Coupling failure is a possible outcome, as are de-blading in the LP stage, winding movement in the stator, bearing movement anywhere along the shaft, all manner of bad things. I think this was a Skoda machine of around 120MVA if memory serves correctly. Coupling failure resulted in loss of the entire unit.
Thanks for links so far. It was stator winding movement I was interested in. We had failure of Diesel engine piston and suspect error in sync. Only small set 525kva. Currently investigating and looking for clues, no power management system to back up records of what was going on at time and nobody putting there had up to admit error
I'll defer to the experts on smaller sets - on a big set I'd be looking for loose blocks and cracked resin around the endwinding region, change in PD signature, change in impulse signature, plus anything which 'looked wrong'. Hard to quantify, but sometimes things just don't look the way they should.
I can't link to any photos but we had a 7MW set synchronise out of phase a few years ago, the stator winding and core were both damaged (there were flashovers between the winding and core) and the turbine/gearbox coupling bolts failed causing consequential damaged and a fire. Not pretty, the areas on the stator that flashed over could be easily seen by a casual observer and were seperate to the fire damage.
Usually in machines that size an out of phase close usually results in generator coupling damage. Sometimes a phase overcurrent trip (just had a landfill unit suffer that a couple of times after a switchgear repair), usually a blown surge suppressor, and sometimes along with a failed suppressor a failed diode or two. A lot depends on how "stiff" the network is you're paralleling to, a small unit to a large stable utility seems to suffer the worst damage in my experience, units of similar size with an out of phase close seem to suffer severe mechanical damage some of the time. Other times I have had units simply stop/stall, I have units bend generator flex plates and after phase correction and restart, failed a crank from thrust failure. Once have seen a crank crack at the rear main bearing journal.
On the tail end, have seen cracking of the insulation on the stator end windings, especially in mush wound machines, fairly rare in form wound. Have also seen amortisseur bars crack or break away from the collector ring in severe events. Also had two events of the generator rotor shaft at the drive coupling twist and bend.
But NEVER have seen a piston failure that I could blame on an out of phase close, and unfortunately I've seen quite a few. Depending on the type of piston failure, they are typically related to an over fueling, overload or lack of cooling type problem.
