Optimizing Your Potential on Permissive Schemes

As Inverter-Based Resources (IBRs) become more prevalent on the Transmission System, stakeholders are shifting from blocking schemes that have historically been utilized for pilot protection, to permissive schemes. Permissive schemes provide weak-infeed logic and a more secure pilot protection scheme compared to dependable blocking schemes historically installed on the power system. This paper provides a high-level overview of the historical use of Directional Comparison Blocking schemes and the subsequent industry transition to permissive pilot protection schemes like Directional Comparison Unblocking and Permissive Overreaching Transfer Trip. As industry stakeholders transition to permissive pilot schemes, it is important to understand the impact to design, especially the location and reliance upon the line polarizing voltage potential. This paper shares lessons learned on optimizing the use of the polarizing voltage for permissive schemes and issues to identify when implementing these permissive pilot protection schemes to maintain dependability.