NERC MOD-026-1: Verification of Models and Data for Generator Excitation Control System or Plant Volt/Var Control Functions

Excitation system dynamic models must match actual machine behavior, verified through field testing

What Is MOD-026-1?

Dynamic models of generator excitation control systems (the AVR, the PSS, and plant-level volt/var controllers) are used by transmission planners to simulate how the grid will behave during disturbances. Those studies drive decisions about system stability margins, protection settings, and operational limits. If the models don't match reality, the studies are built on flawed data, and the decisions made from them may not hold up when an actual event occurs.

MOD-026-1 requires generator owners to validate their excitation control system models against recorded field test data. The process involves staging a small disturbance (typically a voltage step test) and comparing the system's recorded response to the model's predicted response under the same conditions. If the match is within the standard's acceptance criteria, the model is considered verified. If not, the model must be updated and re-tested.

This is one of the more operationally intensive compliance requirements for generator owners, because it requires on-site testing that must be coordinated with the transmission system operator.

Who Must Comply?

MOD-026-1 applies to Generator Owners (GO) of synchronous generators above defined nameplate thresholds connected to the BES. The threshold is generally 100 MVA aggregate nameplate for synchronous generators. Also applicable to:

Synchronous generators at or above the applicable nameplate threshold
Facilities below the threshold but designated for compliance by the transmission planner
IBRs with plant-level volt/var control functions, as specified by the applicable transmission planner

Re-verification is required at defined intervals and following modifications to the excitation system, including AVR replacements, PSS tuning changes, or control system software updates that could affect dynamic response.

Key Requirements

Staged Field Testing

Perform voltage step tests or other approved excitation to excite the control system's dynamic response. The test must be conducted at the operating conditions specified in the standard's test procedures, with the PSS in and out of service as applicable.

Model-to-Field Comparison

Simulate the same test condition in the dynamic model and compare the model response to the recorded field response. The comparison must cover defined performance metrics (response time, overshoot, settling time) within the acceptance criteria specified in the standard.

Model Update if Required

If the model doesn't match the field response within the required tolerance, the model parameters must be adjusted and the comparison repeated until the criteria are met. Submitting an unverified model is not compliant, regardless of how well-intentioned the effort was.

Model Data Submission

Submit the verified model data to the applicable transmission planner in the required format. Maintain records of the test data, comparison analysis, and submission confirmation as evidence of compliance.

Common Compliance Challenges

Test Scheduling Takes Months

Voltage step tests affect the reactive power seen by the transmission system, so they require coordination with the system operator for an acceptable test window. Getting that window scheduled, and completing the test without the unit being redispatched or the test aborted mid-way, takes advance planning and sometimes multiple attempts.

Older AVR Platforms That Can't Be Modeled Accurately

Some AVR platforms (particularly older analog systems or first-generation digital platforms) have dynamic behavior that can't be represented accurately within the standard model structures used in PSSe or PSLF. When the model structure doesn't fit the machine, you can't get a verified model without either accepting inaccuracy or replacing the AVR.

PSS Tuning Changes Trigger Re-Verification

When a PSS is tuned or re-tuned (which is increasingly common as units comply with VAR-501 or make AVR upgrades), the excitation system dynamic response changes, and MOD-026-1 re-verification is required. The re-verification obligation isn't always recognized at the time of the PSS work.

Model Format Requirements Vary by Planner

Different transmission planners use different simulation tools (PSSe, PSLF, PowerWorld) and require model submissions in specific formats. A model verified in one format may need conversion and re-validation before submission to a planner using a different tool.

How TWC Can Help

TWC performs MOD-026-1 model verification for generator owners, with experience across AVR platforms, simulation tools, and the system operator coordination process.

Test Planning and Operator Coordination

We develop the test procedure and manage coordination with the system operator to secure a test window, including pre-test communications and the technical justification for the test parameters.

On-Site Testing

We conduct the voltage step tests on-site, capture high-resolution data from the appropriate measurement points, and confirm that the recorded response is usable for model comparison before leaving the site.

Model Development and Comparison

We develop or update the excitation system model, simulate the test condition, and perform the model-to-field comparison using the appropriate simulation tool. If the model doesn't match, we tune the parameters and rerun until the criteria are met.

Model Submission and Documentation

We prepare the model data in the format required by the transmission planner, coordinate the submission, and produce compliance documentation covering the full verification process.

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