Considerations and Benefits of Using Five Zones for Distance Protection
This paper discusses application considerations for communications-assisted line protective relays using five distance zones. This discussion includes how modern microprocessor-based relays can benefit the power system when properly applied to pilot protection and backup step-distance schemes. Different problematic scenarios—like mutual coupling, series compensation, long lines with adjacent short lines, current transformer saturation, and the use of coupling-capacitor voltage transformers (CCVTs)—are discussed. The paper analyzes using multiple zones in step-distance and pilot protection schemes such as permissive overreaching transfer trip (POTT), directional comparison unblocking (DCUB), and directional comparison blocking (DCB).