Discussion
A 20 MVA, 50 Hz generator delivers 18 MW over a double circuit line to an infinite bus. The generator has kinetic energy of 2.52 MJ/MVA at rated speed. What is the value of inertia constant?
Answer: A
The inertia constant \(M\) in seconds squared per electrical degree is related to the stored energy per MVA, \(H\), by
\[ M = \frac{GH}{180 f} \]
Taking the stored energy of 2.52 MJ/MVA as H on a per MVA basis (G = 1):
\[ \begin{aligned} M = \frac{2.52}{180 \times 50} \\ &= 2.8 \times 10^{-4}\ \text{s}^2/\text{elec degree} \end{aligned} \]
This is the key, option A. If the 20 MVA rating were put in as G, the value would be 20 times larger (5.6e-3), which is not offered, so the item is read per unit MVA. The load of 18 MW is not needed.
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