Why Dynamic Modelling Is Required for PRC-029-1 Compliance

A settings review cannot demonstrate that an inverter-based resource meets PRC-029-1's performance requirements, and not all dynamic modelling approaches are equally capable

Why a Settings Review Is Not Sufficient

PRC-029-1 imposes two distinct types of requirements. Protection settings must not operate within the defined ride-through zones, but the standard also requires that the facility meet specified performance criteria during and after a disturbance.

Performance requirements include:

Active power recovery trajectory following fault clearance
Reactive power response during the voltage event
Post-fault behavior of inverter controls and plant controller

None of these can be read from a settings sheet. They are determined by inverter control logic and plant controller configuration, and can only be assessed through dynamic simulation. A facility with compliant settings may still fail performance requirements if controls curtail output incorrectly, inject reactive current in the wrong direction, or recover too slowly after the fault clears.

Positive Sequence Modelling: Permitted, But With Important Caveats

The standard accommodates positive sequence dynamic simulations (PSSE, PSLF) as a compliance modelling approach. For IBR compliance under PRC-029-1, however, the limitations of positive sequence tools are more consequential than they are for bulk system studies.

Key limitations of positive sequence models for IBR compliance:

Generic transfer function representations, not built from actual inverter control code
May not capture control mode transitions or protection interactions during fault events
Phasor approximations are not designed for millisecond-scale inverter dynamics
A positive sequence simulation may show apparent compliance while actual inverter behavior would not

EMT simulation, specifically real-code models built from actual inverter control software, provides a substantially more defensible compliance basis.

Why EMT Simulation Provides a Stronger Compliance Basis

EMT tools such as PSCAD operate at a level of detail that positive sequence tools are not designed to provide. The following capabilities are specifically relevant to PRC-029-1 compliance.

Control Interaction Representation

Models the full inverter and plant controller control system as-built
Captures interactions between inverter-level and plant-level controls during fault events
Real-code models reflect actual control software behavior, not a simplified approximation

Ride-Through Mode Behavior

Accurately represents activation of dedicated fault ride-through modes
Captures enhanced reactive current injection beyond normal operating capability
Models mode transition timing as implemented in firmware, not approximated by gain blocks

Phase-Lock Loop Dynamics

Includes the PLL algorithm as implemented in inverter firmware
Represents PLL instability during voltage distortion, a known IBR failure mode
Positive sequence models typically assume ideal synchronization and do not represent PLL degradation

DC-Side Representation

Models DC bus voltage, DC protection functions, and BESS management interfaces
Enables assessment of DC overvoltage and overcurrent protection within the compliance scope
DC-side protection is invisible to positive sequence tools, which model only the AC network

Time-Domain Evaluation

Solves network and control equations directly in the time domain, with no phasor approximation
Accurately resolves millisecond-scale inverter control responses
PSSE/PSLF phasor approximations are designed for bulk system dynamics, not inverter transients

Time Step Resolution

Typical PSCAD time steps of 1–50 microseconds resolve fast inverter control dynamics
Captures protection responses and control mode transitions within individual switching cycles
Positive sequence tools use 5-10 ms time steps, too coarse to resolve these behaviors

Negative and Zero Sequence Analysis

Calculates positive, negative, and zero sequence responses simultaneously
Enables evaluation of unbalanced faults (SLG, LL) that produce significant negative sequence voltage
Positive sequence simulation cannot represent inverter response to asymmetrical conditions

Instantaneous Values vs. RMS

Resolves instantaneous waveforms, not RMS phasors
Accurately evaluates protection functions that operate on instantaneous values (common in inverters)
PSSE/PSLF RMS simulations cannot reflect instantaneous protection triggered by voltage peaks within the ride-through boundary

What Are Your Options Without a PSCAD Model?

Not every facility has an EMT model available. This is common for newer sites whose owners never commissioned one, and it is nearly universal for older inverter platforms where the manufacturer no longer supports the equipment or never developed an EMT model at all. The compliance path depends on which situation applies.

Request a Model from the Manufacturer

For inverters that are still actively supported, the OEM may have an EMT model available that was not provided at the time of commissioning. This is the first step for any facility without a model, particularly as manufacturers have developed or improved their EMT models since PRC-029-1 became effective.

Manufacturer-provided real-code models are typically the highest-fidelity option available
Model availability and licensing terms vary by manufacturer and firmware version
TWC can assist with manufacturer coordination and evaluate the model's suitability once obtained

Develop a Model from Available Documentation

Where a manufacturer model does not exist or cannot be obtained, an EMT model can be developed from inverter technical documentation, control block diagrams, and published specifications. The fidelity of this approach depends on the completeness of the available documentation.

Suitable where sufficient technical documentation is available from the OEM or commissioning records
Model assumptions and limitations must be clearly documented in the compliance evidence
Can be validated against field measurements or manufacturer test data where available

Positive Sequence Modelling with Documented Limitations

When no EMT model is available and development is not feasible, positive sequence dynamic simulation using generic IBR models (PSSE, PSLF) remains a permitted compliance path. The limitations discussed above apply, and the compliance evidence must acknowledge them.

Requires use of an appropriate generic dynamic model with parameters matched to the actual inverter where possible
Modelling assumptions and known gaps must be explicitly documented
Does not resolve instantaneous protection, PLL dynamics, negative sequence response, or DC-side behavior

Legacy and Unsupported Inverters

Older inverter platforms present the most challenging compliance path. When the manufacturer no longer supports the equipment, no model exists, and control documentation is incomplete, the assessment must be built around what can be directly measured and documented from the installed equipment.

Field measurements of inverter response to disturbances can provide direct evidence of actual behavior
Protection settings review remains possible regardless of model availability
Where full compliance cannot be demonstrated, the documentation path shifts to identifying and recording specific limitations, which may require coordination with the regional entity
Firmware update or hardware replacement may ultimately be required for sites that cannot demonstrate compliance through analysis alone

How TWC Approaches PRC-029-1 Modelling

TWC performs PRC-029-1 compliance assessments using EMT simulation where the facility and its compliance risk warrant it. Our modelling approach is matched to what the compliance question actually requires.

Model Review and Selection

We review the models available for your facility (manufacturer-provided EMT models, generic positive sequence dynamic models, or both) and assess their suitability for the specific compliance determination at hand. Where model gaps exist, we work with the manufacturer or develop supplemental modelling to address them.

EMT Simulation in PSCAD

For facilities where EMT simulation provides a material capability advantage (complex control schemes, instantaneous protection elements, asymmetrical fault evaluation, or DC-side protection scope), we perform the compliance simulation in PSCAD, using real-code or high-fidelity manufacturer models where available.

Performance Requirement Analysis

We evaluate the simulated active and reactive power response of the facility against the PRC-029-1 performance requirements for each applicable fault condition, documenting the results in a format traceable to the standard's requirements.

Audit-Ready Documentation

Each study is documented with the model description, simulation inputs and conditions, results plots, and a requirement-by-requirement compliance determination. The package is organized for regional entity review and designed to support the audit process without requiring the reviewer to reconstruct the engineer's analysis.

NERC PRC-029-1 Compliance Overview

Frequency and voltage ride-through requirements for IBRs, covering what the standard requires and how TWC approaches compliance assessments.

PRC-029-1 vs PRC-024-4: Why They Are Not the Same Standard

Both standards address ride-through, but they impose fundamentally different compliance obligations. Understanding the distinction matters for IBR owners with prior PRC-024 experience.

Need a PRC-029-1 Compliance Assessment?

Contact us to discuss your facility's ride-through and performance requirements. We work with IBR owners and operators across North America.

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