Generator breakdown
Generator breakdown
(OP)
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.
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.





RE: Generator breakdown
Hydro generator or turbo or diesel?
Wolf
RE: Generator breakdown
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.
RE: Generator breakdown
Generator is salient pole, gas engine driven, 1000RPM.
I have also asked for log from the protection units, hope to have them very soon.
RE: Generator breakdown
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.
RE: Generator breakdown
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.
RE: Generator breakdown
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.
RE: Generator breakdown
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
RE: Generator breakdown
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..
RE: Generator breakdown
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.
RE: Generator breakdown
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..
RE: Generator breakdown
CTs outside of generator are 200/1 (for diff.relay)
RE: Generator breakdown
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.
RE: Generator breakdown
(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
RE: Generator breakdown
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.
RE: Generator breakdown
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
RE: Generator breakdown
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.
RE: Generator breakdown
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
RE: Generator breakdown
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:
RE: Generator breakdown
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
RE: Generator breakdown
I wonder if the other breaker (grid breaker, utility breaker) might have some faults, because in log of prot. unit, we could see that this breaker did change status many times lately (before the fault) which is not normal there. Also, several times, there was an alarm we call Grid breaker fault, also before the fault. It might have been that the breaker did not change status after opening while engine was running, causing high voltage on the generator (AVR is in PF mode when parallel with grid).
Also, several times before the fault log shows High voltage on the generator..
Unforunatelly, the generator is already on the way to some workshop for dismantling. But the breakers are still there, I think that they are ABB make.
But I have got few pictures from site:
h
RE: Generator breakdown
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RE: Generator breakdown
The protection should have tripped on earth fault (apparenly did) resulting in minimal damage at the point of fault in the winding.
It would seem that the control system has some/many issues. The breaker (grid breaker) may have been giving false indication due a faulty aux switch on it.
The PF control has to be only enabled when the Grid Tie is closed, otherwise the voltage may/will go too high if the unit is actually in PF control due the false indication from the grid tie breaker which was actually open (the unit in island operation.) However, there should be an overvoltage and under/over frequency protection to trip the unit in this case.
What is not clear in the recording is why there is still stator current even though the excitation had tripped. Where are the CTs located for the recorder input? If the gen brkr had actually opened, there would be no possible stator current. It may be that the unit stayed on line without excitation. The further oscillations in current seem to indicate this.
There may be a lot more damage as well as blown diodes in the rotating rectifier.
What is the OEM for the protection and what relay type? (unless that is better left out of the discussion...)
just some ideas...
rasevskii
RE: Generator breakdown
Particularly in the AVR sensing circuits, loose PT secondary connections or bad contacts on the HV side in drawer type PTs can drive the excitation system wild, and it will happen when nobody is around to see it...
rasevskii
RE: Generator breakdown
RE: Generator breakdown
unfortunately I'm not able to get any suggestion by the SCADA logs...
As nothing was found in protection's alarm log, I'll start to consider the possibility of a fault which has occurred after generator CB was open while the machine was de-excited (but with residual flux) and starting to decelerate....
Ciao
Erminio
The difference between overload and short circuit lies in the nature of the fault, not in the value of the current.
RE: Generator breakdown
From the log it looks like both Gen. and Grid CB had multiple operations, apparently without anything initiating it. So, it may be that the generator got the damage on 03.sep. at 18:58, according to the Gen log. Breaker closed at 13:40 and did not open until 18:58. Protection unit reacts at 13:40:55 with low freq. and low volt, as well as grid CB fault, but the breaker did not open.
Note also what happened 02.09. at 14:53 and again 15:07 - Phase sequence error. For us it is not clear why this happened.
On 02.09 there was no CB OPEN until late in the afternoon but there were lines without voltage present, in which case it would normally be without generator running. Can it be that the breaker closed when engine was not running?
Rasevskii,
We are also thinking of damaged diodes, since that excitation rose high while voltage was not too high.
CTs are mounted inside the generator, near the neutral bar.
potteryhard,
I don't know what type of breaker is used there, assume it is ABB but not sure.
RE: Generator breakdown
CTs mounted at the neutral bar (neutral side of the stator winding) may well account for the current that existed in the stator after the Gen CB opened (if it did), that was current flowing from the earth fault in the stator to earth over the neutral. (possible scenario)
It really sounds like the control equipment is faulty in several ways and the protections also. If we can believe the logs.
