PRC-026 Compliance Guide for Transmission Relay Performance During Stable Power Swings

Introduction

Modern transmission networks face an increasing challenge: maintaining system stability while protecting critical infrastructure. As the complexity of Bulk Electric Systems (BES), grows, so does the risk of inadvertent relay operations during stable power swings. NERC PRC-026 was introduced to mitigate this risk by ensuring that load-responsive relays do not operate incorrectly under stable power swing conditions.

In this comprehensive article, Keentel Engineering explores the core concepts, practical evaluation methodologies, and lessons learned from implementing PRC-026 compliance.

What is PRC-026?

The PRC-026 standard (Relay Performance During Stable Power Swings) ensures that protective relays in the Bulk Electric System remain secure and stable during non-fault conditions that involve stable power swings.



Its main objective is to improve relay security while preserving proper fault detection and enabling reliable out-of-step tripping when necessary.

Key Terms and Concepts of  PRC-026

Understanding PRC-026 requires familiarity with several foundational terms:

Power Swing

Variations in three-phase power flow caused by changes in generation/load, line switching, faults, or system disturbances.

Stable Power Swing

A swing in which generators do not experience pole slip and the system settles into a new equilibrium.

Unstable Power Swing

Occurs when a generator or group of generators slips poles, requiring corrective action.

UPSR (Unstable Power Swing Region)

A geometrically defined area on the impedance plane where stable operation must be maintained. Relay trip characteristics must not intersect this region.

Two-Port Equivalent Network

A reduced equivalent circuit representing the BES element under study and the source impedances at both terminals (sending and receiving).

System Separation Angle (δ)

Angle difference between source voltages at the two ends of the BES element. A separation angle of 120° defines the stability boundary in PRC-026.

BES Elements and Relays Subject to PRC-026

Elements

• Transmission lines
  
• Transformers
• Generators
  

Relays

Load-responsive relay elements that can trip in <15 cycles:

• Phase distance relays (including those in schemes like DUTT, DCB, POTT, PUTT, DCUB)
  
• Out-of-step protection elements
• Certain overcurrent relays (when not excluded)
  

Noteworthy Exclusions

• Relays supervised by power swing blocking
  
• Relays enabled only during failure of other relays
  
• Switch-onto-fault relays
  
• Voltage-restrained/controlled overcurrent relays
  

How to Evaluate PRC-026 Compliance

1. Building the Relay List

Start by identifying all relay elements that could trip a BES element. This usually requires reviewing:

• Breaker tripping schematics
  
• Protection documents
• Relay settings
  

Tip: Don't exclude phase distance elements with time delays >15 cycles if they participate in high-speed communication-aided schemes (such as DUTT).

2. Transmission Line Relay Evaluation

Phase Distance Relays

a. Instantaneous Zones

• Check if the relay characteristic fully fits inside the UPSR boundary.
  
• If not, reduce reach, change to a lens characteristic, or use blinders.
  

b. Delayed Zones (<15 cycles)

• Same evaluation as instantaneous zones.
  
• Alternatively, increase delay to ≥15 cycles if coordination and stability allow.
  

c. DUTT Schemes

• Evaluate remote relay characteristics at the local terminal UPSR.
  
• Example: Remote Mho circle in DUTT must be fully inside local UPSR.
  

d. DCB Schemes

• Evaluate the local zone first.
  
• If out of UPSR, check reverse element contribution. Be cautious with infeed effects.
  

e. POTT/PUTT/DCUB Schemes

• The intersection of local and remote zones must be fully inside UPSR.

Out-of-Step (OOS) Protection

• Typically implemented in line relays.
  
• The tripping region (defined by blinders or circles) must be inside UPSR.

Phase Overcurrent Relays

• Generally excluded unless improperly supervised.
• If evaluated, set pickup > computed threshold per PRC-026 guidelines.
  

Relays That Tripped on a Power Swing

• Must be evaluated against Attachment B criteria.
  
• Use COMTRADE records for analysis.
  

Time-Domain Phase Distance Elements

• New and challenging to evaluate.
  
• Contact relay manufacturer for guidance.
  

3. Transformer Relay Evaluation

• Differential relays are excluded.
  
• Evaluate any phase distance elements that protect both the line and transformer.
  
• Carefully adjust impedance levels across different voltage sides using per-unit conversion or transformer ratio-based conversion.
  

4. Three-Terminal Lines

• Construct three two-port equivalents.
  
• Evaluate combinations of local/remote relay characteristics.
  
• POTT: Overlapping region of local and remote zones must stay in UPSR.
  
• DCB: Local zone (excluding common region covered by remote zones) must stay in UPSR.
  

5. Impedance Conversion at Different Voltage Levels

When evaluating relays installed at different voltages (e.g. low-voltage side of transformer), scale impedance values properly:

• Use per-unit conversion
• Adjust primary settings based on PT/CT ratios and voltage ratio.
  
  

6. Challenging Real-Life Example

A 230kV three-terminal line protected with a relay on the 13.8kV side of a transformer.

• Required careful model reduction and impedance conversion.
  
• Used blinders to correct DUTT scheme violation at remote terminal.
  

7. Common Corrective Actions

• Reduce relay reach.
  
• Change from Mho to lens or quadrilateral characteristic.
  
• Add blinders or power swing blocking.
  
• Use remote supervision in communication-aided schemes.
  
• Replace machine reactance with a less conservative value if stability allows.
  
• Use transient stability studies to justify reduced UPSR if possible.
  

For system modeling and transient simulations, see our Power System Studies service.

Automating PRC-026 Compliance

A major contribution of this paper is describing a software tool that automates compliance evaluation:

• Computes two-port equivalents
  
• Models relay behavior (phase comparator models)
  
• Evaluates overcurrent and distance relays
  
• Generates pass/fail reports with safety margins
  
• Supports large batch processing for utilities

Also read: NERC PRC-002-5 and PRC-028-1 Compliance Explanation for GOs and TOs and PRC-029-1 for Inverter-Based Resources.

Conclusion

• Review breaker schematics carefully to build the correct list of relays.
  
• DUTT relays must be evaluated even though the standard doesn't explicitly list them.
  
• Multi-terminal elements require special care.
  
• Use correct impedance scaling when evaluating across different voltage levels.
  
• Automated tools can greatly streamline compliance studies.

Need Help with PRC-026 Implementation?

Keentel Engineering offers complete support in PRC-026 studies, relay modeling, transient stability analysis, and compliance documentation. Our NERC Compliance team works closely with utilities and asset owners to maintain system reliability and avoid regulatory risk.

Contact Us today for technical consultation or a compliance evaluation quote.

Why Choose Keentel Engineering

At Keentel Engineering, we bring deep technical expertise in protection system modeling, transient stability analysis, and relay coordination. Our engineers have hands-on experience with PRC-026 compliance audits and software automation tools that streamline complex evaluations. Whether you’re a transmission operator, generator owner, or protection consultant, we ensure your systems remain secure, stable, and audit-ready. We don’t just interpret standards — we help you implement them with precision.

FAQs: PRC-026 for Electrical Engineers