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:
- Operating the generator at a defined operating point (typically 70-100% rated MW, rated power factor)
- Applying a defined step change (typically ±2-5%) to the AVR voltage setpoint
- Recording terminal voltage, field current, and reactive power response
- 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:
- Identify the model parameters responsible for the discrepancy
- Update model parameter values to improve simulation fidelity
- Re-run simulations to confirm the updated model matches test data
- 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
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