NERC PRC-024-4: Frequency and Voltage Protection Settings for Generator Owners

NERC Standard PRC-024-4, Generator Frequency and Voltage Protective Relay Settings, addresses one of the most consequential reliability challenges facing the modern power system: protective relays that trip generators off-line when the system needs them most.

The standard establishes mandatory requirements for the settings of frequency and voltage protective relays on generators connected to the bulk electric system. Its fundamental premise is that generator protection relay settings must be coordinated with the power system’s normal operating range so that protection does not operate during system disturbances that the generator should ride through.

The Problem PRC-024-4 Solves

Prior to PRC-024-4, generator protection relay settings were typically determined by equipment protection considerations alone what settings would prevent the generator from being damaged during abnormal conditions. These settings were rarely coordinated with:

• The range of frequency and voltage that the bulk electric system legitimately experiences during normal operations and disturbances
• The protection relay settings of adjacent generators and loads
• The UFLS (under-frequency load shedding) and UVLS (under-voltage load shedding) programs that deliberately allow frequency and voltage to deviate from nominal before shedding load

The result: generators tripping off-line during frequency and voltage events at precisely the moment when their continued operation was most needed for system stability.

PRC-024-4 Requirements: The No-Trip Zone

PRC-024-4 defines a “No-Trip Zone” a region of frequency and voltage within which generator protection relays must NOT cause the generator to trip. The No-Trip Zone boundaries are:

Frequency No-Trip Zone:

• Frequencies between: Generators must remain online indefinitely
• 57.0 – 59.4 Hz: Generators must remain online for at least 2 minutes before any trip is permitted
• 60.6 – 61.7 Hz: Generators must remain online for at least 30 seconds before any trip is permitted

Voltage No-Trip Zone:

• Voltages between 0.90 per unit and 1.10 per unit at the generator terminal: Protection must not trip
• During transient low-voltage events, more complex tripping time criteria apply based on voltage severity

These requirements define the minimum performance generator owners may set their protection to ride through more extreme conditions, but may not set it to trip within the No-Trip Zone.

Relationship to PRC-026-1: Transmission Relay Loadability

PRC-024-4 focuses on generator protection. The companion standard PRC-026-1 (now in its latest revision) addresses transmission relay loadability ensuring that load-responsive transmission line relays do not trip lines during stable power swings or heavy loading conditions that are within the system’s reliable operating range.

Together, PRC-024-4 and PRC-026-1 constitute NERC’s strategy for ensuring that protection systems support rather than undermine grid stability during stressed system conditions.

PRC-024-4 Compliance Process

Step 1: Applicable Relay Inventory

Identify all frequency and voltage protective relay functions that are applicable to PRC-024-4. These typically include:

• Under-frequency relay functions (ANSI 81U)
• Over-frequency relay functions (ANSI 81O)
• Under-voltage relay functions (ANSI 27)
• Over-voltage relay functions (ANSI 59)
• Rate-of-change-of-frequency (ROCOF, ANSI 81R) functions

Note that the standard applies specifically to protective functions that can cause a generator to trip — not to alarm functions, monitoring functions, or protection functions that perform other actions (such as switching to voltage regulation mode).

Step 2: Settings Review Against No-Trip Zone

For each applicable relay function, compare the current setting to the PRC-024-4 No-Trip Zone criteria. Document each setting and its compliance status.

Step 3: Remediation

For non-compliant settings, develop and implement a remediation plan. Common remediation approaches:

• Modify relay setpoints to comply with PRC-024-4 while maintaining adequate equipment protection
• Replace legacy relays that cannot provide the required setting flexibility
• Coordinate with interconnecting transmission owner where IPR settings are involved

Step 4: Documentation and Evidence

Maintain documentation of applicable relay inventory, settings, compliance assessment, and any remediation actions. This documentation is required for NERC audit response.

PRC-024-4 for IBRs: Special Considerations

For IBRs, PRC-024-4 compliance has additional complexity:

Anti-Islanding Protection: As discussed in our PRC-029-1 guide, anti-islanding algorithms can cause IBR tripping during grid disturbances that superficially resemble islanding conditions. PRC-024-4 compliance for IBRs requires confirming that anti-islanding functions do not operate within the No-Trip Zone.

ROCOF-Based Protection: Rate-of-change-of-frequency (ROCOF) protection is particularly problematic because large system disturbances produce rapid frequency rate changes that can trigger ROCOF protection prematurely. PRC-024-4 compliance analysis must assess ROCOF settings against the maximum dF/dt expected during system disturbances.

OEM Firmware Constraints: Some inverter OEMs implement protection functions that cannot be modified by the generator owner. PRC-024-4 compliance may require firmware updates or, in some cases, changes to inverter hardware.

Final Thoughts

PRC-024-4 exists because protection settings that make sense for a single generator can undermine reliability for the whole grid. A relay that trips at the first sign of frequency or voltage deviation might protect that unit — but if it fires inside the No-Trip Zone during a system disturbance, it removes exactly the generation the grid needs to stay stable, and can turn a manageable event into a cascading one.

For generator owners, PRC-024-4 compliance isn’t a one-time exercise. Relay settings drift out of compliance after equipment replacements, firmware updates, or IPR-driven setting changes, and IBR owners face additional complexity from anti-islanding logic and OEM firmware constraints that aren’t always transparent. Building a documented, repeatable process for relay inventory, settings review, and remediation is what keeps a fleet audit-ready rather than scrambling to reconstruct evidence after an NERC data request lands.

FAQs

What is the No-Trip Zone in PRC-024-4?

The No-Trip Zone is the range of frequency (59.4–60.6 Hz) and voltage (0.90–1.10 per unit) within which generator protective relays must not cause a trip. Outside that core range, PRC-024-4 defines minimum ride-through times for example, 2 minutes between 57.0–59.4 Hz and 30 seconds between 60.6–61.7 Hz — before a trip is permitted.

Does PRC-024-4 apply to alarm and monitoring functions?

No. The standard applies specifically to protective functions that can cause a generator to trip, such as under/over-frequency (81U/81O) and under/over-voltage (27/59) relay elements. Alarm-only functions, monitoring functions, and functions that trigger a mode change rather than a trip (such as switching to voltage regulation mode) fall outside its scope.

How is PRC-024-4 different from PRC-026-1?

PRC-024-4 governs generator protective relay settings. PRC-026-1 addresses transmission relay loadability, ensuring load-responsive transmission line relays don’t trip during stable power swings or heavy loading. The two standards work together so that neither generation nor transmission protection undermines grid stability during a stressed system event.

Why is PRC-024-4 compliance more complex for inverter-based resources (IBRs)?

IBRs introduce anti-islanding algorithms that can misinterpret a grid disturbance as an islanding condition and trip inside the No-Trip Zone. They also frequently rely on rate-of-change-of-frequency (ROCOF) protection, which can trigger prematurely during large system disturbances, and some OEM firmware restricts which settings a generator owner can actually modify.

What documentation is needed to demonstrate PRC-024-4 compliance?

Generator owners should maintain a complete inventory of applicable relay functions, their current settings, a documented comparison against the No-Trip Zone criteria, and records of any remediation actions taken. This evidence package is what’s typically requested during a NERC audit.

What happens if a relay setting is found non-compliant?

Non-compliant settings require a remediation plan typically adjusting setpoints to meet PRC-024-4 while still protecting the equipment, replacing relays that lack the necessary setting flexibility, or coordinating with the interconnecting transmission owner where IPR (Interconnection Protection Requirements) settings are involved.

PRC-024-4 Compliance Assessment | NERC Compliance Services