External directional protection unit capable of realizing directional zone selectivity interlocking

The invention brings forward an external directional protection unit (EDP) capable of realizing directional zone selectivity interlocking (DZSI) of a circuit breaker (CB), wherein the protection unit can be connected to the existing circuit breaker with the zone selectivity interlocking (ZSI) function externally. The protection unit comprises an input interface (IE) connected to the circuit breaker and an output interface (OE) connected to the circuit breaker, a forward interface pair (IF/OF) connected with protection units of other circuit breakers to transmit an interlocking signal, and a reverse interface pair (IB/OB). A control unit (CTRL) arranged in the protection unit is configured to determine whether the direction of the current flowing through the circuit breaker (CB) is consistent with a predetermined current reference direction; if so, the forward interface pair (IF/OF) are enabled; and if not, the reverse interface pair (IB/OB) are enabled.