What models are verified?

Excitation system models (AVR), power system stabilizer (PSS) models, and voltage control loop parameters.

Is field testing required?

Testing or disturbance data analysis is required to verify model response matches actual excitation system behavior.

How often is verification required?

Verification required when units are placed in service or when excitation system modifications occur.

NERC MOD-026 Compliance | Excitation System Model Verification

Q: Who must comply with MOD-026?

MOD-026 applies to Generator Owners of BES generators with excitation systems requiring dynamic model verification.

What is NERC MOD-026?

MOD-026 ensures excitation system dynamic models accurately represent actual generator voltage control response.

Excitation System Models

Verifies AVR and PSS dynamic models match actual generator voltage control response during disturbances

Testing or Disturbance Data

Verification through field testing, staged tests, or analysis of actual system disturbance recordings

Voltage Stability Impact

Accurate excitation models critical for voltage stability studies and reactive power planning

Model Response Validation

Model vs measured response comparison validates parameters and ensures planning study reliability

Who Must Comply with MOD-026?

MOD-026 applies to Generator Owners responsible for excitation system model accuracy.

Generator Owners

Entities owning BES generators with excitation systems requiring model verification

BES Generators

Generators meeting size/voltage thresholds requiring dynamic models

Trigger Events

New units, excitation system changes, or AVR replacements

Important: Verification required when units placed in service or excitation system modifications occur. Accurate models prevent voltage stability study errors affecting planning decisions.

Excitation System Model Verification Fundamentals

Understanding excitation system dynamics is essential for model validation.

Excitation System Response

Excitation systems regulate generator terminal voltage by controlling field current. AVR response determines reactive power output during voltage disturbances. Field forcing capability, ceiling voltage, and control loop dynamics affect voltage stability. Models must capture transient and steady-state behavior accurately.

AVR Control Parameters

Key parameters include voltage regulator gain, time constants, limiters (over-excitation, under-excitation), and stabilizing feedback loops. Parameter accuracy determines model fidelity. Small parameter errors create large response discrepancies. Verification validates all critical parameters through measured data comparison.

Power System Stabilizer (PSS)

PSS adds damping to generator oscillations improving system stability. PSS tuning affects inter-area oscillation damping critical for reliability. Models must include PSS gain, washout filters, phase compensation, and output limits. PSS verification requires dynamic disturbance data showing oscillatory response.

Model Errors Impact Voltage Stability

Incorrect excitation models cause voltage stability study errors. Overstated field forcing predicts adequate voltage support unavailable during emergencies. Understated capability wastes reactive resources. Both create planning errors requiring accurate model verification to prevent reliability or economic problems.

What eGridSync Delivers for MOD-026

Applicability review and model scope determination

Field test planning and execution support

Test data vs model response comparison

Model parameter tuning and optimization

PSS®E/PSCAD model validation and updates

Verification reports with technical justification

Evidence packages for audit readiness

Ongoing compliance and testing support

Inputs Required for MOD-026 Verification

To perform excitation model verification, eGridSync requires:

Item	Examples	Why Required
Excitation System Data	AVR manufacturer/model, control diagrams, parameter settings, firmware version	Identify model type and baseline parameters
Field Test Data	Step response tests, frequency response, voltage reference changes, disturbance recordings	Measured response for model validation
Existing Dynamic Models	PSS®E .dyr files, PSCAD models, manufacturer models	Baseline for comparison and tuning
Generator Parameters	Synchronous reactances, time constants, saturation curves	Generator model affects excitation response
PSS Configuration	PSS settings, tuning parameters, enable/disable status, input signals	PSS affects overall excitation system response
Planning Models	Power flow cases showing operating conditions during tests	Simulation initial conditions must match test conditions
SCADA/PMU Data	High-resolution voltage, current, MW, MVAr recordings during disturbances	Accurate measured data for validation
Prior Verification Results	Previous test reports, model updates, NERC audit findings	Baseline and address known deficiencies

Common MOD-026 Failure Points

Default Parameters Not Validated

Using manufacturer default parameters without field verification. Defaults represent typical settings, not site-specific tuning. Field conditions vary. Models require validation against actual measured response to ensure accuracy.

