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 59.4 Hz and 60.6 Hz: 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.
CTA: PRC-024-4 Compliance Assessment | NERC Compliance Services