Was the equipment ever properly commissioned and tested? Are there test reports and signed docs from a consultant or the power company? It is very unlikely that the power company would allow any connection to there system at the HV level if the equipment had never been properly tested.
This will be the case when the unit is recommissioned as well.
Is there a PLC involved that is suspect?
rasevskii
RE: Generator breakdown
I have already asked my colleagues at local office to ask the company that installed these breakers to check them all for possible faults. We will probably have to do the same with our part after the generator is brought back.
I don't think there is any issues with a PLC, there is one, but it does not do anything directly with interface with the breakers.
By the way, I forgot to mention there is apparently no earth fault relay in the system (at least on our side). On similar plants, we have broken delta detection, but I can not see it on the SLD from this plant. We should maybe add it, right? Generator neutral point is not connected to the ground.
RE: Generator breakdown
I assume that you are the OEM for the genset, and the switchgear and remaining plant is from "others". Always a sticky situation after a fault has occurred. The question is mainly about the relay protection for the unit, did you supply it or "others"? In any case the protection has to be retested, including all tripping and alarm functions, and it is in your interest to get a signed protocol from these tests.
On the earthing, the generator neutral is unearthed? (is this true), in that case a bad idea. The 10.5 Kv system must be resistance earthed somewhere, at the (unshown) grid tie transformer neutral?
If the gen neutral is left unconnected, then the only earth point is over the wye connected PTs, with broken delta additional secondaries conn to the earth fault relay to give some degree of protection before the unit is connected to the grid. (from your explanation, I think this is what you have?)
Look up some of the threads on this Forum regarding neutral point earthing on industrial systems.
rasevskii
RE: Generator breakdown
yes, we supplied the genset, and also control system, including protection. Not the switchgear etc..
About generatro beeing unearthed, that is common in the Netherlands, we have several plants like this one, and none of the generators are earthed in any way (only PE of course). But as you say, usually thafo neutral point is earthed, but in this case I have no idea where the trafo is located, how far from the genset.
We don't have any earth fault relay in our system, I realise that we probably should have it, but that is how it is now. I see that broken delta PT is available (shown on the SLD). I will try to find out if others have it in their scope.
On the later plants we had earth fault relay from broken delta PT (either we or the others).
RE: Generator breakdown
A possible solution would be to disable this protection after the unit is online to avoid this situation. In that case the generator would be protected against E/F only by the protection elsewhere in the system.
A protection study of the whole system should really be done to co-ordinate all the protections. It is likely that the 10.5 KV system is resistance earthed at the neutral of the supply transformer that has to exist to connect with the utility system.
It is quite possible to get by with uncoordinated or not-thorughly-studied systems as long as nothing ever goes wrong or no fault ever occurs...
Since you are not telling us what kind of industrial situation this is, we can only make some educated guesses from experience elsewhere with cogen and industrial generators connected to large grids. In other words, the more large, extensive, and complicated the 10.5 KV system is, the more difficult the situation with neutral earthing and earth fault protection becomes.
rasevskii
RE: Generator breakdown
I have asked to find out more about E/F elsewhere in the system, no replay yet.
RE: Generator breakdown
the public utility, which the plant is connected to, should have requested to install some protection functions on the grid breaker which are needed both to protect the plant from grid faults and to protect the grid from plant internal faults.
Both actions are done by means of opening the grid breaker:
- in the first case (trip for grid faults) the plant will keep on running in islanded operation
- in the second case (trip for internal fault), the trip of the grid breaker isn't sufficient to clear the fault: trip of the generator is necessary.
In order to work correctly in the first case, the grid breaker protections, requested by the public utility, should be faster than internal protection.
On the other hand, the grid breaker protections aren't sufficient to clear internal faults, unless they are equipped with two stages: the faster one will be used to trip the grid breaker and the slower one to trip the generator.
This solution, in my opinion, can be used (or maybe is already used) in your plant to achieve selective behavior in case of earth fault as the generator isn't equipped with stator earth fault protection.
I hope this can help.
Ciao
Erminio
The difference between overload and short circuit lies in the nature of the fault, not in the value of the current.