The existing Overcurrent Protection Function (OCPF) of distribution systems is vulnerable to Multiple Feeder Faults (MFF), which involve at least two feeders simultaneously, leading to the incoming feeder relay trip faster than the faulted feeder relays, although the OCPF of the relays can be coordinated for all fault current values. In this case, all feeders connected to the power transformer are de-energized, even the healthy ones, directly impacting the network reliability, service quality and availability of the electricity utilities and ultimately leading to outage costs for the customers. In light of this, an IEC 61850-based OCPF scheme with the directional element is proposed to support existing OCPF to deal with MFF and encompass the integration of distributed generation, and can be directly implemented in the digital substations without requiring further equipment installation. Moreover, an hardware-in-the-loop platform is developed for testing the interoperability of the proposed scheme with multi-vendor protection relays, which can assist electricity utilities also in the validation of their protection and control schemes based on their requirements before the field implementation. Test cases are specifically elaborated to study the interoperability of the proposed scheme with three real devices—whose configuration details are also provided to aid protection engineers—in a real distribution system, under different conditions such as switching operations, MFF types, resistance and location, integration of distributed generation. The results show that the proposed scheme manages to enhance the selectivity and reliability of the digital substation protection system, hence representing a valuable complement to existing OCPF schemes.