)

 

[eeprom]

Just some thoughts...

If it were a short, the excitation current would have stayed at max value. The decrease in excitation current suggests that the loading (after the transient) is within range of the generator. The waveforms provided look pretty much exactly what you'd expect of a motor starting.

Prior to transient, current mag is about 200. After transient, current mag is about 250. So, we have a 50 A load.

Transient time is about 12 cycles. Transient max current is 500A, or about 300 above pre-transient conditions.

A 50A induction motor would normally draw 5-6 times its FLA on start up. This fits the max value of the transient. Depending on the load on the motor, and the generator capacity, a 12 cycle transient is reasonable.

 

[rasevskii]

At 1000 RPM and 3 MVA this is likely an open circuit air cooled unit
(or is it closed circuit water cooled?) driven by a diesel on an unattended site. If these assumptions are true (OC air cooled)and the environment is a desert area (oil patch) or a tropical site, look for accumulation of dust/dirt/sand or moisture/oil in the stator windings. This would result in poor cooling and a possible hot spot, resulting in an eventual failure after many weeks/months of continuous operation.

On the site, get someone to remove some covers from the generator and do an interior inspection and make photos.

Also the info on what relays tripped would be useful.

How heavily was the set loaded? Temperature readings from RTDs in the stator ans air/in out would be useful.

just some ideas.

rasevskii

 

[Denan]

Thanks guys for sharing your thoughts!

Rasevskii, you are partially right, it is air cooled generator, driven by gas engine. It runs unattended, but it is in Europe (the Netherlands). It is around 6 years old. It could be dust as you say, I have been in the area before, and it can be pretty dusty around there.

I will probably be travelling to site on friday, and hopefully I can get some log from protection relays, to see what tripped. We intend to check both the generator itself, but also control system and the breaker.

 

[rasevskii]

It sounds like a landfill gas generator setup. If so, likely that the landfill operations are/were stirring up a lot of dust, coupled with dampness may have coated the stator windings such that the air cooling passages became partially or largley blocked resulting in overtemperatures (my guesstimate). The stator winding RTDs should have signalled this situation and shut down the unit. The actual temperatures as measured on the stator-embedded RTDs should all be within about 5 deg C of each other in normal service if the cooling air is properly distributed inside the stator.

I may be all wrong on this as some other fault may have occurred.

The temperature limits should be set according to the OEM specs, but this setting may be in fact too high. For Class F insulation the limit is something like 110 deg. C (or more) but this is already asking for trouble. (my opinion only)

Others may want to comment here...

rasevskii

 

[Denan]

Hi again,

Now I got alarm list from SCADA, and it shows following alarms:
Under frequency and under voltage, that is all. Unfortunatelly, alarm log from the protection unit dows not show anything from that day for some reason, nor several days around that day.

I attach one more picture showing the event. I can not find the attachments from earlier, do they get deleted from the thread?

If someone needs it please let me know, and will attach again.

 

[rasevskii]

The last attachment is missing from your post.

If you are on the site or have been, were the generator casings/covers removed and the stator inspected? Are there any photos of the damage? Megger readings? Switchgear/relays/controls inspected? Rotating diodes/excitation system tested? ...etc...

rasevskii

 

[Denan]

I try again...

I have not been at site yet, I have not seen any pictures etc.
Maybe during this week I will get a chance to travel there.

http://files.engineering.com/getfil...3-dcd3-42d6-914f-50756190ef6a&file=curves.jpg

 

[rasevskii]

From the last attachment picture, a fault occurred that caused the excitation current to go high and the stator current also to go high, just before the unit tripped (excitation went off)then the stator current dropped to a low steady value some 5+ seconds afterward.

This rather looks like an external fault as there was still stator current after the excitation trip, but that depends on where in the system the current reading takes place. Did the generator breaker actually open? If so why is there still some stator current?

When you get to the site, better have the OEM rep present with you and have a thorough inspection done of the entire plant.

If it is, for example, ABB or Siemens, get their service dept in NL involved.

rasevskii