NERC PRC-027-1: Coordination of Protection Systems for Performance During Faults

Protection systems must clear faults correctly and selectively across all applicable zones

What Is PRC-027-1?

When a fault occurs on the bulk electric system, protection should operate in a defined sequence: primary protection clears the fault first, quickly and close to the fault location. If primary protection fails, backup protection clears it, removing slightly more of the system but still minimizing the outage extent. When this sequence breaks down, with backup protection operating before primary or coordination failing across ownership boundaries, a single fault can take out far more of the system than necessary, potentially removing generation and lines needed to prevent cascading effects.

PRC-027-1 requires generator owners and transmission owners to perform protection coordination studies, document the results, and keep that documentation current. The standard formalizes a practice that protection engineers have always known is important, and makes it an auditable compliance obligation.

The standard was developed alongside PRC-023-6 and PRC-025-2, and is intended to work with them. PRC-027-1 addresses fault performance coordination; the other two address loadability.

Who Must Comply?

PRC-027-1 applies to Generator Owners (GO) and Transmission Owners (TO) with BES-connected protection systems. For generator owners, the applicable protection systems include:

Generator protection relays and associated step-up transformer protection
High-side bus protection at the point of interconnection
Backup protection systems that provide coverage for faults beyond the generator's immediate zone

The scope of the coordination study must cover the full protection chain from the generator terminals through the interconnecting transmission line, including coordination with the transmission owner's protection at the remote end. Coordination across ownership boundaries is specifically required.

Key Requirements

Protection Coordination Study

Perform a coordination study covering all applicable protection zones. The study must address primary and backup coordination, cross-ownership coordination, and confirm that protection operates correctly and selectively for faults throughout the applicable zone.

Short Circuit Analysis

Coordination studies require fault current data for the maximum and minimum system conditions at the facility's interconnection. This requires a short circuit model that reflects current system configuration, including contributions from the generator and from the transmission system.

Periodic Coordination Assessment (R2)

The standard requires periodic reassessment on one of three pathways: a full coordination review every six years, a baseline comparison method that requires reassessment only when fault current levels deviate more than 15% from established baselines, or a combination of both across different sections of the system. Sites that did not have an established baseline before the standard's April 2021 effective date must complete a full coordination review as their first R2 assessment.

Communication with Adjacent Entities (R1.3)

A formal communication protocol is required when developing or changing settings that could affect coordination with electrically connected entities. Generator owners must notify adjacent protection owners of proposed settings, address any coordination concerns raised, document the resolution, and update adjacent owners when settings change. This is an auditable obligation, not just good practice.

Common Compliance Challenges

Scope Is Broader Than Existing Commissioning Studies

Most facilities have coordination studies from when the facility was built, but those studies often don't cover the full scope PRC-027-1 requires, particularly backup coordination across ownership boundaries and coordination for minimum system conditions. They also frequently predate the current relay configuration.

Short Circuit Data From the Utility

The coordination analysis requires fault current data at the interconnection bus for current system conditions. Getting that data from the transmission owner requires a formal request, and the data doesn't always arrive quickly or in a format that's directly usable in the coordination tool.

Transmission System Changes Outside the Owner's Control

New generation, retired lines, or switched capacitors added to the transmission system can change fault current levels at the generator's interconnection point significantly. The generator owner isn't always notified, and the existing coordination study becomes outdated without anyone realizing it.

Cross-Ownership Coordination Gaps

Coordinating with the transmission owner's relays at the remote end of the interconnecting line requires access to those relay settings, which the generator owner doesn't have direct access to. When settings at the remote end change, the generator owner's coordination may be affected without their knowledge.

How TWC Can Help

TWC performs PRC-027-1 protection coordination studies for generator owners, with experience in both the technical analysis and the utility coordination required to complete the work.

Short Circuit Analysis

We develop or update the short circuit model for the facility, including obtaining system impedance data from the transmission owner, and calculate fault currents for the conditions required by the study.

Coordination Study

We perform the full protection coordination analysis covering primary and backup zones, cross-ownership coordination, and both maximum and minimum system conditions, using industry-standard coordination tools.

Setting Recommendations

Where coordination gaps are identified, we develop relay setting recommendations that resolve the coordination issue while maintaining adequate protection performance across the applicable fault types.

Audit-Ready Documentation

We produce a coordination study report with full short circuit calculations, time-current coordination plots, and a requirement-by-requirement compliance summary organized for regional entity audit review.

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