Calculating centre of inertia frequency, scaling?

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Hey there,

I have code that acquires all frequencies and is used to find COI frequency to find ROCOF on COI frequency, so I can use the ROCOF to estimate system inertia when certain contingencies occur.

To make COI frequency, I need to scale the machines' frequency contribution. I do not understand what Mbase is. Is it equivalent to inertia constant or rated power? If not, how can I get these two values from PSSE? I've spent so long looking around.

Thank you for your time.