A modified mutual impedance based backup protection for series-compensated transmission lines

This paper proposes a modified mutual impedance based backup protection for series-compensated transmis-sion line. In case of failure of current-differential scheme, distance relaying scheme provides protection by calculating and comparing positive-sequence impedance up to fault point. However, non-linear operation of metal-oxide varistor (MOV) result in continuously varying equivalent impedance across series-capacitor during fault which affects the reach and directional discrimination feature. Inter-phase mutual impedance of line up to fault point is proportional to the distance of fault from relaying point and therefore can be exploited as a reliable relaying parameter for backup protection purpose. The reason is that the equivalent impedance across MOV protected series-capacitor does not alter inter-phase mutual impedance up to fault point. This mutual impedance can be computed utilizing sequence components of voltage and current phasors measured by phasor measurement unit (PMU) at line ends. Existing mutual impedance based scheme is highly influenced by fault resistance and synchronization error. The proposed modified algorithm is developed to overcome the effects of fault resistance and synchronization error as compared to previously developed scheme. The performance is assessed for different single line-to-ground (SLG) and double line-to-ground (LLG) fault cases with variations in fault resistance, fault distance (FD) and compensation level. Simulation results affirm the advantages of proposed scheme as an extra layer of protection over conventional schemes and prove superiority in performance over existing mutual impedance based scheme. Experimental results also demonstrate the feasibility of deployment for practical applications.