basically this inverter incorporates high frequency intermediate stage , however the output frequency is still 50/60 Hz,
In this inverter , basicallly there are 2 stages ,
1) DC-DC stage
2) DC-AC stage
in first stage a lower voltage dc say 12V is converted to 311VDC now in second stage the 311VDC which is a peak value is converted into 220V AC sine wave,
why 311VDC ?
because if we consider the output waveform to be sinusoidal the rms value of 311 (peak) is 220 i.e 311 divided by root 2