Beyond Positive Sequence Modeling: The Future of Power System Studies for High DER Grids

A Technical Deep Dive for Utilities, Developers & Grid Operators

Introduction

The rapid growth of Distributed Energy Resources (DERs)—solar, wind, BESS, and inverter-based resources has fundamentally changed how power systems behave. Traditional simulation methods, especially positive sequence RMS (Root Mean Square) models, are increasingly insufficient for capturing real-world grid dynamics under high DER penetration.

The NERC technical report highlights a critical industry shift:

• Moving “beyond positive sequence” toward T&D co-simulation and EMT-based modeling.

At Keentel Engineering we specialize in helping utilities, developers, and asset owners transition to these advanced modeling frameworks ensuring compliance, accuracy, and grid reliability.

What is “Beyond Positive Sequence” Modeling?

Traditional positive sequence models assume:

• Balanced three-phase systems 
• Aggregated loads and DERs 
• Simplified system behavior 

However, real grids especially distribution systems are:

• Unbalanced 
• Highly dynamic 
• Spatially diverse 

“Beyond positive sequence” refers to simulation approaches that:

• Model phase-level behavior 
• Capture unbalanced faults 
• Integrate transmission + distribution (T&D co-simulation) 
• Include EMT (electromagnetic transient) dynamics 
  

Key Limitations of Traditional Positive Sequence Models

1. Inability to Capture Unbalanced Fault Behavior

Positive sequence models:

• Use averaged voltages 
• Ignore phase-level differences 

Result:

• Underestimation of DER tripping during faults 

2. Aggregation Masks Real DER Behavior

DER_A models aggregate:

• Thousands of distributed devices into one equivalent 

Problem:

• Cannot capture location-specific voltage variations 
• Misses feeder-level dynamics 

3. Voltage Profile Variability Across Feeders

Voltage varies significantly:

• Along feeder length 
• With DER placement 

In some cases:

• Voltage increases with distance (reverse power flow scenarios) 

Positive sequence models cannot represent this behavior accurately.

4. Transformer Configuration Impacts Are Ignored

Transformer winding (Δ-Y, Y-Y, etc.):

• Changes phase voltages during faults 

Leads to incorrect DER tripping predictions in simplified models .

5. Motor Stalling & FIDVR Not Accurately Modeled

• Fault-Induced Delayed Voltage Recovery (FIDVR) 
• Induction motor dynamics 

Require detailed modeling beyond aggregated approaches. 

When Should You Go Beyond Positive Sequence?

Based on NERC findings, advanced modeling is required when:

• High DER penetration (>20–30%)
• Unbalanced fault studies are critical
• DER tripping accuracy impacts planning decisions
• Motor stalling / FIDVR risk exists
• Mixed DER vintages (IEEE 1547-2003 vs 2018)
• DERs located near disturbance points

Key Insight:

• Distant buses → positive sequence is adequate 
• Nearby buses → detailed modeling required 
  

T&D Co-Simulation: The Industry Solution

What is T&D Co-Simulation?

A combined simulation of:

• Transmission system (PSSE, PSLF) 
• Distribution system (OpenDSS, CYME, etc.) 

Enables:

• Phase-level modeling 
• Real DER behavior representation 
• Accurate fault response 
  

Key Use Cases

1. Motor Stalling & Load Recovery

• DERs can mitigate or worsen FIDVR 
• Requires EMT-level accuracy 

2. Mixed DER Modeling (IEEE 1547)

• Different ride-through settings 
• Different trip thresholds 

Single aggregated model may not be sufficient.

3. Voltage Profile Analysis

• Feeder-level voltage diversity 
• DER location impacts 

4. DER Tripping Accuracy

• Phase-specific tripping 
• Realistic system response
  

Key Technical Observations from NERC

Aggregated models are still useful but limited

• Good for bulk planning trends 
• Not for detailed local behavior 

DER_A model is “adequate but not perfect”

• Works well in many cases 
• Fails in: 

• Mixed vintages 
• Unbalanced faults 
• Localized studies 

EMT studies are the most accurate but expensive



Challenges include:

• Data requirements (2x–10x increase) 
• Long simulation times (months) 
• High computational cost 
  

How Keentel Engineering Helps

At Keentel Engineering  we bridge the gap between traditional studies and next-generation grid modeling.

Our Core Services

Advanced Power System Studies

• RMS + EMT simulations 
• PSSE, PSCAD, PowerFactory 

T&D Co-Simulation

• Transmission + distribution integration 
• DER impact analysis 

DER Modeling & Compliance

• DER_A parameterization 
• IEEE 1547 compliance 
• NERC PRC / MOD / TPL support 

Grid Code & Interconnection Studies

• ERCOT, CAISO, PJM, WECC 
• Dynamic and transient stability 

EMT & Inverter-Based Resource Studies

• Fast control dynamics 
• Grid-forming / grid-following behavior 

Protection & Control Integration

• Relay coordination 
• System stability under faults 
  

Why Choose Keentel Engineering?

• 30+ years of engineering expertise
  
• Deep expertise in NERC & ISO requirements
• Advanced modeling tools (PSSE, PSCAD, TSAT, PowerFactory)
  
• Proven experience in renewable & BESS projects
  
• End-to-end engineering + compliance support
  

Frequently Asked Questions (FAQs)

Final Thoughts

The power grid is no longer simple, balanced, or predictable.

The future belongs to advanced simulation frameworks that capture real-world complexity.



Keentel Engineering is your partner in that transition.