Take the LMS100 gas turbine for example. Why does the efficiency change when generating power in 50 or 60hz waveform?
There are multiple effects how net-frequency impacts the GT-efficiency. But it also depends on the GT-architecture.
The following effects also impact the efficiency even if the 50 and 60Hz version would get their unique design in order to operate them at their design-point (at peak performance)
Aero-derivatives for 50 and 60Hz usually use the same „gas-generator“ but differ in the power-turbine. The power-turbine is not on the same shaft/rotor as the gas-generator. This means the 60Hz-version will spin faster. Without going into the aerodynamic details the efficiency of a turbine is higher with a higher speed. Besides this you will also see that the 60Hz version has a higher power-output as well.
Industrial GTs (for higher power-output) are designed so that generator and GT are on the same rotor/shaft. In order to use as many designs in a 50 and 60Hz version the manufacturers usually ‘scale’ the GTs which means the Mach-Number is kept constant. This means that a 50Hz version of a given Model will have a higher power-output and will also have a slightly higher efficiency. This is because bigger engines have relatively smaller tolerances compared to smaller (60Hz) engines.
In gas turbines, peak efficiency is reached at a given design speed. When a gas turbine is used to drive an alternator, the reduction gearing is designed according to the best design speed of the turbine and the frequency and number of poles of the alternator, In the case of two poles and 50Hz, the alternator speed is 3000 rpm, but if the frequency is 60 Hz, the speed is 3600 rpm. If the same reduction gearing is used, the working speed of the turbine will vary by 20%, thus, when designing the reduction gearing, a choice of frequency must be made.