Complex sentence: In electrical engineering, power engineering and the electric power industry, power conversion is converting electric energy from one form to another, converting between AC and DC, or just changing the voltage or frequency, or some combination of these.
Split into simpler sentences: In electrical engineering, power engineering and the electric power industry, power conversion is converting electric energy from one form to another. It may be converting AC to or from DC, or the voltage or frequency, or some combination of these.