Directional Elements – How Fast Can They Be?
Directional comparison schemes provide dependable and secure power line protection. Directional elements are at the heart of directional comparison schemes. Voltage-polarized directional elements require voltages and currents to declare forward or reverse fault conditions. Typically, coupling capacitor voltage transformers (CCVTs) supply voltage signals to directional elements in transmission line protection applications. Recently, traveling wave-based (TW-based) directional elements have been introduced into line protective relays. Because of their construction, CCVTs present challenges to these TW-based directional elements. This paper shows that TW-based directional elements can work with CCVTs by relying on the stray capacitances of the tuning reactor and step-down transformer. In this paper, we use field events captured during forward and reverse faults to analyze the performance of a permissive overreaching transfer trip scheme that combines incremental-quantity and TW-based directional elements for achieving end-to-end scheme operating times on the order of 4 to 6 ms.