Sheared Flywheel Bolts on Generator 2

[IOC-AUS]

Hello All,
We recently had a Generator Failure - High Tensile Flywheel Bolts were sheared on the AC Generator.

The Generator Set has been in operation and we understand the system includes Solar.

My question is could this Generator Failure be caused from incorrect Synchronisation.

I would like to understand the Failure and have it explained with logical possibilities.

Any help and advice much appreciated.
Regards, Ian.

 

[itsmoked]

That is certainly the most likely cause of shearing those bolts.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[IOC-AUS]

Thanks Keith,

I would be interested in a more detailed explanation of what occurs when Synchronisation goes wrong.

Torque is no doubt a key ingredient in the failure but can it be explained how the Process unfolds leading up to the sheared bolts.

 

[byrdj]

First, Is the generator synchronous?
what type of copntrols for closing the breaker in on an operating "grid"

the generator not be in pase would be a sudden torque, either with or agaisnt rotation depending on angle when breaker closed.
If the "grid" has a larger "inertia" it will suddenly pull the generator the partial rotation to synch the two
using the terminoligy of a "clock", being even 5 minutes off can casue damage

https://www.youtube.com/watch?v=3W7oSN_zHjQ

 

[waross]

You were probably more than 30 degrees or 40 degrees out of sync when the generator breaker was closed.
If this was the first attempt at paralleling it may be a miswired synchro-scope.
It may have been operator error.
When the breaker is closed, there is a very high torque tending to bring the set into sync.
To come into sync, the generator rotor must instantly move to reduce the phase error to near zero.
The rotating mass of the prime mover must also make this step change in position.
The instantaneous torque developed is capable of doing mechanical damage.
I encountered a small diesel generating plant with a 60 degree error in the syncro-scope wiring.
When the 'scope indicated synchronization, the incoming set was actually 60 degrees out of sync.
The circuit breaker would often trip several times before an incoming set would stay on-line.
Once or twice a year, the key in the coupling key-way would be sheared.

An out of sync closure on a large set may do additional damage including the possibility of foundation damage.
Transformers have been damaged by out of sync closures.
The worst case mechanically is a 90 degree out of sync closure.
The worst case electrically is a 180 degree out of sync closure.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[desertfox]

Hi IOC-AUS

I found this regarding synchronising a generator and what happens if the voltage, frequency and phase angle doesn’t match.

https://electrical-engineering-portal.com/preparing-to-synchronize-a-generator-to-the-gridHi

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[FacEngrPE]

link above has a typo, use this

https://electrical-engineering-portal.com/preparing-to-synchronize-a-generator-to-the-grid

. really concise explanation.

 

[desertfox]

Thank you FACEngrPE

I must have not completely copied the link, yes it’s a good article for sure

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[IOC-AUS]

Thanks Kindly for the information. That link is excellent.

 

[waross]

The statement that the frequencys must be equal is valid for that automatic equipment but is not a universal condition.
Manually operated stations and less sophisticated syncro check relays never intentionally try to sync at equal frequencies.
I have had problems with Very fast transfer Automatic Transfer Switches.
The very fast ATSs need a sync check relay and should be in sync before transferring to the grid.
There are two reasons:
With the very close internal clearances, there is a possibility of flash-over if the transfer is made too far out of sync.
A sudden change in phase angle may cause mechanical damage to motor shafts, couplings or driven loads.

Normally a standby generator seldom runs at 60 Hz. The set will run at a frequency that varies from 60 Hz at full load to 61.8 Hz at no load.
I had a set that ran at very close to full load. (Uncommon for a standby set, but it happens in commercial applications.)
At times it would take five minutes or more to drift into synchronization and transfer back to the grid.
The moral: If the frequency is exactly 60 Hz, you may never be able to sync.
The solution? Easy, set the full load speed to about 60.5 Hz. The ATS will be able to sync and transfer in a few seconds.

Another possible issue with exact frequency transfers is reverse power protection and loss of load protection.
By no means not all, but some protection schemes do not like to see a set running with no load. An exact frequency transfer will not pick up any real load.

Common practice in manually operated plants is to run the incoming set 10 RPM or 20 RPM fast, so that the set immediately picks up a little load when the breaker is closed.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

Can block loading or a line fault cause drive bolts to shear?
Block loading;
Very unlikely. Thousands of standby sets are hit with 200% to 250% overloads when the breaker closes. This is motor starting and inrush of various loads and is never a mechanical problem. In the event that the bolts sheared due to block loading, I would suspect that the bolts were improperly installed, were the wrong grade or became loose in service.
Line faults:
Possible but not likely. The maximum torque transient under fault conditions will be dependent on the generator characteristics.
A good design will have considered this and selected mechanical components able to withstand the worst case torque transient.
BUT
An out of sync connection may easily impose torque transients several time the magnitude of the design worst case transients.
An easy question:
Did the bolts fail in serve? Check that the bolts were the proper grade. Look for signs or wear on the bolts.
Did the bolts fail during a synchronization?
There is your answer.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[catserveng]

The attached is a paper on the subject, while for a much larger unit, the cause and effect explanations would apply to any synchronous generator I would expect.

