Composition for heat cycle system, and heat cycle system

A composition, comprising

• a working fluid for heat cycle containing difluoromethane as a saturated fluorinated hydrocarbon compound, trifluoroethylene and 2,3,3,3-tetrafluoropropene as unsaturated fluorinated hydrocarbon compounds, and optionally at least one other unsaturated fluorinated hydrocarbon compound which is a compound comprising at least one carbon-carbon unsaturated bond and represented by formula (I), and
• a refrigerant oil having a breakdown voltage of at least 25 kV, a hydroxyl value of at most 0.1 mgKOH/g, and a minimum temperature on a high temperature side of a phase separation temperature from the working fluid for heat cycle of at least 35° C. and a maximum temperature on a low temperature side of at most −60° C.:
• CxFyRz (I),
• where R is H or Cl, x is an integer of from 2 to 6, y is an integer of from 1 to 12, and z is an integer of from 0 to 11, provided that 2x≥y+z≥2.