Missing Test Documentation

Models exist but lack supporting test data or verification evidence. Auditors require documented model-vs-measured comparisons. Missing documentation cannot demonstrate compliance even if models are accurate.

Models Not Updated After Equipment Changes

Excitation system upgrades, AVR replacements, PSS additions without model reverification. Equipment changes invalidate previous verification. Any excitation modification triggers MOD-026 compliance obligation.

Frequently Asked Questions

What is MOD-026?

MOD-026 requires Generator Owners to verify excitation system and voltage regulator dynamic models against actual field performance through testing or disturbance data analysis. Ensures planning studies use accurate voltage control models.

Who must comply with MOD-026?

Generator Owners of BES generators with excitation systems requiring dynamic model verification. Applies when units placed in service or excitation system modifications occur.

What excitation models are verified?

AVR models (voltage regulator), PSS models (power system stabilizer), exciter models, field control loops, and limiters. All components affecting generator voltage control response.

Is field testing always required?

Testing or analysis of disturbance data is required. Staged field tests provide controlled verification. Actual system disturbance data can validate models if sufficient quality and instrumentation available.

How often is MOD-026 verification required?

When units placed in service, excitation system modified, AVR replaced, PSS added/tuned, or periodically per entity procedures. Most perform verification every 5-10 years minimum.

What test methods are acceptable?

Step response tests, frequency response tests, voltage reference changes, staged fault tests, or analysis of actual system disturbances. Method selection depends on equipment access and disturbance data availability.

How long does model verification take?

2-4 months typical including test planning, field execution, data analysis, model tuning, and documentation. Complexity depends on system type and model accuracy requirements.

What evidence is required?

Test data, model-vs-measured comparison plots, parameter justifications, tuning methodology, verification reports, and traceability to planning models. Complete documentation package required for audits.

Can manufacturer models be used directly?

Manufacturer models provide starting point but require validation against field data. Site-specific tuning accounts for actual generator, transformer, and system conditions affecting excitation response.

How does MOD-026 relate to MOD-025?

MOD-025 verifies steady-state capability (MW/MVAr limits). MOD-026 verifies dynamic excitation response. Both ensure complete generator model accuracy. MOD-026 focuses on transient voltage control behavior.

What tools are used for verification?

PSS®E, PSCAD, MATLAB/Simulink for simulation. PSAT, DSATools for disturbance analysis. Oscillography equipment, PMUs for high-resolution measurements. Specialized excitation test equipment for controlled tests.

What are common audit findings?

Default parameters not validated, missing test documentation, models not updated after excitation changes, poor model-vs-measured fit, unclear verification methodology, no traceability to planning models.

How accurate must models be?

Industry practice targets 5-10% error in voltage response magnitude and phase. Critical voltage stability studies may require tighter tolerances. Acceptable accuracy depends on study purpose and model intended use.

What if model doesn't match test data?

Requires parameter tuning or model structure changes. Sometimes indicates equipment problems (controller degradation, instrument errors). Resolving requires iterative adjustment and possible additional testing. Persistent mismatch may indicate incorrect model type selection.

How does eGridSync support MOD-026 audits?

Provide verification reports, test data analysis, model-vs-measured comparison documentation, parameter justifications, and technical evidence demonstrating compliance. Support during NERC data requests and technical discussions with auditors.

Official References

For complete requirements, refer to official NERC resources:

NERC MOD-026-1 Official PDF

Current version of MOD-026

NERC Reliability Standards Library

Complete library

Important: This page summarizes MOD-026 in original language for educational purposes. Always refer to official NERC standard for authoritative requirements.

Excitation System Model Verification