How does Gas Turbine Generator Output Change when GT Frequency Changes


Thread Starter


A GE frame 6B gas turbine had frequency conversion from 3600 RPM to 3000 RPM to facilitate frequency conversion from 60HZ to 50Hz machine with the use of Load reduction gearbox, to what extend will this affect the Generator output (KVA, KW) after the conversion was effected? How can this change be computed to reflect the change?

A generator is a device for converting torque into amperes, which can be transmitted using wires to remote locations to perform work (motors; lights; computers and computer monitors (virtual work)).

Since the speed of the gas turbine has not changed (the input to the reduction gear box) the ability of the prime mover to produce torque has not changed.

So, the "new" generator, even though it's at 50 Hz, will still produce the same amount of electrical power as the 60 Hz version--assuming the "new" generator is rated similarly to the 60 Hz generator. The 50 Hz generator should have nearly the same KW rating as the 60 Hz generator; the KVa rating may differ depending on generator construction and type of excitation system.

But, for all intents and purposes, the new "unit" should be capable (presuming the 50 Hz generator is rated similarly to the 60 Hz generator) of producing roughly the same power (KW, primarily) as the 60 Hz generator--because the gas turbine power output (torque) has not changed. Only the speed of rotation of the reduction gear box output has changed. And the KVa rating (i.e., the amount of reactive current the 50 Hz generator can handle) is a function of the construction of the generator and the excitation system.

Hope this helps!

By the way, the GE-design Frame 6B heavy duty gas turbine must rotate at approximately 5100 RPM because the axial compressor and turbine are designed to produce rated power output (torque) at that speed. That's why a reduction gear box is used to "change" the speed of rotation to a speed that a synchronous generator of either 50- or 60 Hz can use (3000- or 3600 RPM, respectively).
Know-How ... your question, "... to what extent will this affect the generator output ... ?" led me to conclude that your intent is to use the original 60 Hz machine, but operate it at 50 Hz!

Please confirm if my conclusion is correct! Is so, the conversion will have a negative impact on the generator's output-performance! The corrective actions (if possible) will be discussed in detail!

Regards, Phil Corso

Ps: A similar discussion can also be found at:
Correct frequency with reduction gearbox to control it. Gas Turbine Controls Corporation maintains the world's largest stock of GE Speedtronic Turbine industries.