Transmission line protection using POTT scheme
Statement of Problem: Permissive Overreaching Transfer Trip (POTT) is a scheme that enables fast tripping of breakers at the local and remote end of a protected equipment. POTT schemes are typically used for high voltage line protection against faults in areas with coordination and power stability issues. Three-phase fault is the most severe type of fault that can occur and fast tripping at high voltages is paramount for personnel safety, to avoid equipment destruction, and fires. POTT schemes require a secure form of communication between both relays (local and remote). Sources of Data: For this project, I followed guidelines provided by Pacific Gas and Electric (PG&E) Transmission Line Protection Manual and Relay Application Guideline. I used Aspen software package to model the power system (transmission lines and buses). I also performed analyses such as fault-duty study, relay-setting testing, and coordination study using Aspen. I used Mathcad to create relay-setting using PG&E guidelines. In general, the settings depend on the fundamentals of POTT scheme, the fault-duty study, and the current transformer (CT) and voltage/potential transformer (PT) ratios. I used Microsoft Visio to model other diagrams and figures required to better explain how the POTT scheme works. Conclusions Reached: POTT scheme is an effective scheme for fast tripping. It requires a secure form of communication between both relays (local and remote) protecting the equipment. Using relevant manuals, guidelines and papers, I modelled and set relays to protect a 230kV line using a POTT scheme. I also provided Phase distance and Ground overcurrent settings for SEL-411L relays.