Since We use PLL for grid connected converters. Does it actually need modification in PWM for the converters
A PLL is only essential when one uses self commutating power devices such as thyristors. When one uses PWM and forced-commutating devices there is no need to synchronize the PWM.
However, the controller needs to be synchronised as do the voltage waveforms to allow for synchronous operation with the grid. Normally in the controller the micro has a different frequency to that of the PWM but these may be synchronized by using synchronous PWM where the PWM generates a synchronous interrupt for the controller and the controller in turn is synced to the mains / grid using a mains based PLL based interrupt.
However the actual PWM and mains need not be synchronous. This effect may be seen by the PWM ripple running across the screen of an oscilloscope scope when synced to the mains or the mains waveform doing same when synced to the PWM.
This does not happen in thyristor controlled inverters where both the controller and firing are synced to the mains / grid. In general the mains / grid frequency, magnitude and phase change constantly due to varying loads, switching and faults.
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