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Carlos Melim
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Join Date: Feb 2018
Posts: 9

Inertia Constant of a Geared Synchronous Generator

07/25/2018 2:39 PM

Good evening everybody.
I want to calculate the inertia constant of a 1500 rpm synchronous generator with a 8304 rpm steam turbine prime-mover, both attached through a gear box.
I have the J figures for both. The generator has a J of 789 Kgm^2 and the turbine a J of 40 kgm^2.
I’ve come across the Equivalent moment of inertia concept. The constant inertia H the of generator alone is 0.9 seconds but, for the whole turbine-generator set, the H is around 2.3 seconds.
Some colleagues told me that for grid disturbances studies I should consider the generator alone.
That a steam turbine does not respond to a grid disturbance since its typical governor is slow.
Does it make sense? The governor can be slow but there is the synchronous inertial response of the generator that depends on the inertia constant of the set, right?
Am I missing something? The steam turbine is small but runs at a high speed so its inertia is not negligible.
I wish to hear your experts informations.
Thanks for your help.
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Tornado
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#1

Re: Inertia Constant of a Geared Synchronous Generator

07/25/2018 3:01 PM

I think you are right and some of your colleagues wrong.

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Carlos Melim
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Join Date: Feb 2018
Posts: 9
#3
In reply to #1

Re: Inertia Constant of a Geared Synchronous Generator

07/25/2018 4:02 PM

Thanks Tornado.

It’s great when people support us when we are right.

I work for an utility in a Portuguese island and our main prime-movers are hydro, fuel and natural gas.

We have two steam turbines that are quite small (5 and 8 MW) but, due to its speed, have inertia constant figures greater than the other fuel and gas generators.

I think their contribution to grid stability has been neglected.

I don’t find many white papers focused on the synchronous generators inertia subject.

Do you any technical info that might be useful?

Best regards.

Carlos Melim
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SolarEagle
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#2

Re: Inertia Constant of a Geared Synchronous Generator

07/25/2018 3:21 PM

..."I'd say that anytime you are interested in calculating how the shaft speed of a rotating generator changes with time during or after a disturbance, you are calculating the dynamic response. The response is a function of the rotational inertia of the shaft and the sum of all torques acting on the shaft."...

Ref link...

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RAMConsult
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#4

Re: Inertia Constant of a Geared Synchronous Generator

07/25/2018 7:01 PM

1. "... I should consider the generator alone...", wrong, that's the same as saying that a steam turbine has the same inertia as a hydro turbine, for the same rating.

2. "...a steam turbine does not respond to a grid disturbance since its typical governor is slow...", wrong, that's like saying a turbine whose steam supply is from a once through boiler has the same response as a drum boiler plant; it's overall response depends upon the governor, its type and settings, plus all the dynamics of the steam system. There's also the excitation system's contribution to stability. Plus, as you correctly state, the rated speed and type of turbine play a major role; i.e., a high speed, long, thin turbine has a different inertia than a low speed, short, wide turbine. And don't forget the inertia of the the gearbox

Use of the per unit system will assist in understanding the individual contributions from each source and form of generation. It will normalize the ratings of each unit against a common base (usually 100MVA); a 10MVA generator with an H of 2 on its own base will have an H of 0.2 on a 100MVA base, while a 15MVA unit with an H of 2 on its own base will have an H of 0.3 on a 100MVA base. Same rated H, two different contributions to stability for the same system..

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Anonymous Poster #1
#5

Re: Inertia Constant of a Geared Synchronous Generator

07/26/2018 4:21 AM

https://cr4.globalspec.com/thread/121496/Validation-of-the-Inertia-Constant-of-a-Synchronous-Generator
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Rixter
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#6

Re: Inertia Constant of a Geared Synchronous Generator

07/26/2018 4:02 PM

Inertia constant is the ratio of rotational kinetic energy to delivered power (or rated power) and has the dimensions of Energy/Power = seconds. Since the turbine is directly coupled to the alternator rotor, IMHO, it only makes sense to include it in the inertial constant calculation.

Inertia constant = kinetic energy of rotation / power.

The rate of change of frequency due to a change in power (generated or consumed), or frequency stability, is inversely proportional to the Inertia Constant of the entire power system, and a power system with greater percentage of alternative sources (wind, solar) would have a lower inertia constant and less frequency stability.

https://www.nationalgrid.com/sites/default/files/documents/16890-Meeting%208%20-%20Inertia%20presentation.pdf
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67model
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#7

Re: Inertia Constant of a Geared Synchronous Generator

07/30/2018 6:15 AM

I make the J value for the turbine 1226 kgm2 and the genset 2015, equivalent at generator speed. Total stored energy 24.86 MJ. It is not possible to verify your H value because you have not told us the MVA of the generator, only the MW (8 or 10)!

It would be 10.8 MVA if H were exactly 2.3 seconds. This seems an 8MW unit - which would be 10 MVA at standard 0.8 power factor - but such numbers can be moved around by the "site ratings" for your nominated ambient rather than ISO standard conditions. As has been commented, it is best to convert all values to same base value like 100MVA as 1.0 unit for your system model, relative contributions to H and short circuit levels are then clear in model.

I agree the comments from other posts.

Tell your erroneous colleagues they have been reading too much "Harry Potter" and not enough "Isaac Newton"!
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