I've found a Simulink model for a PLL for 3 phase grid. It works very well, but I don't understand the purpose of a block called 'actual amplitude detection'. It computes the followings: square of the magnitude, low pass filter the result (this is done for all the 3 phases), and then the maximum between the 3 signals. This value is then used to normalize the signals that represent the input grid. Can you please tell me why is this done, why I cannot normalize the input grid with a fixed value (for example 400V)? Why is this method better, I don't understand the principle, the theory behind this?
Many thanks, Monica