PRC-029-1 and PRC-024-3: Why They Are Not the Same Standard

Both standards address IBR ride-through during grid disturbances, but the compliance obligations they impose are fundamentally different

The Common Misconception

Inverter-based resources were previously subject to PRC-024-3, which addressed frequency and voltage ride-through for a broad range of generating technologies including IBRs. PRC-029-1 was developed specifically for IBRs and replaced PRC-024-3 as the applicable standard for those facilities. A common assumption is that PRC-029-1 is simply an updated version of PRC-024-3 for IBRs. It is not.

The two standards share a common goal: keeping generating facilities connected during grid disturbances. Beyond that, the compliance obligations differ in scope, analytical approach, and practical complexity. Key differences include:

How the ride-through obligation is framed: no-trip zones versus must-ride-through zones with performance requirements
What analytical work is required to demonstrate compliance
The complexity of the protection hierarchy that must be evaluated
Constraints introduced by inverter firmware with no equivalent in relay-based protection
A broader applicability threshold that captures facilities below the BES definition

Treating PRC-029-1 as a structural equivalent of PRC-024-3 leads to compliance gaps. The sections below explain each difference.

Where the Standards Diverge

1. The Ride-Through Obligation Is Framed Differently

PRC-024-3 defines no-trip zones. PRC-029-1 defines must-ride-through zones and adds performance requirements.

PRC-024-3: settings must not operate within defined frequency and voltage boundaries. Compliance is a settings verification exercise.
PRC-029-1: the facility must also meet active power recovery and reactive power response requirements during and after the disturbance
A facility that trips no protection but curtails output or injects reactive power incorrectly may still be out of compliance with PRC-029-1

2. The Analytical Work Is Fundamentally Different

PRC-024-3 compliance is demonstrated through a settings review. PRC-029-1 requires dynamic simulation.

PRC-024-3: collect relay settings, translate to the correct voltage reference, confirm they are outside the no-trip boundary
PRC-029-1: simulate the facility's dynamic response to each applicable fault condition using models that represent inverter control logic and protective functions
The choice of modelling tool and inverter model fidelity are part of the compliance methodology in a way that has no parallel in PRC-024-3

3. The Protection Hierarchy Is More Complex for IBRs

PRC-024-3 evaluated protection at the machine terminals and GSU high-side. PRC-029-1 requires evaluation at multiple levels through the collection system.

Protection functions exist at individual inverter terminals, medium-voltage collector equipment, and the plant controller
Voltage at the inverter terminals during a fault differs from the point-of-interconnection voltage due to collector system impedance, requiring case-by-case translation
Plant controller frequency and voltage limits must also be evaluated, as they can curtail or disconnect the entire facility independently of inverter-level protection

4. Firmware Constraints Have No PRC-024-3 Equivalent

PRC-024-3 relay settings are adjustable. IBR ride-through behavior under PRC-029-1 is governed by inverter firmware.

Frequency pickup, voltage pickup, and time delays in discrete relay elements are generally adjustable within a defined range
What can and cannot be adjusted in an inverter varies by manufacturer, model, and firmware version; some behavior cannot be changed without a manufacturer firmware update
When settings cannot be freely modified, the compliance path shifts to documenting actual inverter behavior and demonstrating whether it satisfies the standard's requirements

5. PRC-029-1 Has a Broader Applicability Threshold

PRC-029-1 captures a larger population of IBR facilities than PRC-024-3 did.

PRC-024-3 applied to BES-connected generators above defined nameplate thresholds
PRC-029-1 adds a Category 2 classification for IBRs with aggregate nameplate greater than 20 MVA interconnecting above 60 kV, even if they do not individually qualify as BES elements
This lower threshold reflects a reliability concern specific to IBRs: large numbers of smaller facilities tripping simultaneously can cause significant generation loss even when each individual unit has limited impact

6. Legacy Sites Face a More Complex Transition

Facilities that previously demonstrated compliance under PRC-024-3 cannot assume that compliance carries over to PRC-029-1.

Inverter configurations acceptable under PRC-024-3 may not meet PRC-029-1 performance requirements
Bringing legacy sites into compliance often requires firmware updates, hardware modifications, or coordination between the Generator Owner, inverter manufacturer, utility, and regional entity
Facilities with exclusions must maintain auditable documentation identifying the exclusion and the level of the standard they can comply with

Side-by-Side Comparison

Dimension	PRC-024-3	PRC-029-1
Ride-Through Frame	No-trip zones: settings must not operate within boundaries	Must ride-through zones plus performance requirements during and after the event
Core Compliance Task	Settings verification with voltage translation	Dynamic simulation of facility response to fault conditions
Protection Hierarchy	Machine terminals and GSU high-side	Inverter-level, collector system, and plant controller, each requiring separate evaluation
Setting Adjustability	Relay elements generally adjustable within range	Constrained by firmware; varies by manufacturer, model, and version
Applicability Threshold	BES-connected generators	BES IBRs plus Category 2 (greater than 20 MVA, greater than 60 kV interconnection)
Legacy Site Risk	Prior compliance does not carry over to PRC-029-1	Full reassessment required; firmware or hardware changes may be needed

Buyer Beware: Does Your PRC-029-1 Assessment Actually Cover the Standard?

If a PRC-029-1 scope of work, deliverable, or report from an in-house team or external consultant does not include an evaluation of facility performance under each applicable system deviation, carefully consider whether it satisfies the standard's requirements.

A PRC-029-1 assessment limited to settings verification is not complete. A complete assessment must demonstrate:

That protection settings do not operate within the ride-through zones (the settings review component)
That the facility's active power response meets the standard's performance requirements during and after each applicable voltage and frequency deviation
That the facility's reactive power response meets the standard's performance requirements during and after each applicable deviation
That protection and performance have been evaluated at both the inverter level and the plant controller level

A report that verifies ride-through settings but does not simulate and document the facility's performance response to each system deviation leaves the second half of the standard unaddressed. Under audit, that gap is a compliance finding.

Need a PRC-029-1 Assessment for Your IBR Facility?

TWC performs PRC-029-1 compliance engineering for inverter-based resources across North America. Contact us to discuss your facility's requirements.

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