This work describes the modeling of busbar differential protection function by using the MODELS language available in the software Alternative Transients Program (ATP). This type of modeling includes data acquisition and signal conditioning modules for suitably implementing the commercially used 1-out-of-1 and 2-out-of-2 logics. This algorithm is based on instantaneous values of currents and considers an additional zone selection logic to keep up with the dynamic reconfiguration of flexible busbar arrangements. The results show that this type of modeling is a powerful tool for assessments of protection relays for any substation configuration in operational conditions, not readily available in actual records.