A refrigeration system using CO2 as a refrigerant includes a receiver having a liquid outlet connected to expansion valves, which are connected to evaporators, which are connected to the suction side of the compressor. The power conditioning unit includes a power injection control block having a sense input coupled to an energy storage capacitor on a dc link for controlling a dc-to-ac converter to control the injected mains power. The second ramp voltage generating circuit generates a second ramp voltage increasing with a gradient gentler than that of the rising edge of a sustain pulse and steeper than that of the first ramp voltage, at the end of each sustain period.