The University of Manchester? United KingdomIn general, distance relays are most appropriate for both local and remote backup protection since they require only local voltage and current measurements. However, as transmission lines and substations flourish, problems occur in the coordination and setting of distance relays. [1-2] stated that one of the major causes for cascading outage was the maloperation of backup protections employing the Zone-2 and Zone-3 distance elements. Adoption of communication systems can improve the sensitivity and selectivity of a distance relay. But it inevitably degrades one of the advantages of distance protection because it now requires remote data.
Unlike distance relays, current differential relays (CDRs) have a much better selectivity. Furthermore, with the introduction of GPS, all measured data can be time-tagged and compared to those at other terminals with minimum timing errors [3], thereby enabling the protection systems to cover a wider area. Hence, the GPS current differential protection is more suitable for wide area backup protection than distance protection in terms of communication requirement, selectivity and protection scale.
This paper proposes an innovative multi-zone differential protection (MDP) scheme for protecting wide area transmission networks. The term multi-zone indicates that this scheme covers more than one protection zone whilst differential suggests that current differential protection (CDP) algorithm is used. The MDP can serve as a backup protection system, or operate as a main-3 protection in Extra High Voltage transmission systems. It consists of distributed CDRs and these CDRs will interact with each other via high-speed communication networks. In normal situations where all data are available, the MDP is a collection of CDP schemes and each scheme operates individually. Whilst when the measurement data are missing from a particular end of a transmission line, the MDP does not block the related CDRs like conventional protection schemes. Conversely, the MDP can reconstruct the lost data and detect faults using the Representative Data (RD) technique. After the fault detection, the integrated Tripping Priority Strategy (TPS) can be deployed to minimize the outage area. Moreover, the MDP also provides a Terminal Exclusion Function (TEF) to check whether a fault still exists or a CB fails. Therefore, the MDP is more advantageous than conventional protection schemes (main or backup) since it can protect larger scale power system with high selectivity and at the same time solve the problem of data loss. The MDP may encourage development in the transmission network protection and greater use of WAN based protection and control concepts.
