AVR Applicability to PRC-005

By Leland McMillan, NERC Supervisor

The North American Generator Forum (NAGF) authored a Standard Authorization Request (SAR) in 2019 which ultimately resulted in the following industry need:

The SAR proposes revisions to PRC-005-6 to provide clarity that the protective functions enabled within excitation systems (including analog/digital AVRs), and other control systems, that respond to electrical quantities and act to cease injecting current (75 MVA or greater) or trip Bulk Electric System (BES) elements either directly or via lockout or auxiliary tripping relays are within the scope of the standard. Without clear applicability, the industry is struggling with how to implement PRC-005 and what testing is acceptable to meet the required maintenance activities prescribed by PRC-005. The lack of clarity presents a reliability gap in the application of PRC-005.

Additionally, there are Protection System Station direct current (DC) supply technologies that do not currently have maintenance activities established in PRC-005. The standard needs to address battery-based station DC technologies that are not covered by PRC-005 and consider other emerging technologies, both battery-based and non-battery-based.

A technical rationale document has been developed to clarify applicability and in-scope protective functions:

The SDT recognized the need to clarify which functions within protective relays and control systems meet the criteria of a protective function for inclusion in the Protection System definition. The SDT used the following criteria to establish which functions meet the criteria of protective functions for inclusion in the definition. Functions that are implemented to initiate or prevent the automatic isolation of Facilities:

  • To protect power system Elements;
  • To maintain Stability; or
  • In response to detected faults.

Functions not applicable to the definition include those which do not initiate or prevent automatic isolation (such as limiters or functions which only provide indication) or devices which do not respond to the aforementioned scenarios; such as those detecting malfunctions of an excitation system, or automatic switching of capacitor banks for the purpose of voltage-control).

Protective functions focus on the action being performed and not the equipment itself, which allows for exclusion of components or functionality within the relay or control system that are not performing a protective function.

A Supplementary Reference and FAQ was also created to provide additional guidance. Ultimately, Draft 1 of the revised standard PRC-005-7 did not pass, but here is a link to the redline of the currently approved standard:

Microsoft Word – Project 2019-04 PRC-005-7 – redline to currently enforced (nerc.com)

While this draft standard revision has not yet passed, it will be important for generators to stay abreast of the progress made as some version of PRC-005-7 will likely go into effect in the future, requiring some form of maintenance for AVR devices. You can learn more and follow the progress on the Standard Page.