system RoCoF calculation 1

[Carlos Melim]

Good night experts.

I work for the utility in Madeira island. Our grid is small and not connected to Portugal mainland.

I want to validate our RoCoF figure. I have calculated all the synchronous generators (Si) inertia constants (Hi).

I´ve used Hi= (J*Wi^2)/(0.5*Si).

I´ve read somewhere that we should refer all the powers to, for example, 10 MVA.

So I have multiplied the final figures: Hifinal=(Si/10)*Hi.

I´m a newbie on the subject and I´m having problems with the system inertia constant (Hsys).

I have came across two formulas:

One formula multiplies the grid nominal frequency f0 and the other doesn´t. I don´t understand the difference.


The system inertia constant (Hsys) will be referred to the sum of all synchronous powers online.

But the individual Hi figures are 10 MVA based. Something is wrong.

What am I missing here?

All your help will be greatly appreciated.

Best regards.

Carlos Melim

 

[electricpete]

The first equation looks like a very simple and familiar form of the fundamental motion equation (rotational equivalent to F = m*a = m*dv/dt) involving one generator. All the quantities are uniquely associated with that one generator.

I'm not familiar with the second equation, but it appears to involve multiple generators.
The notation indicates derivative at time 0+, typically the beginning of a transient.
If I have to guess the numerator Pk is the generator of interest (whose instaneous rate of change at t=0+ will be computed). The denominator sum involving Hi * Si would involve a set of connected machines with inertia Hi and complex power flow (or rating?) Si. That's all just a guess, and I can't get any further without reading more of the context. Maybe you could have guessed better than me.

You'd probably do better on this question in the electric power forum than the electric machine forum, since this is more of a power system study type of question.





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(2B)+(2B)' ?