NERC PRC-026-2: Relay Performance During Stable Power Swings

Protection must not operate on stable power swings that the system would otherwise recover from

What Is PRC-026-2?

A power swing is an oscillation in generator rotor angle that occurs after a system disturbance: a fault, a sudden load change, or a generation trip. In a stable swing, the system naturally damps the oscillation and returns to a normal operating point without requiring additional intervention. In an unstable swing, the angles continue to diverge until the system separates.

The problem is that distance relays measure impedance, and during a power swing, the measured impedance traces a path through the R-X plane that can enter a relay's operating zone, even during a stable swing that the system would recover from on its own. If the relay trips in that situation, it removes a generator or transmission element during a recoverable event, potentially converting it into a cascading outage.

PRC-026-2 requires facilities with applicable relay types to demonstrate that their settings will not operate during the stable power swings identified by the transmission planner. The standard is notable because compliance depends on external study data that the generator owner doesn't control, making proactive engagement with the transmission planner essential.

Who Must Comply?

PRC-026-2 applies to Generator Owners and Transmission Owners with phase distance relays that could operate during a stable power swing. The exposure is determined by the transmission planner through system studies, not solely by facility type. Applicable situations include:

Facilities with phase distance relays on transmission lines or GSU high-side connections at 100 kV and above
Facilities identified by the transmission planner as potentially exposed to stable power swings of concern
Facilities with out-of-step tripping relays where the operating boundary could be entered during a stable swing

The first step for any generator owner with distance relays at BES voltage is to contact the transmission planner to understand whether a stable power swing study exists that covers the facility's interconnection.

Key Requirements

Obtain Stable Power Swing Data

The transmission planner must provide the applicable stable power swing data: the impedance trajectory and associated time duration that characterize the swing of concern. This data drives the entire evaluation.

Relay Setting Evaluation

Plot the stable power swing trajectory on the relay's R-X characteristic to determine whether any zone of the distance relay (including time-delayed zones) would operate during the swing. The evaluation must account for the timing of the swing relative to relay time delays.

Mitigation Where Required

If the relay would operate during the stable swing, the settings must be modified or power swing blocking (PSB) logic must be applied to prevent operation. The technical tradeoffs between swing security and fault coverage must be analyzed and documented.

Documentation

The analysis, the source swing data, the relay characteristics, and the conclusion must be documented. If no applicable stable power swing has been identified by the transmission planner, that determination should also be on record.

Common Compliance Challenges

Not Knowing You Have an Exposure

Most generator owners don't know whether a stable power swing of concern exists at their interconnection until they ask, and many haven't asked. By the time an audit reveals the gap, there may be a finding to manage. Getting the information from the transmission planner should be done proactively.

Swing Data That's Hard to Obtain

Transmission planners don't always have PRC-026-2 swing data readily available for every facility. Requesting it can initiate a study that takes months, and the data format and level of detail vary by planner. Understanding how to request and interpret the data requires familiarity with both the standard and the planning community.

Tradeoff Between Swing Security and Fault Coverage

Pulling a distance relay zone back to avoid the swing trajectory may reduce the zone's fault coverage. In some cases, there is no setting that satisfies both PRC-026-2 and adequate backup protection, requiring power swing blocking logic, zone configuration changes, or a formal documented exception.

System Changes That Invalidate Prior Analysis

When the transmission system changes (new generation added, lines reconfigured, fault current levels shift), The stable power swing trajectories change too. Prior compliance analyses tied to old system conditions may no longer be valid, and the generator owner may not be informed of system changes that affect them.

How TWC Can Help

TWC supports generator owners through the full PRC-026-2 process, from initial transmission planner engagement through relay evaluation and corrective action.

Transmission Planner Coordination

We help formulate the right data request to the transmission planner, interpret the swing data received, and assess whether the data adequately characterizes the facility's exposure.

Relay Characteristic Analysis

We plot the stable power swing trajectory against the relay's operating characteristics and determine whether any relay function would operate, accounting for time delays and multiple zone reaches.

Mitigation Development

Where settings need to change, we develop specific recommendations including zone reach adjustments and PSB logic configuration, with explicit analysis of the impact on fault coverage.

Compliance Documentation

We prepare the analysis documentation in a format organized by requirement, with the swing data source, relay characteristics, evaluation conclusion, and any corrective actions clearly documented.

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