Oil distribution in multiple-compressor systems utilizing variable speed

A refrigeration system comprising:

• a plurality of compressors connected in series with each other, each compressor having an oil sump located in a gravitational bottom of the compressor, and having one or more oil flow conduits coupled between adjacent compressors of the plurality of compressors, the oil flow conduits configured to facilitate a distribution of oil from an upstream compressor to a downstream compressor;
• a common supply line for supplying refrigerant and oil to each of the plurality of compressors;
• wherein the plurality of compressors includes a lead compressor and one or more non-lead compressors, and wherein the common supply line is configured to return more oil to the lead compressor than to the one or more non-lead compressors;
• and
• a controller coupled to each of the plurality of compressors;
• wherein the controller regulates the speed of each of the plurality of compressors to control oil sump pressures for each of the plurality of compressors such that the lead compressor has a higher oil sump pressure than any of the one or more non-lead compressors in order to facilitate oil distribution from the lead compressor to the one or more non-lead compressors.