Refrigeration system with oil control system

A controller for a refrigeration system that provides cooling for a refrigeration load using carbon dioxide (CO2) as a refrigerant, the controller comprising:

• one or more processors;
• and
• one or more non-transitory computer-readable media storing instructions that, when executed by the one or more processors, cause the one or more processors to perform operations comprising:
• controlling a plurality of compressors to circulate the CO2 refrigerant within the CO2 refrigeration system;
• obtaining a measurement of an amount of oil within a suction line coupled to an inlet of the plurality of compressors, the suction line configured to contain a mixture of oil and the CO2 refrigerant and to deliver at least a portion of the mixture of the oil and the CO2 refrigerant to at least one of the plurality of compressors positioned downstream of the suction line;
• operating a plurality of oil equalization valves in a plurality of oil equalization lines connecting the plurality of compressors to an oil return line to control the CO2 refrigerant flow through the plurality of oil equalization lines to the plurality of compressors;
• comparing the measured amount of the oil within the suction line to an oil threshold;
• based on a result of the comparison, periodically opening the plurality of oil equalization valves to cause a flow of excess oil from the plurality of compressors into the plurality of oil equalization lines to equalize an amount of oil within each compressor of the plurality of compressors;
• opening the oil control valve in response to a determination that the amount of the oil within the suction line is less than the oil threshold;
• and
• keeping the oil control valve closed in response to a determination that the amount of the oil within the suction line is greater than or equal to the oil threshold.