Recently DC control power for the output breaker on our combustion turbine generator was inadvertently removed. The combustion turbine generator set was sent a trip signal. However, without control power, the output breaker could not open. The set was reverse powered for several minutes. In the case of steam turbines, prolonged reverse powering (current flowing from the bus bars to the generator) can result in LP blade failure. The question that I am having trouble finding the answer to: what is the concern for combustion turbine generator sets if they are exposed to prolonged reverse power? The turbine section? The compressor section? Thanks in advance for any insight.
Reverse Power Combustion Turbine
- Thread starter dmlew
- Start date
Gas turbines (combustion turbines) are very different from steam turbines. Steam turbines rely on the flow of steam through the turbine to cool them (yeah; it sounds weird, but it's true). Without steam flowing through the turbine if the blades turn they can heat up.
In a gas turbine there is this huge thing on the front of the machine called an axial compressor. It takes a LOT of power to turn that compressor. And as long as that compressor is turning it is pushing air in the same direction through the turbine--all of the turbine and exhaust. The combustors, the turbine nozzles and blades, the exhaust frame, the exhaust stack. AND, it's cooler air than when there is fuel being combusted to heat the air that is flowing through the turbine and exhaust.
There's more likelihood of damage to the generator than to the turbine (and axial compressor). If the excitation for the generator rotor was not working during the reverse power operation then it can get very hot very quickly (induction), and that can cause the rotor to expand and rub and damage the insulation of the rotor windings.
But damage to the turbine (and axial compressor)? Not really likely. Not impossible as we really don't know much about the turbine, but, again, in a steam turbine losing steam flow and keeping the blades rotating at rated speed can result in real problems for the turbine buckets. I've always thought that losing steam in a steam turbine while the turbine was running at rated speed would result in the turbine trying to "pull" air/steam through it, putting a reverse stress on the buckets. And in the absence of steam flow, if there is air those buckets will heat it (the air) up. A gas (combustion) turbine being rotated at rated speed by the generator (asking as a motor) still has air flowing through the machine--in the same direction--and at a cooler temperature than if fuel were being burnt. All the way through the turbine and exhaust. In the same direction, with pressure in the same direction (because it's still being compressed by the axial compressor).
But, the generator doesn't like reverse power--particularly if the excitation isn't on.
In a gas turbine there is this huge thing on the front of the machine called an axial compressor. It takes a LOT of power to turn that compressor. And as long as that compressor is turning it is pushing air in the same direction through the turbine--all of the turbine and exhaust. The combustors, the turbine nozzles and blades, the exhaust frame, the exhaust stack. AND, it's cooler air than when there is fuel being combusted to heat the air that is flowing through the turbine and exhaust.
There's more likelihood of damage to the generator than to the turbine (and axial compressor). If the excitation for the generator rotor was not working during the reverse power operation then it can get very hot very quickly (induction), and that can cause the rotor to expand and rub and damage the insulation of the rotor windings.
But damage to the turbine (and axial compressor)? Not really likely. Not impossible as we really don't know much about the turbine, but, again, in a steam turbine losing steam flow and keeping the blades rotating at rated speed can result in real problems for the turbine buckets. I've always thought that losing steam in a steam turbine while the turbine was running at rated speed would result in the turbine trying to "pull" air/steam through it, putting a reverse stress on the buckets. And in the absence of steam flow, if there is air those buckets will heat it (the air) up. A gas (combustion) turbine being rotated at rated speed by the generator (asking as a motor) still has air flowing through the machine--in the same direction--and at a cooler temperature than if fuel were being burnt. All the way through the turbine and exhaust. In the same direction, with pressure in the same direction (because it's still being compressed by the axial compressor).
But, the generator doesn't like reverse power--particularly if the excitation isn't on.
Thanks for the kind words. They are much appreciated.
