Its not my field, having worked with picowatts rather than Terawatts, but I believe grid level converters - at least the big ones used on HVDC links - do not run at high speed. They are quite crude and some even simply chop DC at Mains frequency to produce a square wave and rely on subsequent L-C filtering to get a sinusoid. More sophisticated ones manage a sort of multilevel boxcar waveform to approximate a sinewave more closely. They too operate at low speed, a few multiples of Mains frequency.
Having now read up on MMC high voltage converters, its a concept I had totally discounted for the grid because I didn't think capacitors could exist for that function at that combination of frequencies, voltage, power and charge, they must be physically massive which in itself must create other complications.
Likewise on power, one of my specialisms was down at the < -130dBm signal levels - but not at audio frequencies. Those days have well gone as has the grey matter.
