Are ripples and harmonics the same thing?
Are "power ripples" cause by hysteresis controllers considered
  "harmonics" ?


No, not all of them, harmonics are specific frequencies that are resonant, all noise ripples such as switching noise or hysteresis add up to the total noise 'ripple' of the circuit. Ripple is less applicable in AC systems because the signal (the AC waveform itself) is much higher than the noise. To see the noise in an AC system you need to look at the frequency content or subtract out the first harmonic (ie 60 or 50Hz)


Source: http://www.compliance-club.com/archive/keitharmstrong/design_techniques6.html


  What are the frequencies of those power [harmonics]?


Typically multiples of the carrier, if your on 60Hz, you get one at double and triple so 120Hz and 180Hz and so on.


  Do they affect the value of THD ?


THD only takes into consideration the harmonics to the fundamental:

$$ %THD = \frac{\sqrt{v_2^2+v_3^2 \ddots v_n^2}}{v_1^2} $$