The application of voltage source converter(VSC)based interconnection devices in the AC system makes the distribution network change from a radial network with one-terminal power supply to a multi-terminal power supply network. However

the conventional three-section current protection applied in distribution network is not able to determine the direction of the fault. Therefore it is necessary to install directional elements to ensure the reliability of three-section current protection. The phase relationships between sequence currents under phase-to-phase faults that happen at different locations in the AC distribution network connected with the back-to-back VSC-based interconnection device is deduced. Then a novel criterion for fault direction recognition is proposed based on the phase relationship between the negative-sequence current flowing through the protected line and the positive-sequence current flowing through the passive branches emanating from the same bus. Through theoretical analysis

the action interval of direction element under forward faults and backward faults are derived. The proposed directional element is applicable to multi-terminal power supply networks containing VSC-based interconnection devices or VSC interfaced distributed generators(DGs)

where there is passive branch emanating from each bus. The proposed directional element does not require potential transformers(PTs)

and is scarcely affected by the transition resistance. Under a three-phase symmetrical fault

the correct operation of the protection can still be guaranteed

and the dead zone problem of conventional power directional elements when the fault happens at the outlet of the relay can be avoided. The criterion is verified by a large number of PSCAD simulation results of phase-to-phase faults that occur at different locations of the distribution network. This work is supported by National High Technology Research and Development Program of China(863 Program)(No. 2015AA050102)

National Natural Science Foundation of China(No. 51577129)and Tianjin Natural Science Foundation(No. 14JCYBJC21000).