Actually, the generator doesn't really care about reverse power--as long as the excitation system is running. The only difference between a motor and a generator is the direction of current flow (and the direction of torque flow). What makes most AC (Alternating Current) synchronous generators--properly called alternators--different is the way the magentic field is developed on the rotor, using DC current from some source (PMG (Permanent Magnet Generator); static exciter; brushless exciter; etc.). BAD things happen to synchronous generators (alternators) when there is no excitation for the rotor. They can slip a pole (VERY bad!) when operating as a generator, or the rotor can overheat when operating as a motor if there's no excitation due to induced magnetic fields on the generator rotor.
But, reverse power isn't good for steam turbines or reciprocating engines being used to drive alternators (synchronous generators). Gas turbines, not really a problem. In fact, when one initiates a normal STOP on a GE-design heavy duty gas turbine fuel is reduced gradually until reverse power is detected--and THAT'S what opens the generator breaker. (Freaks some people out when they see that for the first time--especially people with steam turbine experience!) It's just for a short period of time (a couple of seconds, if that, but it's the normal shutdown (STOP) sequence for GE-design heavy duty gas turbines. And, one reason that is even possible is because reverse power doesn't hurt the gas turbine or axial compressor.
Remember, though, as a general rule, two of every three horsepower developed by the turbine section of a gas turbine is required to turn the axial compressor. That's why the simple-cycle efficiency of many gas turbines is only about 33-35%--because so much energy is required just to drive the axial compressor. In the old days when there were large steam-turbine power plants, they were never much more than about 35% efficient, so gas turbines weren't that different But, now with combined cycle gas turbines, using the exhaust heat from the gas turbine to produce steam to drive a steam turbine, combined cycle power plants have efficiencies in the high 50% ranges--and manufacturers "fight" over tenths of a percent of efficiency numbers (in advertising literature, of course)!
So. turning an axial compressor when there's no fuel being burned in the gas turbine (using the generator acting as a motor with reverse power) is pretty "wasteful" (of electricity).
Actually, the generator doesn't really care about reverse power--as long as the excitation system is running. The only difference between a motor and a generator is the direction of current flow (and the direction of torque flow). What makes most AC (Alternating Current) synchronous generators--properly called alternators--different is the way the magentic field is developed on the rotor, using DC current from some source (PMG (Permanent Magnet Generator); static exciter; brushless exciter; etc.). BAD things happen to synchronous generators (alternators) when there is no excitation for the rotor. They can slip a pole (VERY bad!) when operating as a generator, or the rotor can overheat when operating as a motor if there's no excitation due to induced magnetic fields on the generator rotor.
But, reverse power isn't good for steam turbines or reciprocating engines being used to drive alternators (synchronous generators). Gas turbines, not really a problem. In fact, when one initiates a normal STOP on a GE-design heavy duty gas turbine fuel is reduced gradually until reverse power is detected--and THAT'S what opens the generator breaker. (Freaks some people out when they see that for the first time--especially people with steam turbine experience!) It's just for a short period of time (a couple of seconds, if that, but it's the normal shutdown (STOP) sequence for GE-design heavy duty gas turbines. And, one reason that is even possible is because reverse power doesn't hurt the gas turbine or axial compressor.
Remember, though, as a general rule, two of every three horsepower developed by the turbine section of a gas turbine is required to turn the axial compressor. That's why the simple-cycle efficiency of many gas turbines is only about 33-35%--because so much energy is required just to drive the axial compressor. In the old days when there were large steam-turbine power plants, they were never much more than about 35% efficient, so gas turbines weren't that different But, now with combined cycle gas turbines, using the exhaust heat from the gas turbine to produce steam to drive a steam turbine, combined cycle power plants have efficiencies in the high 50% ranges--and manufacturers "fight" over tenths of a percent of efficiency numbers (in advertising literature, of course)!
So. turning an axial compressor when there's no fuel being burned in the gas turbine (using the generator acting as a motor with reverse power) is pretty "wasteful" (of electricity).
It would be good to inspect the Auxiliary gear box ( flender graffestanden these gear boxes are built at 50 km for my home)
We also proud and happy to have Asltom transport and GE Gas power/Steam power nearby...
