A Novel Method to Distinguish Internal and External Faults During Power Swing

The immunity to external faults is very important for any power swing unblocking scheme to improve its reliability. However, most of the available schemes to differentiate faults from power swing have poor reliability as they are not immune to external faults and switching. Phase comparison and current differential protection schemes can be suitable for this purpose, although their accuracy depend on the removal of sub-synchronous frequency and decaying dc component which is very prominent in current signals especially after close-in faults. In this context, the present article proposes a new method to distinguish internal and external faults during power swing on the transmission line by determining the orientation of Lissajous figure of two terminal current signals with the objective to improve the reliability without removing decaying dc component. An index is developed mathematically based on the change of orientation of Lissajous figure due to a fault to distinguish internal and external faults at swing condition and to detect the faulty phase accurately at various conditions. Performance of the method is also evaluated for close-in internal and external faults, switching events, noisy conditions and in presence of decaying dc component and is compared to the available methods to prove its merit.