We are opearting 5 GE frame-6 gas turbine, for refinery load. plant load remains around 120 MW. these 5 GTs are distributed between two power plants. Powerplant -1 which operats 3 GTs and Powerplant-2 which operats 2 GTs. Many times due to miscommunication or load variations we operate GTs with system frequency as high as 50.22 (ous is a 50 Hz system).

I have following doubts-

1. Does running GTs at lower frequencies around 50.1 will help deceasing the fuel cost.

2. when calculating the efficiency why dont we take into account the speed of the machine.

3. Can i show it in numbers?

Thanks in advance.

 

I've been thinking about this for several days, and I'd just like to point out the following.

First, a 50 Hz system is supposed to be operated at 50 Hz. Sure, a little high or a little low doesn't hurt very much, but that's the intent of an AC system--to keep the frequency constant at the nominal, desired setpoint. Otherwise, why would the frequency of an AC system matter at all? Why not just generate "power" at any frequency?

Second, heavy duty gas turbines are mass flow machines, meaning that the more mass flow they can get through them the more power they can produce. <b>However,</b> in your case with five Frame 6 machines, nominally rated at approximately 40 MW (some are rated lower in higher ambient conditions) none of the units is probably running at or near Base Load--which is the optimal power output and efficient running condition. When heavy duty gas turbines are operated at Part Load (less than Base Load) quite often the IGVs are not fully open (as they would be at Base Load) and so mass flow through the machine is restricted. Especially if the gas turbines are being used to produce steam for a plant or process.

Third, heavy duty gas turbine axial compressors are designed to run an a particular speed for optimal efficiency, and that's usually the speed that corresponds to the desired, nominal frequency of the generator being driven by the gas turbine (and its axial compressor).

I don't really understand why "efficiency" is being considered when the units are being operated at Part Load anyway. They are, by their nature, inefficient at Part Load--unless the exhaust heat is being used to produce steam, and then the overall plant thermal efficiency is higher at Part Load when the IGVs are being modulated to increase exhaust temperature to maximize steam production.

What's the real purpose behind this question?

I think someone is looking for a reason to justify an inability to maintain plant frequency by saying, "Oh, well, it's more "efficient" to operate off-frequency!" Why can't the units be operated to properly control frequency?

 

Maintaining the system frequency at the nominal frequency is not as "simple" as what you are proposing. If you think it is simple probably because you have very big system is very close to an infinite bus system. You can take for granted that your system frequency stays close the the scheduled frequency. To the most extreme even plant operators may want to believe all the parallel generators just simply "locked to synchronism and cannot move away from it". Whether such believe is right or wrong makes no significant meaning. They don't really need to know what is the value of their "system inertial of rotating mass" but still believe they can operate the plants at the scheduled frequency. It is a matter of fact they did! They don't even need to know the safe ramp rates for their generators but still can claim they know the safe way to load the plants. Sometimes size does matter. This is true for a grid system.

If you have a 20,000MW system with the biggest per unit generator capacity 1,000MW, you can't expect frequency characteristic is similar to a 250,000MW system with the biggest per unit capacity 1,000MW too. The later ratio 1,000 to 250,000 is closer zero, i.e. closer to an infinite system. (Theoretically an infinite bus system has such ratio to be zero. You can't find even one of such system in world. But many systems practically close to an infinite bus system. To name few, US, Canada, Japan and France grid) The former is much closer to finite system but it can be managed more or less as an infinite system. Obviously with difficulty.

Note: A 250,000MW might have inertial energy from rotating generators and prime movers alone as big as 2350GJ at 60Hz. A 20,000MW system might have a value as small as 235GJ at 60Hz. These figure tells us how simple the frequency for the later system can be influenced by incremental load/demand.

 

Thanks for your reply CSA.

I agree that when operating machine at part load we are deliberately running machine at lower efficiency, but this is done to maintain a more reliable power generation. This captive power plant is being used for refinery power demand, so although four GTs are sufficient to deliver the desired load, five GTs are run to keep the system unaffected if one GT trips. Also all these GTs are run in Co-generation cycle, so steam generation is another factor which decides the loading of the gas turbine. So you see the part load operation of Gas turbine is more of a compulsion than a choice.
I asked this question because there is a practice in my plant to run GTs at relatively very high frequency i.e. 50.22 Hz, again in the name of reliability. I just wanted to counter this practice but not without a solid reasoning, that's why i finally logged into this forum.

Here is some calculation i have made, in case it could clear my question-

In droop characteristic mode-
Base load speed
100.000 % (50Hz)
No load speed
104.000 %
Base load
30.150 MW
Percentage speed Decrease from no load to Base load 3.846%

Speed will decrease 0.128% For every MW increase, so at any frequency 0.128% decrease in frequency will take care of 1 MW Load per Machine

Suppose System Frequency
50.200 Hz
No. Of GTs on Bar 5
Total Plant Load
120.000 MW
If we decrease frequency from 50.2 to 50.1 total decrease 0.199%
That will take care of total (Considering all 5 machines)
7.808MW

So effectively the fuel we are burning is sufficient to generate 127.8 MW (120+7.8 MW) at 50.1 Hz.
In other words we can generate 120MW at 50.1 Hz and save the fuel equivalent to 7.8MW.

Thanks in advance for your time.

 

Yes; you're wasting fuel by operating above nominal frequency. How much? That's a question I don't have the maths for the answer. Remember, the gas turbines are driving axial compressors and auxiliaries and their loads in addition to the generators and their loads. I don't know if it's so easily quantified as you indicated.

But, if you can't run reliably at 50.0 Hz, what makes you think you can run reliably at 50.1 Hz?

Is there some kind of "load sharing" system at the site that's supplying a reference to the Speedtronic control panels? Is the load relatively stable or does it have large swings during normal operation?

Best of luck with your situation.

 

If you have the commissioning active power correction curve for frequency correction you can use them as the basis to calculate corrected MW output for two frequency deviations namely at 50.22Hz and 50.1Hz. From there you can calculate equivalent fuel cost. You still can use typical correction curve from other GT suppliers too to get a rough estimate.

I think I got the idea why your management keeps the frequency at 50.22Hz. Let me try to figure it out. The frequency oscillation for your electrical system may vary between +/-0.4Hz. If the system frequency is kept at the nominal frequency of 50 Hz then the lowest frequency dips could be 49.6Hz. This lowest limit might not be good for some of the equipments and appliances. Therefore in order to limit the lowest frequency dips to 49.8Hz only the scheduled frequency is set at 50.22Hz.

If you want to put the scheduled frequency at 50.1 Hz, theoretically you can get similar the lowest frequency dips at 49.8Hz if you increase the speed droop sensitivity set point to says 3.3% (Just simple estimate). Practically you may see load hunting, frequency oscillation more often than before. You have to judge on your own whether you can except the new operating characteristic or not.

Reducing the scheduled frequency to 50.1Hz reduces fuel cost to deliver the same output. Introducing additional number of frequency oscillation within the same period increases fuel cost for the same load. Whether net cost will increase or reduce, you have to be the judge.

 

I think you get very close to it. You can approximate as the following calculation

Total plant output = 120MW

Total plant input =120/0.3 =360MW (assuming 30% thermal eff)

At 50.22Hz the total input required is 360MW (Baseline assumption)

At 50.1Hz the total input required = 50.1*360/50.22 = 0.717MW _input per unit...

If you don't have to have any additional frequency oscillation for keeping the scheduled frequency at 50.1Hz, I would say the calculated saving this.