Current source inverters (CSI) associated to rectifiers are used in variable speed drives for the propulsion system onboard ships. The complete structure will generate frequency perturbations in the entire supply system which could become undesirable. In a first part, the modulation theory is applied to the ideal structure of a converter. The designs of the propulsion transformers and of the propulsion motor are considered in order to calculate the injected currents on both sides of the converters. Finally, those results are confronted to experimental ones obtained onboard an electric ship. Those results show a complete validation of the theoretical predictions.