Novel unit protective relaying techniques for teed circuit based on sequential overlapping derivative transform

This paper presents a novel protective concept for EHV Three-terminal transmission line, teed circuit. The proposed protective relaying techniques use the fourth sequential overlapping derivative, SOD, concept at all line terminals. The SOD signals represent the transient components initiated due to fault occurrence. Such signals are free of both dc and fundamental components. Two protective relaying techniques are proposed here. The first applies the SOD to current signals only at all relay locations. As soon as these signals exceed a predetermined threshold limit, a discriminator compares their polarities to determine whether the fault is internal or external. The comparison is done by examining the polarities similarity/dissimilarity at the three terminals. The other applies the SOD to both current and voltage signals. The multiplier of the produced transient components is used as a relaying signal. Polarity of the initial change of such relaying signal is an indicator of fault direction. Negative polarity indicate forward fault direction while positive polarity indicate backward fault direction. Exchanging data between relaying stations using a suitable communication channel enable us to discriminate between internal and external faults and send a tripping signal to the prober circuit breakers. Testing and evaluation of the proposed protective techniques has been performed with fault data obtained from comprehensive ATP simulations. The simulation results confirmed that the proposed techniques have high reliability for internal faults and do not mal-operate for external faults and load switching conditions. The suggested techniques overcome most problems facing the other protective techniques such as synchronization between the three relaying stations, CT saturation compensation. Also they are not affected by the distributed shunt capacitance of the long line and they offer very simple detection method that can easily be practically implemented.