Frequency of natural oscillation is given by, fn = {((dPe/dδ)at(δo))/M)} dPe/dδ = ((V1*V2)/X*(cosδ)) = (1.1/0.6)*cosδ = (1.1/0.6)*0.5 = 0.91 M = (H*s)/(πf) = 4/(50π)
Here auto transformer change voltage from 600 V to 200 V. Voltage ratio (a) = 600/200 = 3 kVA rating of the auto-transformer = (a/(a - 1))*(kVA rating of auto transformer) = (3/2)*20 = 30 kVA
EXAMIANS