Can we couple turbine 1,7MW with generator 7,5MW? What could be the problem?

Is it better to have similar power? Why?

 

A generator is a device for converting torque into amperes. (And a motor is a device for converting amperes into torque.)

If you have a device which is capable of producing more torque than the generator can convert into amperes then it's very likely the generator will be damaged and/or the coupling between the prime mover and the generator will be damaged if not properly sized.

If you have a device which cannot produce more torque than the generator can convert into amperes then there will be no damage to the generator (and likely not the coupling either).

Most prime movers are rated for slightly less power than the generators they supply torque to. Or, most generators are rated for slightly more power than the prime movers which supply torque to them. In this way, it is hoped the generators will not be damaged by excessive current flowing in the stator windings (which causes excessive heating which leads to the damage), and that if for some reason the power output of the prime mover should exceed its rating for some (usually short) period of time that the generator will not be damaged.

If the prime mover was rated for more than the generator then it would be necessary to have some kind of load-limiting scheme to prevent damage to the generator when the prime mover power output continually exceeded the generator rating.

You can think of your situation in this way: A prime mover only producing a portion of rated power can drive a generator rated for higher power output for years and years and years with no ill effect on the generator. In this example, you would only be using a portion of the generator's capability while using all of the turbine in your example, which is akin to a larger turbine driving the same generator at a lower power output (equivalent to your turbine's).

No problem at all.

 

Good Response,

IF you can uprate your Turbine, you can generate more MWatts as your Generator capable to generate 7.5 MW.

Maintain ISO conditions to your turbine so that it can give full output at all ambient conditions.

 

Thanks a lot.

But what about excitation? Can excitation be a problem?
Or if voltage regulator is not stable?

 

Okay; now I'm worried (not that I wasn't before).

Generator terminal voltage is a function of generator field flux and speed. Speed, for a synchronous generator is constant regardless of the turbine power output. (I hope the turbine you want to couple to the generator is rated for the generator speed/frequency.)

Again, if you had a turbine that was rated for precisely, or even slightly less than the generator was rated for, and you were operating the turbine (and generator) at less than rated load the excitation should be capable of providing sufficient excitation to the generator field to produce the required field flux to achieve rated terminal voltage (at synchronous speed/frequency).

And, if you couple a turbine that is rated much less than the generator <b>at the proper speed</b> the excitation system of the generator will still be capable of providing the proper excitation (presuming the excitation system is the proper one to achieve rated generator output if the prime mover was rated at or very near generator rating).

So, I should have qualified my previous response by stating that the turbine you want to couple to the generator must be rated for the same speed as the generator requires to produce rated frequency (or there must be some kind of gear box to decrease or increase the speed output of the turbine to match that required by the generator).

Presuming the exciter is the one provided with the generator to produce nameplate rating, again, using a turbine with a lower rating to drive the generator (at the proper speed) is exactly like running a turbine with the nearly the same rating as the generator at less than rated output. Which happens all the time.

Exciter output is unrelated to turbine output. Real power (watts) is affected by turbine output. Generator terminal voltage (and VArs/Power Factor--imaginary or apparent power) is a function of excitation. Excitation, and the excitation control system, is usually a separate system (why is that, anyway; I've never understood that...!) from the turbine and the turbine control system.

 

no problem, but is this a cost effective choice in general!?

 

> Can we couple turbine 1,7MW with generator 7,5MW? What could be the problem?

I think there is no problem but it's a tremendous uneconomic solution

 

S.Zoghby, is absoluly correct.

1) There are qualified millrights that can properly couple and align the two machines!

2) I'm sure generator loss at the reduced load won't significantly impact efficiency!

3) I'm also sure that as long as generator output does not exceed the turbine's rating, and them achieve speeds are compatible!

Regards, Phil Corso

 

A couple other things to consider:

1. A larger generator can cause a larger torque on the prime mover when synchronized to an infinite grid out of phase. So the larger generator could damage the prime mover with a smaller phase angle error.

2. The available fault current from the larger generator would also likely be much greater. Generator breakers, field breakers, and electrical distribution equipment may not be designed with the larger available fault current in mind. I'm a mechanical guy so I can't talk about details.

 

Dragan... Dave Amundson makes two excellent points that I completely overlooked. Based on his astute observations, I withdraw my earlier support for a plan to couple the two machines without additional study!

Re: CSA's question "(....why is that, anyway; I've never understood that...!) from the turbine and the turbine control system."

CSA, would you actually accept an answer from me?

Phil

 

I agree Dave Amundson made two very important points about the turbine-generator application. The original question was seemingly a very simple one, but as with most things a complete and proper answer is much more involved, and would require many more questions on the part of the respondents and answers from the original poster. And, the subsequent questions from the original poster are cause for some concern, as previously stated. It's sincerely hoped that a complete engineering assessment would be done on any such "coupling."

By copy to Phil Corso, the question was basically rhetorical, but if you have some information other than, "It's been done that way since the beginning of electrical power generation," I would be very interested and grateful to learn of reasons why, in this day and age (not 100 or 50 or even 20 years ago) excitation control systems are integral parts of the overall turbine-generator- or even plant-wide control systems (since plant-wide controls are taking on more and more of the turbine-generator island control functions all the time).

It's understood that when fly-ball governors, or even early mechanical-hydraulic and even electro-hydraulic turbine control systems were used on turbines that incorporating excitation control into them was not feasible. But, with the rich selection of I/O functions available to most control systems today, why are excitation control systems still separate from turbine control systems, or, even plant-wide control systems? As electronic controls came into wider use in the 1980s and 1990s in power plants why weren't excitation control systems incorporated into the turbine control system?

Again, it's basically a rhetorical question, but sometimes there are regulations or laws, not just convention, for these kinds of situation and if that's the case I'd be grateful to know of them.