Another irony : P5 already had extra windings in its main transformer for the provision of HOG. It only required a 'converter'. But IR never made use of it :P. Only now, they made its composite converter although it was proposed in 2012. We're still yet to see its HOG variant rolling out.
P4's HOG is still under research BTW and in due course of time, it'll be equipped with HOG converter.