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PRC-025-2
Generator Relay Loadability
PRC-025-2 requires Generator Owners, Transmission Owners and Distribution Providers that apply load-responsive protective relays at the terminals of listed Elements of BES generating units and plants to set each relay to the criteria in Attachment 1, while maintaining reliable fault protection. The purpose is to keep generators from tripping unnecessarily during a system disturbance for conditions that do not pose a risk of damage to the equipment.
Who it applies to
- Generator Owners, Transmission Owners and Distribution Providers that apply load-responsive protective relays, including low voltage protection devices with adjustable settings, at the terminals of these Elements associated with BES generating units and generating plants, including Blackstart Resources:
- Generating units, generator step-up (GSU) transformers, and unit auxiliary transformers that supply overall auxiliary power needed to keep generating units online.
- Elements that connect the GSU transformer(s) to the Transmission system and are used exclusively to export energy directly from a BES generating unit or plant (they may also supply plant loads).
- Elements used in the aggregation of dispersed power producing resources.
- Attachment 1 excludes eight categories, among them relay elements in service only during start-up, elements armed only when the generator is disconnected, and low voltage protection devices without adjustable settings. The standard does not require using any of the protective functions in its Table 1.
- Category 2 Generator Owners of non-BES inverter-based resources are not subject to PRC-025-2, whose Facilities are Elements associated with BES generating units and generating plants, unless a later revision of the standard includes them.
Key dates
- July 1, 2018: effective date. PRC-025-1 was retired immediately before it.
- October 1, 2019: R1 compliance where the entity determined that relay replacement or removal was not necessary (the later of this date or 12 months after the effective date), except for the Table 1 options with their own phased dates. For example, Option 5b (asynchronous and inverter-based units, phase overcurrent 51 or 51V-R) was due July 1, 2020, or July 1, 2022 where replacement or removal was necessary.
- October 1, 2021: R1 compliance where replacement or removal was necessary (the later of this date or 36 months after the effective date).
- July 1, 2025: the last of the option-specific phased dates in the implementation plan (84 months after the effective date for the 50 element under the listed options where replacement or removal was necessary).
- Newly applicable relays: relays that become subject to the standard other than through the entity's own actions, such as a change in registration criteria or the BES definition, comply within 60 months of becoming applicable, or 84 months where replacement or removal is necessary.
Source
NERC PRC-025-2 (standard page)October 29, 2026
Requirements
What each requirement asks for, in plain English.
R1 · Set load-responsive relays to Attachment 1
What it means. Apply settings that are in accordance with Attachment 1 on each load-responsive protective relay while maintaining reliable fault protection. Table 1 of Attachment 1 gives one or more options for each application (generating unit, GSU transformer, unit auxiliary transformer, export Elements) and relay type (for example 21, 50, 51, 51V and 67), each with a bus voltage and setting criteria. Synchronous generator criteria derive from the unit's maximum gross Real Power capability reported to the Transmission Planner and its nameplate MVA at rated power factor. Asynchronous generator criteria, including inverter-based installations, derive from the site's aggregate maximum complex power capability in MVA as reported to the Transmission Planner, including the Mvar output of any static or dynamic reactive power devices. If different seasonal capabilities are reported, the maximum capability is used as a minimum requirement, and the Generator Owner may base settings on a higher capability than it reported.
Evidence an auditor expects. For each load-responsive protective relay, evidence that the settings were applied per Attachment 1, such as summaries of calculations, spreadsheets, simulation reports or setting sheets.
For inverter-based plants
Inverter-based plants, including Type 3 and Type 4 wind and photovoltaic solar, use the asynchronous generator options of Attachment 1 Table 1 (Options 4, 5, 6, 10, 11, 12, 17, 18 and 19), as do Type 1 and Type 2 induction wind turbine installations.
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