Answer Posted / 7anoter4

If the change in load will be the same Pa1=Pa2 and the
rated frequency fo1=fo2

for the same time interval dt1=dt2

IF Pa=H*MVA/180/fo*d2R/dt2 [See IEEE-399 ch. 8.6.2 How
stability programs work]

and d2R/dt2=2*pi()*df/dt

then (df/dt)1=Pa/2/pi()*180*fo/MVA1/H1 and

(df/dt)2=Pa/2/pi()*180*fo/MVA2/H2

(df/dt)2/(df/dt)1=MVA1*H1/MVA2*H2

If MVA1=MVA2 then (df/dt)2=(df/dt)1*H1/H2

df2=12*30/42=8.57 Hz

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