Inertia Constant 'H' 1

[GDL90958]

All,

A few weeks ago I posted a question about this subject which seemed to cause confusion over the SI units being utilised. Since my question I have clarified it and am posting for all's info.

Inertia Constant 'H' can be defined as the ratio of stored energy at a rated speed to the rated apparant power of a given machine such:

Inertia Constant 'H' = K.E. (kW.sec)/kVA

where K.E. = 1/2.I.w^2 = 1/2.(Wr^2).w^2

=1/2.Wr^2.(rpm.2pi/60)^2

= Wr^2.rpm^2.(5.483)/1000 Joules

= Wr^2.rpm^2.(5.483)/10^6

such it follows the injertia constant

= Wr^2.rpm^2.(5.483)/10^6 kVA

This figure is related to the size and weight of the rotor and the higher the figure for a particular machine the greater it's capability in dealing with sudden increase in load and greater stability.

Regards

G

 

[electricpete]

A less precise but more intuitive way of viewing would be as follows:

If you cancel KVA and KW you get the units of H are seconds.

H corresponds to the time it would take to accelerate the machine inertai from zero to rated speed using full rated power (assuming rated mva can actually be output as mw... not really true but close enough).

High H corresponds to a machine which changes speed over long time periods (responds slowly to transient mismatch of power input to power output). Low H corresonds to machine which changes speed faster.

You are correct the High H is more stable (although on the surface it sounds backwards from a pure control perspective). In the case of generators we don't want the speed (more precisely the angle) to change quickly.