DER Interconnection & IEEE 1547 - Steering Clear of Compliance Pitfalls

U.S. distributed energy resources (DER) capacity is expected to increase by 217 GW by 2028, according to a Wood Mackenzie Report. A staggering number, even to the seasoned solar industry professional. One of the challenges is connecting the massive solar energy supply to the electricity grid. IEEE 1547 is a substantial piece of regulation in that context. 

If you're designing solar systems that connect to the grid, you’ve probably come across IEEE 1547—maybe in an interconnection agreement, maybe in a utility email, or maybe just in conversations with your engineers. Based on the standard, even a 1.5% error in voltage sensing can put you out of compliance and trigger interconnection delays or retrofits. So, what exactly is IEEE 1547? When does it apply? And what does it require from the solar developer?

What Is IEEE 1547 and Why Does It Matter?

Some utilities require IEEE 1547 compliance for systems of 300 kW AC and larger (for example, Eversource in Massachusetts). Most others enforce it for systems over 1 MW AC. While often not front-and-center in interconnection agreements, many utilities still reference IEEE 1547 in the fine print, essentially requiring compliance whether they review it or not.

The main takeaway is that utilities generally have full discretion as far as the application of the standard is concerned.

Where Is It Enforced?

Compliance must be demonstrated at the point of common coupling, that is, where your system connects to the utility.
•    In grid-tied ground mount systems, this is usually on the medium-voltage side of the transformer.
•    In behind-the-meter rooftop systems, it’s often the AC disconnect or main service panel.

How Do You Implement It?

IEEE 1547 compliance isn’t just about choosing the right inverter; it’s about how the entire system behaves at the point of interconnection. That includes how voltage and frequency are detected, how the system reacts to disturbances, and how utility requirements are coordinated.

Grid-Tied (MV) Systems need to include:

•    A medium voltage recloser (installed on the MV side of the transformer)
•    A utility-grade relay to perform grid protection functions (e.g. 27, 59, 81O/U)
•    Voltage and frequency sensing via PTs or LEAs, with ±1% accuracy

This setup ensures that all protective decisions are based on real conditions at the point of common coupling, where the system meets the utility grid.

Behind-the-Meter Rooftop Systems should include:

•    A low-voltage disconnect or breaker
•    A utility-grade relay, often located in the main electrical room or near the service entrance
•    PTs with ±1% accuracy, installed close to the point of interconnection

Design Considerations with Compliance in Mind

Sensor placement matters. If you have long runs of PT cable, the potential for measurement error increases. Keep sensors and relays physically close.

Equipment defaults aren’t always compliant. Some reclosers ship with sensors that are only ±2% accurate, so it pays to verify the specs before ordering. Also, relay settings must match utility standards and may need to be coordinated with existing protective devices on-site.

What Does “1% Accuracy” Mean?

IEEE 1547-2018 Article 4.4 specifies the required accuracy of measurement systems that perform voltage, frequency, and power monitoring. It doesn’t mandate that each individual component (sensor, wire, relay) be 1% accurate, but rather that the system as a whole must meet that accuracy level.

Here’s a simplified version of the relevant part of the standard:

What Protective Functions Are Required?

IEEE 1547 requires several standard relay functions that ensure the system disconnects or rides through grid disturbances appropriately. The protections must operate accurately at the point of interconnection and rely on quality sensor data.
•    27 – Undervoltage
•    59 – Overvoltage
•    81U – Under-frequency
•    81O – Over-frequency
•    79 – Automatic reclose

What’s the Difference Between IEEE 1547 and UL 1741?

UL 1741 was essentially established for inverters. It’s based on IEEE 1547 requirements and ensures that an inverter can perform certain functions, like ride-through and anti-islanding, at its terminals. IEEE 1547 applies to the full system or project, not just the inverter. In other words, UL-listed inverters are not a free pass for IEEE 1547 compliance.

•    UL 1741 ≠ System compliance
•    Inverters may be certified, but measurement and control at the point of interconnection still matter
•    Transformers and long cable runs can introduce variations that the inverter can’t account for

What Is Islanding—and Why Does IEEE 1547 Care About It?

Islanding occurs when a distributed energy system continues generating power even after the utility grid goes down, creating what is called a “power island.” That sounds harmless, but it’s actually dangerous. Here's why:

If utility crews are working on lines they believe are de-energized, live power from a DER system can endanger lives. When the grid returns, unsynchronized systems may cause voltage or frequency spikes, damaging equipment or tripping other generation sources. IEEE 1547 addresses this by requiring anti-islanding protection, so the DER detects a grid outage and disconnects quickly and safely.

Avoiding Common Interconnection Pitfalls

IEEE 1547 may not be enforced equally by every utility, but it’s often still expected. And retrofitting a system to add compliant sensors or relays later can be a costly surprise. So, make sure to steer clear of these most frequent problems:
•    Assuming UL 1741-listed inverters alone are enough
•    Specifying sensors with poor accuracy
•    Letting the distance between sensors and relays create measurement loss
•    Ignoring buried IEEE 1547 references in interconnection agreements

Confidently Navigating Compliance

If your system is 300 kW or more, assume IEEE 1547 applies, even if the utility doesn’t bring it up. Designing with compliance in mind helps you avoid costly delays, failed inspections, or expensive retrofits. By selecting accurate equipment, keeping your measurement points close to your relays, and understanding where enforcement actually happens, you’ll be well-positioned to navigate compliance without headaches. And if you’re still unsure, work with engineers who’ve been through it. For more information on our solar + storage engineering services please email info@PurePower.com.