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# What Makes Turbine Output Difference Between 50 Hz and 60 Hz?

03/21/2012 9:30 PM

For example, in the same gas turbine model, I have seen that the ouput of 50 Hz gas turbine is higher than 60 Hz. Could someone explain what makes this difference? Is it like both turbine have the same turbine structure, but 50 Hz turbine is bigger than 60 Hz turbine in size?

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#1

### Re: What makes turbine output difference between 50 Hz and 60 Hz?

03/21/2012 9:50 PM

P = kNT; the 60Hz one should produce more.

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#6

### Re: What makes turbine output difference between 50 Hz and 60 Hz?

03/23/2012 12:00 AM

IF, the same general size, same magnetic flux, I agree.

On the other hand (and it is a not a quite technical answer), if the turbine maker buys off the self standard size generator, all bets are off. And forget about optimum output.

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#2

### Re: What makes turbine output difference between 50 Hz and 60 Hz?

03/21/2012 9:52 PM

The gas turbine produces the same amount of mechanical power at the shaft output. It depends on the generator attached to the turbine what frequency and voltage gets produced by the generator.

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#4

### Re: What makes turbine output difference between 50 Hz and 60 Hz?

03/22/2012 5:13 PM

He didn't mention a generator, though reference to 50 and 60Hz suggests that's the application. 60 Hz of course means 20% higher turbine speed than 50Hz, and it's not unreasonable to expect it to produce more power at higher speed, as the torque probably doesn't change much.

A generator produces a given Hz only at certain speeds, depending on the number of poles, 3000, 1500, 1000 etc rpm for 50 Hz, and 3600, 1800, 1200 etc rpm for 60 Hz. The maximum generator output power obviously depends on the power of the turbine.

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#3

### Re: What makes turbine output difference between 50 Hz and 60 Hz?

03/22/2012 2:10 AM

This is simple electrical theory describe as below:

Generator are generally operated by gas turbines directly. For a given number of poles, Frequency of power generated by generator is given by

f=NP/120,

where N = RPM of machine, P = Number of poles in generator

Assume generator is with 2 pole,

for generating power at 60 Hz, RPM of the gas turbines will be

N1=fx120/P or 60x120/2 = 3600

but for generating power at 50 Hz, gas turbine will run at

N2 = 50x 120/2 or 3000 Rpm.

Now Power generated by Gas turbine

Q(kw) = (torque N.m) x 2 pi x (rotational Speed RPM)/60000

It is evident that for generating same power at 3000 RPM, gas turbine should have higher torque value as compared to Gas turbine operating at 3600 rpm.

Hence Gas turbine for 50 Hz generator will be bigger then 60Hz generator for same power.

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#5

### Re: What makes turbine output difference between 50 Hz and 60 Hz?

03/22/2012 11:38 PM

Thus, the higher the torque developed at lower frequency for same power generated, the sturdy the turbine should be the reason why low frequency rotating machine are bigger compared to high frequency machine.

i also agree with guru that the 60 hz turbine - generator should produce more power than the 50 Hz, the poster must have miss something in his observation..:)

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#7

### Re: What Makes Turbine Output Difference Between 50 Hz and 60 Hz?

03/23/2012 3:18 AM

Why don't you post the name plate details. Think about the mechanical aspects of gas/steam turbine-power vs speed.

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#8

### Re: What Makes Turbine Output Difference Between 50 Hz and 60 Hz?

03/23/2012 11:18 AM

All magnetic devices (transformers, inductors, magnetic heads, rotary machines, etc.) get larger as the frequency goes down because more magnetic material is required to keep the core from saturating. It is all about the magnetic flux density, B and the magnetic field, H

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#9

### Re: What Makes Turbine Output Difference Between 50 Hz and 60 Hz?

03/23/2012 5:48 PM

It is amazing to see how many assumptions there are in this thread. There are two types of gas turbines in use by utilities, Industrial Turbines in which the compressor and output shafts operate at a fixed speed, and those that utilize aircraft-style jet engines where the compressor speeds are much higher and the only coupling between the engine and the turbine driving the generator is the hot gas streaming from the engine.

