This paper analyzes the influence of frequency and voltage variation over the commutation of thyristors in high-power cycloconverters. The analysis demonstrates that frequency and voltage variations can cause commutation failures generating significant damages in cycloconverters. In addition, the paper proposes a method based on the energy required to commutate thyristors to determine the maximum frequency and voltage variations that will not affect commutation between thyristors, information that can be later used for the correct setting of protection relays. The analysis is complemented with simulated results using data obtained from high-power thyristors used in commercially available cycloconverters. Finally, a commutation failure in a 15 MW grinding mill cycloconverter drive is presented and analyzed.