NERC MOD-026-2 Generator Model Validation: Complete Compliance Guide

NERC MOD-026-2 is one of the most technically demanding reliability standards facing generator owners operating in the North American bulk electric system. Formally titled Verification of Models and Data for Generator Excitation Control System or Plant Volt/VAr Control Function, MOD-026-2 requires generator owners to verify that their dynamic simulation models accurately represent actual generator behavior and to update those models when they don’t.

The purpose of this requirement is fundamental: transmission planners and reliability coordinators use generator dynamic models to simulate the behavior of the power system during disturbances. If those models are wrong, the simulations are wrong and planning decisions made on the basis of inaccurate simulations can lead to insufficient transmission reinforcement, incorrect protection settings, and reliability events.

Who Must Comply with MOD-026-2?

NERC MOD-026-2 applies to Generator Owners that own synchronous generators and IBRs connected to the BES that use voltage regulation through an excitation control system or plant volt/VAr control. This includes virtually all utility-scale synchronous generators (steam, combined cycle, hydro, combustion turbine) and most utility-scale IBRs with plant-level reactive power control.

The companion standard NERC MOD-027-1 addresses verification of governor and turbine models for synchronous generators. Both standards are typically addressed together in a combined model validation program.

The Verification Testing Process

MOD-026-2 requires generator owners to perform disturbance-based testing that exercises the excitation control system or volt/VAr control function and records the response. Test results are then compared to simulation predictions using the submitted dynamic models.

Step Voltage Tests (AVR Step Tests)

The most common MOD-026-2 test type is the automatic voltage regulator (AVR) step test, which involves:

1. Operating the generator at a defined operating point (typically 70-100% rated MW, rated power factor)
2. Applying a defined step change (typically ±2-5%) to the AVR voltage setpoint
3. Recording terminal voltage, field current, and reactive power response
4. Comparing recorded waveforms to PSSE simulation predictions using the submitted generator model

If the simulation matches the field recording within defined tolerances (typically ±10% on key response metrics), the model is verified. If not, the model must be updated to improve the match.

Model Update Process

When AVR step test results identify model discrepancies, the generator owner must:

1. Identify the model parameters responsible for the discrepancy
2. Update model parameter values to improve simulation fidelity
3. Re-run simulations to confirm the updated model matches test data
4. Submit updated model files to the applicable Reliability Coordinator and planning authority

The model parameters most commonly requiring adjustment include AVR gain (KA), AVR time constants (TA, TB, TC), AVR output limits, and exciter parameters (KE, TE, SE).

MOD-026-2 for IBRs: The EMT Model Challenge

For inverter-based resources, MOD-026-2 compliance is more complex than for synchronous generators because:

Multiple Model Types Are Required: IBRs need both positive-sequence phasor models (for use in PSS/E stability studies) and EMT models (for detailed studies of control interactions). Both types must be validated.

OEM Model Confidentiality: Many inverter OEMs protect their control system models as proprietary intellectual property, providing “black box” models that generator owners cannot directly inspect or modify. This creates compliance challenges when test results identify discrepancies.

Testing Logistics: IBRs cannot typically perform the same type of step tests as synchronous generators because inverter control systems respond to commands differently. Specialized test procedures are needed for each IBR type.

Our team has developed standardized MOD-026-2 test protocols for all major IBR technologies, including procedures that accommodate OEM black-box model constraints. See our power system studies services and NERC compliance services for details.

MOD-026-2 Compliance Timeline and Schedule

Regional entities typically establish model validation schedules based on population prioritization:

• High-priority facilities: Large generators (>500 MW), generators in constrained areas, recently commissioned facilities
• Standard-priority facilities: Generators scheduled for testing within 5-7 years of commercial operation
• Low-priority facilities: Smaller generators or those recently tested

Generator owners should proactively engage with their Regional Entity to understand their facility’s position in the validation schedule and plan accordingly.

