Aiming at the incoming line switchgear cabinet characteristics of 12 kV distribution network terminal substation and main unit

this paper presents a multifunctional digital protection set of terminal user switchgear based on 16-bit microprocessor.The power of the protector is supplied by itself from the CT.Based on thermal cumulate principles

a rapid and high precision inverse-time phase-sequence over-current protection is implemented.Three stages phase-sequence current and zero-sequence current protection can be implemented.Main circuit phase-lack fault and CT wire-break fault can be identified by the current signs and values.Fault current recording is displayed by liquid-crystal and current curve diagram can be transmitted to PC and therefore analyzed.It can communicate with other protection apparatus based on CAN field-bus.Test results show that time duration and current operating value precision are less than 2%

and the functions of the protection device are reliable.