OP is probably talking about an industrial turbine in which case the limiting factor is not the generator but the materials and design of the turbine blades. As the turbine speed increases by 20% the rotational forces on the rotor blades increase by 40%. If the machine is rated for 50/60 Hz operation then these additional forces have to be factored into the overall design and usually result in a derating of the output shaft power as the speed goes up.

On the other hand jet-engine types are usually constrained by the output capabilities of the generator because designing the final stage turbine for those additional stresses is much more economic than designing all the stages for them. The manufacturer simply designs for 60 Hz operation (the jet engine doesn't care what the final speed is) and allows for 50 Hz operation as long as the output is derated (if necessary) consistent with Volts/Hz and the MVA rating at the lower speed of the generator.

All of the above is predicated upon identical operating conditions (temperature, fuel, altitude, hunidity, etc.), economics (cost, heat rate, availability, etc.), and the fact that any critical frequencies are well below the 50 Hz operational mode.

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#10

### Re: What Makes Turbine Output Difference Between 50 Hz and 60 Hz?

03/23/2012 8:20 PM

Thank you for the elaboration on the turbines. In short, the turbine is a given. The off-the-shelf generator comes in discrete sizes, a given too.

With that, there are roughly 4 areas of deratings:

Turbine power / torque

Local air pressure / temperature

Frequencies / rpm

Discrete generator sizes

The engineering answer comes out after applying these factors to the theoretical calculations.

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#11

### Re: What Makes Turbine Output Difference Between 50 Hz and 60 Hz?

03/24/2012 9:06 AM

The same can happend with diesel engines. Some are not suitable for both frequencies, and other have a derating from 50 to 60 hz. See an example at http://www.cumminsgdrive.com/accelsite/media/1703/QSX15-G8.pdf.

I know it is due to the tunning between turbo and others performance components, but not the details.

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#12

### Re: What Makes Turbine Output Difference Between 50 Hz and 60 Hz?

03/26/2012 8:59 AM

Thanks all of your answers. I really appreciate it. The reference gas turbine was SGT6-8000H and SGT5-8000H manufactured by Simens. SGT6-8000H is 60 Hz and its output is 274 MW, but SGT5-8000H is 50 Hz and its output is 375 MW. SGT5-8000H's output is quite higher than SGT6-8000H, isn't it?

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#13

### Re: What Makes Turbine Output Difference Between 50 Hz and 60 Hz?

03/27/2012 3:58 PM

Your observation is correct but just because the only difference between them is 5 vs 6 in the product name does not make them twins. They are totally different machines, a quick look at the specs on the Siemens websites

http://www.energy.siemens.com/us/en/power-generation/gas-turbines/sgt6-8000h.htm#content=Technical%20Data

http://www.energy.siemens.com/us/en/power-generation/gas-turbines/sgt5-8000h.htm#content=Technical%20Data

reveals that the 50 Hz version weighs 160 tons more than the 60 Hz version, it's also longer, taller and wider by about 20% on all dimensions. Since the linear dimensions are 20% larger one would expect that the area dimensions should be about 40% greater resulting in approximately 40% greater power related parameters. Let's compare...Output 375 vs 275 MW (37%), Mass Flow rate 820 vs 600 kg/s (37%); yep, they're different sized machines based upon the same overall design scaled up.

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#14

### Re: What Makes Turbine Output Difference Between 50 Hz and 60 Hz?

03/28/2012 1:27 AM

Thank you!

If 60 Hz model is only scaled down, will 60 Hz model work well if 50 Hz machine works without problem or 60 Hz has to be tested since what?

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#15

### Re: What Makes Turbine Output Difference Between 50 Hz and 60 Hz?

03/29/2012 11:27 PM

Not necessarily, and probably no. Scaling has its limits, and one of those limits is the law of unintended consequences. The problem of resonance or critical frequencies is always present in the design and operation of a large rotating mass such as a turbine.

A 50 Hz machine may have no critical frequencies below its operating point but start vibrating uncontrollably at say 58 and never reach 60 Hz without throwing a blade and self-destructing. That's what overspeed protection is for. So if you're planning on copying a design for one speed and scaling to the next speed without adequate modeling and testing expertise I wish you luck.

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