Inertia Constant H vs Starting Time Constant T_a

[gkaestle]

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I think Wolf is perfectly right.

The inertia constant H is the ratio between the rotational energy E_rot stored in the turbogenerator and the nominal Power of the generator P_nom.

H = E_rot / P_nom [kWs/kW = s]

It's more or less artificial. You may calculate the value by using moment of inertia J, nominal rotating speed omega w_nom, nominal moment M_nom

H = (1/2*J*w_nom^2)/(M_nom * w_nom) = 1/2*w_nom * J/M_nom

The starting time constant T_a is the time needed to speed up a rotating system with nominal moment for zero to nominal omega.

As M = J*w', w' = M_nom/J
w' applied during a period of T_a will accelerate the turbogenerator to w_nom. T_a * w' = w_nom. So,

T_a = w_nom * J/M_nom

You can see that T_a is twice the time compared to H.

Regards,

Gunnar


In German T_a is called Anlaufzeitkonstante. Is starting time constant the correct expression?

 

[mgtrp]

I'm used to seeing "Ta" written as "Tm", the mechanical starting time (as opposed to Tw, the water starting time). Usually it is expressed in terms of the MVA base of the unit (i.e., S_nom) rather than the rated MW, so the unit becomes MW-s/MVA, not MW-s/MW (or kW-s/kW).

And yes, Tm = 2H.

Regards.