NERC PRC-025-2: Generator Relay Loadability

Generator and step-up transformer relay settings must not operate on load conditions

What Is PRC-025-2?

Protective relays are designed to isolate faults. But under heavy load conditions (particularly during stressed system events when the grid needs generators online most), certain relay types can misinterpret load current as a fault and trip the generator unnecessarily. PRC-025-2 requires generator owners to verify that relay settings on generators and step-up transformers can distinguish between fault conditions and maximum load conditions.

The standard is the companion to PRC-023-6, which addresses loadability on transmission relays. Together they cover the full chain from the generator terminals through the step-up transformer and interconnecting transmission. For generator owners, PRC-025-2 is typically the more directly applicable of the two.

Relay types subject to PRC-025-2 include phase overcurrent elements, certain impedance-based elements used for backup protection, and other relay functions specifically identified in the standard's applicability table. Not every relay function at a facility qualifies; scope determination is the first step.

Who Must Comply?

PRC-025-2 applies to Generator Owners (GO) of BES-connected (generally >75MVA and interconnected at 100kV) generating facilities with relay types covered by the standard. The standard defines applicability by relay function type, not by generator size alone. Common relay functions that create PRC-025-2 obligations include:

Phase time-overcurrent (TOC) elements (ANSI Element 51) on generators and step-up transformers
Phase instantaneous overcurrent (ANSI Element 50) and directional overcurrent (ANSI Element 67)
Impedance-based elements (such as ANSI Element 21) on step-up transformer high-side relays

Differential relays that are inherently load-blind are generally outside scope. A relay-by-relay review of the protection one-line is needed to accurately define the compliance boundary.

Key Requirements

Relay Scope Identification

Identify all relay elements at the facility that fall within PRC-025-2 scope using the standard's applicability table. This is a relay-by-relay analysis that requires the protection one-line and relay settings files.

Loadability Calculation

Each in-scope relay element must be evaluated against the applicable loadability criterion. For overcurrent elements, this requires comparing the pickup current to the maximum load current. For impedance elements, the impedance must maintain a sufficient margin.

Setting Corrections

Relay elements that fail the loadability criterion must have settings corrected. In cases where the required change would reduce protection coverage to an unacceptable level, the situation must be documented and escalated per the standard's process.

Ongoing Maintenance

The compliance analysis must be kept current. When relay settings change, generating unit ratings are updated, or equipment is modified, the loadability analysis must be reviewed and updated to reflect the new conditions.

Common Compliance Challenges

Scope Ambiguity on Modern Microprocessor Relays

Modern relays have dozens of configurable protection elements. Some are in scope, many are not. Facilities that installed new microprocessor relays without a formal PRC-025-2 review often have backup elements enabled in default configurations that create compliance exposure.

Transformer Impedance Assumptions

The loadability calculation for impedance-based elements depends on the transformer impedance, which must be expressed in the correct per-unit or ohm basis for the relay's operating quantities. Small errors in the impedance basis produce incorrect conclusions, and the correct values aren't always easy to find in older facility records.

Interaction with PRC-023-6

Settings changes on GSU high-side relays may affect both PRC-025-2 and PRC-023-6 compliance simultaneously. Addressing one without reviewing the other can create a violation in the standard that wasn't being assessed. The two analyses need to be done together.

Setting Changes That Reduce Backup Protection Coverage

Pulling back an overcurrent or impedance element to pass the loadability criterion sometimes means that element may no longer clear all the faults it was intended to cover. This requires consideration and a documented resolution if a concern.

How TWC Can Help

TWC performs PRC-025-2 assessments for generator owners, with experience across relay types, facility configurations, and the practical tradeoffs between loadability and protection coverage.

Relay Scope Review

We review the facility's protection one-line and relay settings to identify all functions subject to PRC-025-2. Microprocessor relays can sometimes have elements enabled that are easy to miss.

Loadability Analysis

We perform the loadability calculation for each in-scope element with correct transformer impedance and CT ratio assumptions and document results in a format organized for audit review.

Setting Recommendations

Where settings fail, we develop corrected settings that satisfy the loadability criterion while preserving the maximum achievable protection coverage.

Audit-Ready Reports

Our review process includes stagegated reviews and an indentified client source for each setting evaluation, TWC provides reports that we can defend during an audit.

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