Hello,

We recently had one of our 6.6kV Tornado GTGs (gas turbine generator) couplings shear between the turbine and the alternator. I was wondering if anyone else had any experience with this. Initial investigation is suggesting wear-and-tear on the coupling. One of the things I noticed was that the speed and frequency increased after the shearing. And, the incomer breaker did not trip. There is not a lot of information regarding this. however, I wanted to hear other people's stories regarding similar incidents, and where they would advise me to look. The investigation is still in its early stages.

 

Assylbek Mukhanov...

as a start can you at least provide some detail! Did the failure occur to the TG-unit:

a) While the unit was in operation?

b) When the unit was being started?

c) While the unit was being synchronized to another source?

d) When the unit was being stopped?

e) Was the unit inadvertently energized while off-line?

f) If it was connected to the grid, Had something happened electrically such as single-phasing, re closing, insulator flash-over, etc?

g) Other than the coupling, is there any evidence of mechanical damage to the turbine?

h) Other than the coupling, is there any evidence of electrical damage to the generator?

i) What Protective-Devices operated!

j) If grid connected, is the configuration simple, meaning a single transformer between generator and grid, or complex such a "Ring-Bus, or "Breaker and a Half scheme"?

k) Had any shaft torque analysis bec arried out?

And finally,

l) Had you experienced any problem at all, even if an annoying glitch, with the synchronizer?

Regards,
Phil Corso

 

Assyibek Mumhanov,

From the information provided it would seem the frequency was low prior to the coupling breaking (because you indicated speed and frequency increased after the shearing and the "incoming breaker") did not trip. So, it would appear the unit was probably in some kind of overload condition or at least was trying to put more fuel into the turbine to try to get speed and frequency back to rated. Depending on how often this had happened in the past (low frequency and/or overload), how the turbine control system is configured and how much safety margin was designed into the coupling this would explain the failure from the information provided.

A generator is a device for converting torque (from the GT in this case) into amperes. It wants to do so at a constant frequency and when it isn't being operated at a constant frequency the turbine control system should increase the fuel to try to maintain speed (whether in Isochronous speed control or Droop speed control). The turbine should have some kind of "limiter" to protect either the turbine (from over-firing), the generator (from overloading it) or the load coupling (from exceeding the torque rating of the coupling).

Most turbine generator sets are designed such that the generator is rated to be able to produce more power than the turbine can supply and the load coupling is rated for at least the torque the generator can convert into amperes at rated power output. And, also, there is usually some margin in the coupling selected by the packager so that unanticipated load surges can be handled without damaging the coupling.

Hope this helps!