Greetings

While I was reading GEK-107092d, I came across something not very clear. It talks about isolating one cooler (in case it has a failure) of the generator. Fortunately, we haven't had any cooler failures in the past. the quote is:

"If a cooler leak is severe, the defective cooler may be left out of service until repairs can be made. Generators are typically designed to operate at rated power factor at 80% rated load capacity with one hydrogen cooler out of service."

I know that we have to keep monitoring the generator RTDs in case of isolating one cooler.

My question is, what is this rated load? Is it the one on the generator nameplate?

Our generator name plate says the MVA is 228.6 and power factor is 0.85.

By doing small calculations, the rated load will be 155 MW. So 80% would be 124 MWW. Is that correct ?

 

Aptx4869,

If I recall correctly, to determine the possible MW output rating for a synchronous generator nameplate rating given in MVA, one has to multiply the MVA by the rated power factor (from the nameplate). So, 228.6*0.85=193.8 MW, and if you then multiply 193.8 by the 80% (0.80) safety factor for running with one hydrogen cooler out of service you would arrive at 155.04 MW.

It also seems to work the "other" way. If you multiply 228.6 MVA by the 80% safety rating for running with one hydrogen cooler out of service, you get 182.88 MVA. And, if you multiply 182.88 MVA by 0.85 you get 155.45 MW--which is close enough to 155.04 MW for my maths.

But, I'm not a maths wizard. And, I didn't search the World Wide Web for the formula for converting KVA to KW (or MVA to MW). I just used my feeble memory.

In other words, I may be wrong. I would have to refer to my university text, which I don't have with me at this writing.

 

CSA,

Thank you for the reply. I recalculated the MW and reached into the same answer as you had. I seems I made small error in my earlier calculations.

But again, the rated load in the quote, it seems they meant the rated load for the generator. This is what I believe.

 

Aptx4869,

Yes, because the issue is producing electrical power (MVA) without one hydrogen cooler. If necessary, you would have to MANUALLY limit the turbine to keep the electrical power below 155 MW, or 182.88 MVA.

You would be trying to protect the generator, not the turbine. BUT, you would protect the generator by limiting the turbine output (because the generator is driven by the turbine; the generator is "dumb"; it just converts the torque from the turbine to real power (MW)). And you would use the "AVR" to limit the VArs to keep the combination of the watts and the VArs (the MVA) below the limit.

Hope this helps!

 

I have a GE MS7001FA turbine generating at a load of 50 MW. The turbine is a 160 MW ISO turbine. What damage could be occurring to the turbine and generator due to the low load on the machine? According to the manufacturer's recommendations, GE, what is the minimum load that this type of turbine should operate at?

 

@LALEXANDER_22,

Congratulations! I wish I had a 7FA turbine! (Sometimes I wish I had one, other times I don't.)

Let's take the generator first. Presuming the generator is being operated within its Reactive Capability Curve limits of load (MW) and reactive current (MVArs) at 50 MW, there is very likely zero chance of any kind of damage due as a result of the 50 MW load (out of approximately 160 MW capability). We don't know if the hydrogen cooling system is working properly, if the hydrogen purity is within specifications, if the hydrogen is kept at the proper dryness, if the hydrogen sealing system is working properly--but presuming that's all good then it's very likely the possibility of damage to the generator is very low as a result of the "light" load of approximately 50 MW.

Remember, a generator is just a device for converting torque into amperes (and an electric motor is just a device for converting amperes into torque). The amount of load produced by a generator is a direct function of the amount of torque being supplied from its prime mover (in this case a GE-design Frame 7FA heavy duty gas turbine). Lower loads mean the internal heat being generated as a result of current flowing in the machine's windings will be lower--and that's a good thing, generally.

Now, the turbine. We don't know what fuel(s) the machine burns, we don't know what the machine combustion system is (conventional; MNQC (Multi-nozzle Quiet Combustor); DLN (Dry Low NOx); if there is any wet low NOx emissions reductions system in continuous when the machine is operating (NOx Water Injection; NOx Steam Injection) or if the machine has any inlet air cooling system and how well it is operating, how often the machine is started and stopped, how often it is tripped (emergency shutdown) from a loaded condition, etc. All of these factors contribute to the longevity of the machine hot gas path components, and while 50 MW out of 160 MW isn't usually considered a heavy load on the turbine because the IGVs will not usually be fully open it's pretty safe to say that the machine is not operating at optimum efficiency--because it's not at CPR- (or CPD-) biased exhaust temperature control (Base Load), which is the optimum efficiency operating condition for the turbine (maybe not for the plant if it is, or is capable of, Combined Cycle operation using an HRSG (Heat Recovery Steam Generator) on the exhaust. But presuming it is not being operated at some load/condition that causes intermittent and frequent combustion mode switching it's pretty likely that the machine isn't going to suffer much in the way of damage being operated at 50 MW.

But there's a LOT we don't know about the machine and its configuration and we can't say anything with any degree of certainty because we don't know what we don't know. The only thing we can say with any degree of certainty is that the turbine is not operating at optimal efficiency because the IGVs aren't fully open and the machine isn't operating at Base Load (fully open IGVs is one of the conditions that defines Base Load). BUT, if the load the machine is supplying or contributing to supplying doesn't require more than 50 MW and the machine needs to be kept running for some reason (of which there might be several possibilities) then that's how the machine needs to be operated at this point in its lifespan. The unused capability (the difference between the actual load and the rated turbine load) is sometimes referred to as Spinning Reserve--meaning that the machine is at rated speed and producing some power but not rated power, and if something were to happen to require additional load from the machine it could respond very quickly (because it's running at rated speed and already producing some load--just not rated load). Sometimes that capability--being able to quickly supply more load--is more important than machine efficiency. If the machine operates in Combined Cycle and supplies steam to a host facility (a cement plant, or a refinery or some kind of plant that needs steam for its process/product) then that can be more important that turbine efficiency.

But without knowing a lot more about the machine and its primary operating function(s) we can't say much more than as long as it's operating at a steady state without frequency combustion mode changes and excessive NOx emissions reduction flows (water or steam) then it's probably okay to be operated at 50 MW out of a possible 160 MW.

 

To answer the hydrogen question, the 7FH2 model generator has a capacity of 201 MVA. Its hydrogen cooling system works correctly, maintaining internal purity above 90% H2-AIR. The hydrogen seal is normal, with no leaks. There is only a certain amount of venting in the rotameters to maintain purity measurements. The internal hydrogen temperature in the generator remains at 116°F.

The 7FA turbine is a single-cycle turbine powered by gas. It has a dry, low NOx 2.6 (DLN-2.6) emissions system, with four gaseous fuel system distributors: premix 1 (PM1), premix 2 (PM2), premix 3 (PM3), and quaternary (Q). Each combustion chamber has a total of six fuel nozzles. The IGVs currently have an opening of 47.9°, the CPD EN 108 psi.