In order to rapidly clear a fault in all-parallel autotransformer (AT) railway system

the existing protection scheme trips both faulted line and healthy line. To solve this problem

a novel unit protection scheme is proposed in this paper. From fault characteristics measured at substation (SS)

AT station (ATS) and section post (SP)

we can observe that SS protective relays can detect all the faults but could not be selective

and ATS or SP relays can discriminate the faulted line but not sensitively enough; therefore

we make full use of the fault characteristics and propose a novel unit protection scheme. We employ directional distance protections at SS

ATS and SP

and unify them with networked inter-tripping signals and interlocking signals. When a forward distance protection detects a fault and does not receive inter-locking signals from reverse distance protections of other relays in a short setting time

it will trip the relevant circuit breaker and send an inter-tripping signal to other relays in the same protection unit. Field test results show the proposed scheme trips only the faulted line rapidly and never interrupts the power supply of healthy lines; therefore

the proposed protection scheme obtains both selectivity and speed.