Consequences of MOD-026-2 Non-Compliance

MOD-026-2 violations can result in NERC financial penalties and, more importantly, can undermine the reliability of power system planning studies in which your facility’s model is used. Regional entities take model accuracy seriously because inaccurate models contributed to several high-profile reliability events in the 2010s and early 2020s.

Common non-compliance scenarios include:

• Failure to perform required verification testing within the scheduled timeframe
• Failure to update models when test results identify discrepancies beyond tolerance
• Submitting model updates that improve test matching but degrade performance under other operating conditions
• Inadequate documentation of test procedures and results

Final Thoughts

MOD-026-2 compliance is ultimately about one thing: making sure the models planners rely on actually behave the way your equipment behaves in the field. That sounds straightforward, but between AVR step testing logistics, IBR OEM constraints, tolerance-band troubleshooting, and Regional Entity documentation requirements, it’s a standard that rewards early planning far more than last-minute scrambling.

The generator owners who handle MOD-026-2 smoothly are the ones who know their position in the Regional Entity’s testing schedule well in advance, budget for an outage window before it’s mandatory, and treat model updates as a routine part of operations rather than a compliance emergency. For IBR fleets in particular, getting ahead of OEM model access conversations can save months when a discrepancy does turn up.

FAQs

Is MOD-026-2 the same as MOD-026-1?

No. MOD-026-1 covered only excitation control and plant volt/VAr verification for generators. MOD-026-2 is a broader standard that carries this requirement forward for the current generation fleet, including IBRs, while MOD-027-1 continues to address governor and turbine model verification separately. Regional entities are increasingly coordinating both standards under one combined testing and submission schedule, so it’s worth confirming with your Regional Entity which version and companion standards apply to your fleet.

How often do I need to re-verify my generator model?

Verification isn’t a one-time event. Facilities are re-tested on a periodic basis set by the Regional Entity’s prioritization schedule, and re-verification is also typically triggered by major equipment changes — an AVR replacement, exciter retrofit, control firmware update, or inverter hardware change. If your facility hasn’t had any of these changes, check with your Regional Entity on where you sit in the standard testing cycle.

Can I use commissioning test data instead of running a new AVR step test?

Sometimes. Some Regional Entities will accept commissioning-stage test records or disturbance monitoring data in place of a dedicated step test, provided the data captures the required response characteristics and meets documentation standards. This is worth raising early with your Transmission Planner or Reliability Coordinator, since it can save a mobilization and an outage window.

What happens if my AVR step test doesn’t match the model within tolerance?

This is common, not a failure in itself. It simply means the submitted model needs parameter updates — typically to AVR gain, time constants, or exciter parameters — until simulated and recorded responses align within the accepted tolerance band. The compliance issue only arises if the discrepancy is identified and then not corrected, or if the fix isn’t documented and resubmitted.

Our inverters use OEM “black box” models. How do we verify something we can’t inspect?

This is one of the most common IBR compliance gaps. Generator owners generally work directly with the OEM to obtain a validated model or a parameter set tuned to match test results, since the underlying control logic itself typically isn’t editable. Where OEMs won’t fully open the model, documenting the engagement and the best-available validation approach is important for demonstrating a good-faith compliance effort.

Does a small or older generating facility still need to comply?

Applicability is based on whether the unit uses voltage regulation through an excitation control system or plant volt/VAr control and connects to the BES — not on age or size alone, though facility size does affect when your unit comes up in the Regional Entity’s prioritization schedule. Smaller and older units are more often placed in the low-priority testing tier, but they are not automatically exempt.

Who do we submit the updated model to after a discrepancy is corrected?

Updated model files are typically submitted to the applicable Reliability Coordinator and Planning Authority named in your Regional Entity’s process, along with documentation of the test procedure, the parameters changed, and the confirmation run showing the corrected model now falls within tolerance.

What’s the biggest reason generator owners fail MOD-026-2 compliance?

Based on the non-compliance patterns Regional Entities report, missed testing windows and undocumented model updates are far more common causes of violations than the tests themselves. A model that’s technically accurate but poorly documented, or a valid test that’s run outside its scheduled window, creates just as much exposure as an inaccurate model.

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