NERC's Generator Relay Loadability Standard is Now in Effect

NERC’s Generator Relay Loadability Standard PRC-025-2 became effective on July 1, 2018.

The standard establishes criteria for load-responsive protective relays to support the transmission system during transient phases of voltage disturbances. It allows individual generators to provide reactive power within their dynamic capability during transient time periods to help the system recover from disturbances.

While this standard gives Generator Owners some compliance flexibility, it also imposes significant implementation planning obligations. Impacted utilities should review their load-responsive protection schemes under various system conditions and assess whether these existing systems can be set to comply or need to be replaced.

Applicability

All load-responsive protection systems that are affected by increased generator output in response to system disturbances are covered by the standard. The standard applies to protection systems implemented by the Generator Owner, Transmission Owner, and Distribution Provider at the terminals of the generator. This includes a generator step-up transformer, unit auxiliary transformer, and elements that connect a transformer to the transmission system that are used exclusively to export electricity from a generating unit or generating plant. It also covers protection for power delivery elements utilized in the aggregation of dispersed power producing resources.

Requirements

If your company owns a generating facility that applies load-responsive protection systems, the standard sets forth a single requirement and substantial technical guidance which states:

R1 Each Generator Owner, Transmission Owner, and Distribution Provider shall apply settings that are in accordance with PRC‐025‐2 – Attachment 1: Relay Settings, on each load‐responsive protective relay while maintaining reliable fault protection.

Attachment 1

The details in Attachment 1 of the standard specify that synchronous generator relay pickup setting criteria values must be derived from the unit’s maximum gross Real Power capability, in megawatts, as reported to the Transmission Planner, and the unit’s Reactive Power capability based on the unit’s nameplate megavoltampere rating at rated power factor. If different seasonal capabilities are reported, the maximum seasonal capability must be used for the purposes of this standard as a minimum requirement. The Generator Owner may base settings on a capability that is higher than what is reported to the Transmission Planner.

For applications where synchronous and asynchronous generator types are combined on a generator step‐up transformer (GSU) or on elements that connect the GSU to the transmission system, the pickup setting criteria must be determined by vector summing the pickup setting criteria of each generator type and using the bus voltage for the given synchronous generator application and relay type.

The standard includes extensive guidelines and technical materials after Attachment 1.

Implementation Plan

PRC-025-2 will be implemented in phases, allowing up to 84 months to comply with the later stages. However there is a 12 month compliance obligation currently in effect for setting changes in accordance with Attachment 1 of the standard. There are numerous compliance options available with longer time frames if the existing protective relays cannot meet the loadability requirements and need to be replaced. The range of compliance periods is dependent on the relay type. NERC’s implementation plan outlines the compliance phases and options in detail.

Next Steps

Generator Owners should review the details of PRC-025-2 to understand the implementation plan obligations and the extensive technical guidelines that support the standard.

TRC’s protection system experts have developed compliance reports for numerous protection and controls standards including PRC-025-1, the predecessor to PRC-025-2. Our team can help Generator Owners mange the reliability performance and compliance risks related to this standard including protection system assessments and replacements.