If an active PFC follows the standard an draws a perfect sine shaped current from the mains, does it then contribute even more to the flatness of the voltage waveform?
I know what you mean, yes, there is more reactive power if an active PFC draws perfect sine current when the mains voltage is flat-topped.
....But if all PSU's were active PFC'd, then you woudlnt get flat topping (not nowhere near so much anyway).......because AYK, flat topping is mostly caused by the multiple use of "txfmr/rect/smoothing cap" power supplies being used....which have poor PF.
I think the EMC test places have to monitor flat-topping whilst they are testing your product, and if its too much, then they have to invalidate the test......otherwise, 1 or 2 would get you a pass as the test limits are not that tight......PFC approvals is brought in to reduce flat-topping so woudlnt let flat-topping "rain on its parade" so-to-speak.
Yes i think you can draw high current at cos phi = 0 and have no THD and it passes.....but there is no equipment that generally would be available for someone to do that i believe.
AYK, industrial customers are allowed cos phi = 0 but have to pay for all reactive current drawn....Or more accurately, they pay for their total "Apparent power" drawn....Apparent power being the complex combo of active and reactive power.
AYK, the cheapest way to do domestic PFC for residential type products is to use a boost PFC, and use the divided down mains itself as the "sinusoidal reference"...when you do this you pretty much end up with most of your input current (the fundamental) being in phase with the mains voltage, so the points you raise dont really come into it that much, though academically speaking, your points are spot on.
The PLL method you speak of is convenient to do if you are doing PFC with a micro.....but in truth.....who wants to design a PFC for a mass market product that needs a software engineer to be involved in all testing and maintenance of the design, when you can just use a cheap offtheshelf Boost PFC controller...which uses the mains as a reference....and get away cheaply like that , perfectly satisfactorily.