Generator Tripped Shutdown

[HamidEle]

We are investigating the risk of quick shutdown of GE-Frame7 85MW machine. If the machine is triped on a fault condition, it would take roughly 10minutes to come to a complete stop. i would take another 15 minutes to be started from hot conditions. 25minutes might be too long for some process loads in our case. I wonder if it is possible to restart the turbine before it has come to a complete stop. i would deeply appreciate it if someone can shed some light on it.

 

[ScottyUK]

In power generation there are typically a few different contingency responses which have an increasingly severe impact upon both the engine itself and upon the load. these responses are: slowly deload the engine; rapidly deload the engine; load dump (full load rejection); prime mover trip. A trip is in response to a severe fault, or to a fault which remains uncleared by the less onerous contingency actions. You might consider whether there are other options than a full trip which could allow the machine to return to service more quickly. If your process is too vulnerable to survive loss of a turbine then you probably need to consider redundancy.

It is possible with some machine designs to restart before the shaft has reached a stationary state, or dropped onto barring gear. In pretty much all cases the machine will need to drop below cranking speed, which is typically about 20-25% of rated speed, priot ot re-ignition. If you want a definitive answer then GE are probably your best bet. You will also need to consider anything on the back end such as an HRSG which may have purge requirements before re-ignition. Purge times vary depending on HRSG size and fuel type.

 

[HamidEle]

Thanks for your reply, ScottyUK. Some of the process loads highly rely on the quick restore of CO-GEN, which is typically within 20minutes from the complete loss of COGEN and grid. In our case, we have two COGENs, which normally supply the power. In the event of 2 COGENs trips, if grid is not available, we would have to restart one of Co-gens within 20 minutes. HRSG which may have purge requirements before re-ignition as you mentioned above. We will check with GE-Rep. tomorrow and keep you informed.

 

[MiketheEngineer]

Call GE - they would know best. Is it possible to put a "brake" on it if you indeed need to get it to 0 RPM before a restart??

 

[Hoxton]

As an example of what can happen when you do something the designer did not expect:

Some years ago I was running the site acceptance test on a small gas turbine generator set

The set tripped on an external fault, we allowed it to get to standstill and immediately, I then pressed the start button. The set began pre purge and started to crank. It reached firing speed, gas valves opened, NO IGNITION, my fist hit the emergency stop button so hard that you could still see the mark on my fingers the next day!

Subsequent investigation revealed that the ignition was turned off by sensing the power turbine gas temperature. As the turbine was still hot from the earlier run, the temperature was still hot enough to hold off the ignition.

The turbine application engineers told us "but you should not restart until the post lube pump has run and timed out - 30 minutes.... This will give sufficient time for the turbine space to cool below ignition hold off."

There was nothing in the application manual about this!

Admittedly, the controls were basically an electromechanical system, and nowhere near as comprehensive as today's monitoring systems. It does show that one should exhibit caution when operating outside design parameters.