IEEE 1547. IEC 61850. UL 1741. No, these aren’t old computer models from the 1970s, these are standards that are used to ensure that the solar panel on your roof (or the electric vehicle in your garage, or the energy storage battery in your backyard) can safely and reliably interconnect with your local utility’s distribution grid. Each of these standards also covers an increasingly important component in these resources- the inverter. In order to understand why these standards and a smart inverter are important, it is important to first understand the fundamentals of electricity production.
At its most basic, the inverter converts electricity that is generated from the generation resource on your property, such as a rooftop solar panel, from Direct Current (DC) to Alternating Current (AC), allowing the electricity to be delivered to the local distribution grid. This is important because AC is how electricity flows through the grid to the electricity that flows out of your outlet. Our grid is designed to operate within certain boundaries, and when electricity is generated that pushes the grid outside of those boundaries, power quality and reliability are threatened. If you’ve ever gone through a brownout, that is the result of poor voltage quality on the grid. The transformer in your neighborhood usually ensures that voltage stays within that boundary, and technology in that transformer is used to maintain those voltage outputs. This equipment is designed to last decades since it is not expected to be used all that often. A smart inverter is really nothing more than an additional piece of software to control the actions of the inverter itself.
This bit about the physics of electricity is important; electricity generated by rooftop solar panels produces electricity in DC, which means it must be converted into AC in order to be delivered into the distribution grid or used on site. Now, if the sun was always shining and the sky was always clear, the need for a smart inverter would be less evident; but, as clouds pass between the sun and the solar panels, the quality of electricity generated diminishes and the voltage of that electricity fluctuates wildly. This means that the distribution utility must handle and mitigate this poor quality electricity in order to maintain acceptable levels of voltage across the distribution grid and to your home. That transformer is now asked to perform far more often than it was envisioned, resulting in a reduction in its expected useful life. A smart inverter, attached to the panels and on the customer side of the meter, is capable of moderating the voltage or stopping the flow of poor quality electricity to the distribution grid. However, existing standards and interconnection rules utilizing those standards currently do not allow for the smart inverter and its functions to be operational. This is changing.
Starting in 2013, the California Public Utilities Commission (CPUC) initiated an effort to reform its interconnection standards, known as Rule 21. As part of this effort, the parties and the CPUC identified that smart inverters and changing the standards were important to support the development of smart inverters as a means to address physical impacts such as poor quality of electricity generated from rooftop solar. To meet this need, the CPUC and the California Energy Commission created the Smart Inverter Working Group (SIWG) to investigate the feasibility of smart inverters in California and recommendations for modifications to Rule 21 to mandate smart inverters as part of Rule 21. It first identified the need to update the standards governing interconnection and inverters: IEEE 1547 and UL 1741.
Why are the standards important?
The Phase 1 workshop report identified a wide variety of benefits that a smart inverter is capable of providing to the grid including voltage ride-through, islanding, and providing reliability or resilience services when needed by the distribution grid operator. However, these active services were specifically prohibited by the existing standards, because the distribution grid operator wanted to ensure that when a line went down, no electricity was flowing over it. In other words, during any system emergency or event, the standard (and interconnection rule) required that the distributed generation be shut-down.
UL 1741 is the standard that ensures the safe operation and installation of the inverter and smart inverter in conjunction with IEEE 1547. Working together, these standards strived to limit the potential negative impacts of distributed resources, such as rooftop solar; in the same vein, these standards are being updated to allow for these new active services to work with the distribution grid in a way that optimizes the abilities of the smart inverter while also keeping the distribution grid operating safely. A key component of that is recognition of the benefits of a smart inverter in meeting local and grid reliability needs and services. Both standards developed interim solutions to meet the requests of California. Furthermore, the standards development organizations are working to update the whole package of standards to support more widespread use and adoption of smart inverters.
Additionally, the SIWG identified that communications between and with the smart inverter are vital to the smart inverter responding to grid conditions, messages, and signals from the distribution grid operator. As part of that effort, IEC 61850 joins the discussion as the common language for this interaction. IEC 61850 is a key standard supporting the development of the smart grid generally; a part of that standard is a communication standard for devices utilizing 61850 . Recognizing the importance of a common language, that language in 61850 is a vitally important part of any communications between a grid operator and the technology.
When these standards are fully updated and complete, the engineering and physical support for smart inverters beyond California will be ready. Existing interconnection standards across the country will need to be updated to reflect these new functionalities, and new benefits and services can be realized simply through the availability of these standards. Smart inverters, and the work to update the standards, show how important standards are to integrating new distributed resources in a way that doesn’t disrupt or harm the distribution grid, but also doesn’t disrupt the on-going development of rooftop solar, or other distributed energy resources that seek to interconnect with the distribution grid. By allowing for the use of smart inverters to provide these active services, the distribution grid operator has greater certainty that the input of poor quality electricity will be minimized at the source, which relieves the pressure on the transformer, thus, it wrings more efficiency out of the grid and extends the life of the transformer.
On the customer side, the smart inverter reduces utility uncertainty about the customer’s equipment interconnecting with the grid, ensures that total costs will be lower, and has the potential to provide additional services to the distribution grid operator, such as voltage support, helping the distribution company come back up from and outage, and other reliability services. This has the ability to be a true win-win-win for all involved- all enabled by the standards.