NERC PRC-024-4: Frequency and Voltage Protection Settings for Synchronous Generators, Type 1 and Type 2 Wind Resources, and Synchronous Condensers

Protection settings must not operate within the defined frequency and voltage ride-through zones

What Is PRC-024-4?

Grid disturbances cause temporary voltage and frequency excursions. A fault on a transmission line, the sudden loss of a large generator, or a system separation event can push voltage and frequency outside their normal ranges for seconds before automatic corrective action brings conditions back to normal. If generating units trip during that transient period, they compound the problem rather than helping to resolve it.

PRC-024-4 addresses this by defining "no-trip zones": frequency and voltage ranges with associated time boundaries within which generator protection may not operate. The standard applies to synchronous generators, synchronous condensers, and Type 1 and Type 2 wind turbine generators. Inverter-based resources (Type 3, Type 4 wind, solar, and BESS) have ride-through requirements under PRC-029-1 instead.

Compliance requires verifying that every applicable frequency and voltage protection function on the generating unit is set to avoid operating within the no-trip boundaries, at the generator terminal voltage specifically, not at the high-side bus.

Who Must Comply?

PRC-024-4 applies to Generator Owners (GO) of the following BES-connected facility types:

Synchronous generators (steam, gas turbine, combined cycle, hydro, nuclear)
Synchronous condensers used for reactive support
Type 1 and 2 wind turbine generators

Note: Type 3 and Type 4 wind, solar PV, and battery storage are covered by PRC-029-1, not PRC-024-4. Hybrid facilities may have obligations under both standards depending on the generating components present.

Key Requirements

Frequency No-Trip Zone Compliance

Underfrequency and overfrequency protection elements must not operate within the frequency boundaries and time durations specified in Attachment 1 of the standard. Settings that encroach on the no-trip zone must be adjusted.

Voltage No-Trip Zone Compliance

Overvoltage and undervoltage protection elements must not operate within the voltage-versus-time boundaries in Attachment 2. This includes evaluating both the pickup voltage setting and the time delay at each voltage level.

Comparison Between Voltage Levels

The evaluation must consider a voltage excursion at the high side of the GSU. Settings expressed at the generator bus voltage or in terms of relay secondary quantities must be translated to the high side voltage before comparing to the no-trip boundaries.

Documentation of Setting Verification

A documented analysis must exist for each applicable relay function at each generating unit, showing the specific settings, the voltage or frequency basis used, and confirmation that the settings satisfy the no-trip zone boundaries.

Common Compliance Challenges

Conservative Historical Settings on Older Units

Generators commissioned decades ago often have underfrequency protection set tightly (sometimes 59.5 Hz or higher) based on turbine blade resonance concerns that were valid at the time. Widening those settings requires coordination with the OEM and operations staff, and sometimes formal engineering justification.

Little Margin Between Ride-Through and Machine Protection

For some units (particularly older steam turbines and certain hydro configurations), the no-trip zone boundaries are close to the settings needed to protect the machine from damage. Demonstrating compliance while maintaining meaningful protection requires careful analysis and sometimes consultation with the OEM.

AVR Replacements Triggering Re-Evaluation

When an AVR is replaced, the overvoltage and undervoltage protective functions built into the new platform may have default settings that encroach on the PRC-024-4 no-trip zone. This step is frequently missed during AVR commissioning, creating a compliance gap the owner doesn't discover until their next audit.

How TWC Can Help

TWC performs PRC-024-4 compliance evaluations for synchronous generators, wind facilities, and synchronous condensers, with experience across a range of unit types and relay platforms.

Setting Verification

We evaluate each applicable frequency and voltage protection function against the no-trip zone boundaries at the correct voltage reference, and document the analysis in a format ready for audit.

Setting Recommendations

Where settings encroach on the no-trip zone, we develop specific setting recommendations that satisfy PRC-024-4 while maintaining reasonable protection coverage, including coordination with OEM turbine protection requirements where applicable.

Post-Upgrade Assessment

After an AVR replacement or relay change, we perform a fresh PRC-024-4 review to confirm that the new protection configuration doesn't introduce ride-through violations before the unit returns to service.

Portfolio-Level Assessments

For owners with multiple generating units, we organize the assessment across the portfolio, identifying common issues and developing a consistent documentation approach for the entire fleet.

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