Without more information on your particular system hard to provide a good answer. While out of phase paralleling definitely a good candidate, there are some other possibilities causing mechanical damage at the engine to generator connection.

Inadvertent energization, not a common cause but has cropped up as a root cause to a couple coupling failures over the years. Depending on your system and protections this may or may not be a possibility.

Mains failure, in some areas, especially with older services and conditions such as high winds, we have seen what appeared to be damage due to Out Of Paralleling was actually due to short term interruptions of the utility service, sometimes in the span of 100mS up to several seconds. In all cases the units/systems had loss of mains protection, but due to loads and other factors, generator frequency and voltage did not move far enough away from nominal to cause a trip, and when utility returned the results in some cases were catastrophic. So is the system utility paralleled? What kind of loss of mains or out of step protections are applied?

Electrical induced problems like those mentioned above can also cause damage to the generator, so have you inspected the tail end for things like signs of the end turns moving where they come out of the stator slots? How about checking the diodes and surge suppressor? Any electrical event severe enough to cause damage at the would likely cause a problem at the rotating rectifier.

Another possibility not caused but an electrical condition can happen in some generation units, mainly those running on bio-fuels like digester or landfill gas running in parallel to the grid, we have seen severe misfire cause excessive torsional stress and cause coupling failures. In one case with a smaller unit with a disc type coupling it caused the flywheel coupling bolts to fail.

Some additional info about your system could get you some better answers,

What kind of coupling is installed?
What does the unit parallel to, grid, other units, renewable network, or???
What kind of sync controls are being used?
What kind of electrical protections are in place?
Any details on your unit, size, fuel, generator voltage, etc?

MikeL.

 

[catserveng]

Another one I forgot to mention but has actually caused a number of issues in the last couple years, generator breaker slow close. The breaker gets a command to close, but does not actually close as expected. This has been more of an issue in larger units with medium voltage vacuum breakers, but recently a smaller unit with a 1600amp molded case breaker had a slow close problem.

MikeL

 

[desertfox]

I agree with the other posters in that incorrect bolt grade or preload could also contribute and in addition bolt loosening over operating period could also be responsible. Is there any chance that you can post pictures of the failed bolts ie side view and cross section of the failed bolts might get you better answers, very often the failed bolts can provide the clue to why and how they failed.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[TugboatEng]

A misfire can also break flywheel bolts over time.

 

[IOC-AUS]

Have some more information.
Generator Set Rated 30 kVA 1 Phase 240V 50Hz [ Max FLC = 120A ].
Engine Direct Coupled SAE 3/11.5 to a 4 Pole AVR Control AC Generator [ connected Delta Config 60 kVA 3 Ph 36 kVA 1 Ph ].
2x Schneider XW 48VDC Inverters + Schneider Regulator 8600 DC Coupled.
Customer says that "No Gtid Inverter or Gtid Connect Solar" - Generator and PV Solar do not share load". Genset is Basically used off-grid and Charging Batteries on Solar PV and Taking Load [ Domestic Appliances in a few Living Quarter's- Air Conditioners + Hot Water + TV + Microwaves - there is a Welder that is used on the odd occasion)".
Upstream is a 63A Circuit Breaker (to protect cables) which has tripped a few times - interesting as the Generator Set has an 80A Circuit Breaker and capable of 120A Output.
A Manual Transfer Switch is Upstream from Generator Set.
Customer tells us "they can Start the Generator when Inverter sees Load (eg. 30A) or after 5 Minutes (Fully Adjustable) - this function is typically used to prevent Batteries dropping too low on Charge."
2x images attached.

 

[IOC-AUS]

Second image

 

[desertfox]

Hi
Thanks for the photos, can we have a close up on the bolt fracture faces from both the tops of the bolts and the side of the bolts please ? This is so I can see how flat or sloped the fracture faces of the bolts are and to see if they have suffered fatigue. My initial observation on image 2 there appears to be a keyway but I cannot see any damage to it and I would have thought that the key and keyway would have shown so distortion before any bolt failure?
I wonder now if the bolts were overtightened during maintenance installation, is there any info on maintenance work ie how’s those bolts were preloaded?

This link might give some insight into the bolt failures

http://www.croberts.com/bolt.htm

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[JohnMcNutt]

Speaking from practical experience, there is no way a pair of XW inverters has enough stiffness to shear any bolts in a 30 kw genset. I suspect a purely mechanical problem.

 

[FreddyNurk]

Based on the system description I agree with JohnMcNutt. Presuming that its possible to connect the generator whilst the inverter is running in such a manner as to be out of synch, I'd expect blown up inverters as well. If that's not the case then its likely mechanical.

EDMS Australia