Understanding NEC Article 691: Large-Scale Photovoltaic (PV) Electric Supply Stations

NEC Article 691 establishes the requirements for large-scale photovoltaic (PV) electric supply stations, often referred to as solar farms. These systems are designed to supply electricity to utility-operated grids and are characterized by their substantial size and operational complexity.

Scope of Article 691

This article applies to:

• PV electric supply stations with a minimum capacity of 5000 kilowatts (kW) or 5 megawatts (MW).
• Facilities designed for electric supply transfer to regulated utility systems.

Exclusions: Facilities under exclusive utility control (see ANSI/IEEE C2-2017 for such installations).

Key Examples:

• Copper Mountain Solar Facility (802 MW), Nevada, USA.
• Bhadla Solar Park (2245 MW), Rajasthan, India.

Key Requirements for Large-Scale PV Systems

1. Special Requirements (691.4)
   • Access is restricted to qualified personnel only.
   • Systems must connect to the utility grid through approved medium- or high-voltage switchgear.
   • On-site electrical loads are limited to auxiliary equipment essential for PV power generation.
   • A central command center must monitor operations.
2. Engineered Design and Documentation (691.6, 691.7)
   • Engineered Design: All electrical systems require a stamped design by a licensed professional electrical engineer.
   • Construction Conformance: Documentation must confirm adherence to the engineered design before commercial operation.
3. Operating Voltage (691.8)
   • Direct current (DC) voltage calculations are included in the design documentation to ensure compliance.
4. Disconnecting Means (691.9)
   • Disconnects may be located remotely from the equipment.
   • Detailed isolation procedures must be part of the engineered design.
5. Fire Mitigation Plans (691.10)
   • Fire mitigation is required for PV systems that do not comply with 690.11 arc-fault requirements.
   • Plans should include site access roads and emergency response procedures.
6. Fence Bonding and Grounding (691.11)
   • Documentation must detail grounding and bonding for fences, particularly near substations and generation equipment, to address step and touch potentials.

Key Equipment Standards

1. Listing and Labeling (691.5)
   • Equipment must be listed or labeled for the application or validated by engineering review.
2. Installation Standards
   • Field-applied hazard markings must comply with 110.21(B).
   • Medium- and high-voltage switchgear must adhere to stringent safety and performance criteria.

Applications

NEC Article 691 supports the safe installation and operation of:

• Utility-Scale Solar Farms: Facilities designed to supply electricity to power grids.
• Group Solar Installations: Smaller stations collectively exceeding 5 MW in capacity.
• Dedicated Supply Stations: Non-utility-controlled generation facilities.

Conclusion

NEC Article 691 provides critical safety and operational standards for large-scale PV electric supply stations. Its focus on engineered design, restricted access, and robust fire mitigation ensures that these installations integrate seamlessly with utility systems while prioritizing safety.